# Possible to recompose without messing up focus plane?



## caseysrt (Apr 25, 2012)

For example, let's say I'm in a huge open field with nothing but grass as far as the eye can see in front of me, except there is this one tree. I want to take a picture of the tree using the rule of thirds, so, I get the tree in the center of the frame, half press to lock focus at say f11, then recompose to bring the tree to the edge of the frame, but this would move the focus plane would it not?

How can you recompose your image without changing your focus plane?


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## Josh66 (Apr 25, 2012)

You can't.  The question is, will the difference be detectable...?

Edit
If you move the camera, you move the plane of focus.  That's a fact.  That doesn't mean it can't be done.  In a lot of cases, you may never even know the difference.

In your example, I think it would be safe to recompose.

Do try to use the focus point closest to the subject, if possible though.


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## caseysrt (Apr 25, 2012)

So what is the best method for achieving the rule of thirds?


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## Josh66 (Apr 25, 2012)

caseysrt said:


> So what is the best method for achieving the rule of thirds?


Use the focus point that sits right on top of the subject, right where you want it to be.  So that you don't have to recompose.

That isn't always possible though.  When it's not possible, use the closest one.

Sometimes you have no choice but to recompose though.  It's only in a few situations that it will really mess with you.  Close focus, large aperture - you need to nail it.  Long distance or small aperture, you have some 'wiggle room'.


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## MTVision (Apr 25, 2012)

caseysrt said:
			
		

> So what is the best method for achieving the rule of thirds?



Like the above poster mentioned, you probably wouldn't notice a difference in your example. If you were shooting at a large aperture then yeah you would probably notice a difference. 

Toggle your focal points. That's what I usually do instead of using the center focal point all the time.


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## Dominantly (Apr 25, 2012)

Let's say you were shooting that tree with a focal length of 105mm, with the tree being about 100' away at f/11; you would have a total depth of field of about 204'. SO, moving your framing in this scenario, is acceptable.

When you decrease your subject distance and/or use a larger aperture, your room for error drastically decreases. For example with an 85mm lens at f/1.4 shooting a subject 7' away, all you have is 1.32" to work with.


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## Vtec44 (Apr 26, 2012)

You can actually recompose with minimal change of focus plane to a certain extend.  Instead of moving the camera, you can shift your body (up/down/left/right). By doing so, you'll maintain the same focus distance.  It's kind of ridiculous and impractical though.


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## Edsport (Apr 26, 2012)

Set the lens to manual focus and focus until you get focus comfirmation then recompose and take the shot...


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## Kerbouchard (Apr 26, 2012)

Edsport said:


> Set the lens to manual focus and focus until you get focus comfirmation then recompose and take the shot...



That would still shift the plane of focus after the focus had been achieved.

In this case, there are a few options:
1) set your focus to the hyperfocal distance and it won't matter
2) use a focus point over your intended subject
3) manual focus after the composition has been achieved
4) use a tripod, use live view, zoom in to your intended subject(in live view, not with your lens), and then you can adjust critical focus for scenes where you are shooting with a shallow dof


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## MReid (Apr 26, 2012)

Inside of 10 feet this is much more of a concern than shooting a tree out in the distance, in that case don't worry about it.


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## usayit (Apr 26, 2012)

caseysrt said:


> How can you recompose your image without changing your focus plane?



Yes.. there is only one focus plane BUT at reasonable apertures, focal lengths, etc..   the depth of field is large enough that it really doesn't matter.   

If you want to get some of the details of just how small (or large) the focus plane shifts, you can read here:
http://www.thephotoforum.com/forum/...hallenges/262742-focus-recompose-problem.html


In my case, I was shooting with a 50mm @ f/1 and I do notice the shift depending on subject distance... small enough that I can simply lean back a little to compensate.


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## KmH (Apr 26, 2012)

As mentioned, the DoF would be so deep as to make the changing position of the plane of focus from recomposing a non-issue that could be ignored.

But also mentioned was the fact that recomposing likely wouldn't be necesssary either, or at least very minimal. Compose in the viewfinder as you want the final photo, and use a focus point that is on the tree in lieu of recomposing.


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## jmtonkin (Apr 26, 2012)

I'm not sure what camera you have, but you could maybe try focus lock...

From the Nikon website:
"Focus Lock

Focus lock is used to change the composition without changing focus. If you frame the shot so that the main subject is in the selected focus point, focus, and then change the composition while keeping the shutter-release button pressed halfway to lock focus, you can create compositions in which the main subject is not in a focus point but is nevertheless in focus."


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## AperturePriority (Apr 26, 2012)

caseysrt said:


> For example, let's say I'm in a huge open field with nothing but grass as far as the eye can see in front of me, except there is this one tree. I want to take a picture of the tree using the rule of thirds, so, I get the tree in the center of the frame, half press to lock focus at say f11, then recompose to bring the tree to the edge of the frame, but this would move the focus plane would it not?
> 
> How can you recompose your image without changing your focus plane?


Yes, you can:
If you set auto focus mode at AF-S instead of AF-C or AF-A (in nikon cameras), and have AF lock (+ AE lock, if you want to!) enable in the Menu. With that setting, once you haft press to lock focus and then recompose to bring the tree to the edge of the frame; the focus plane will not change untill you release shutter released button later. This is the way I use to take pictures with Nikon camera D70 which has 5 focus points only. With my Nikon camera D300 that has 51 focus points, I rather to use off-center focus point wrapping for faster as others mentioned. 

Tu.


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## MTVision (Apr 26, 2012)

jmtonkin said:
			
		

> I'm not sure what camera you have, but you could maybe try focus lock...
> 
> From the Nikon website:
> "Focus Lock
> ...



Isn't that the same thing as focus and recomposing?


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## jwbryson1 (Apr 26, 2012)

MTVision said:


> caseysrt said:
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Why not just do as Megan suggested and toggle the focus point?  This seems pretty simple to me.  Compose the image with the tree where you want it, and in the viewfinder set the tree directly behind a focus point.  Then use that focus point to focus on the tree and take the shot.  What am I missing?  :scratch:


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## analog.universe (Apr 26, 2012)

Toggling the focus point is the first move, as others have suggested.  Sometimes however, you don't have a focus point where you need it, or the lighting or lens is such that the non-central points don't perform as well as you'd like them to.  In this instance, using a coarse matte focusing screen and focusing manually is what I find to be easiest.  Of course this requires a focusing screen upgrade, but that pays off in a number of ways.

With regard to focus and recompose itself, the longer the focal length you're using, the more recompose you can get away with.  The actually disparity in focal plane depth is proportional to the angular movement of the camera.  You have to swing the camera through a much larger angle to recompose with a wide angle lens, and thus, these lenses will give you more trouble using this technique.


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## Edsport (Apr 26, 2012)

Kerbouchard said:


> Edsport said:
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> > Set the lens to manual focus and focus until you get focus comfirmation then recompose and take the shot...
> ...


Taking a photo like the op is talking about will work with my suggestion. He's not talking about taking a macro shot...


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## Kerbouchard (Apr 26, 2012)

I didn't say it wouldn't work, but here is the OP's question.



			
				OP said:
			
		

> How can you recompose your image without changing your focus plane?


And here is your answer.


Edsport said:


> Set the lens to manual focus and focus until you get focus comfirmation then recompose and take the shot...


I merely pointed out that your answer, which basically tells the OP to focus and recompose doesn't address his question.  Also, you don't need to be in manual to focus and recompose.


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## bhop (Apr 26, 2012)

There are really only two ways to do this.

1. Use a camera that has multiple focus points and move the focus point to the edge of the viewfinder.
2. Use manual focus, ignore the center focus point, but focus by eye AFTER composition so that your tree on the edge is sharp. (hard to do with dim/small viewfinders)

Or heed some other repliers advice stating that it won't really be noticeable at small apertures.


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## dumeril7 (Apr 26, 2012)

There are several ways to accomplish this.  First, the focus lock feature is made to do exactly what the OP asked for -- half-press the shutter button to auto-focus, lock it (either with that same half-shutter press, or with a dedicated focus lock button depending on how your camera works), then move the camera to re-compose.  The original focus object stays in focus but is placed in a different part of the frame.  Using a focus point located in the right area of the frame also works.  And so does manual focus.

D7


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## MTVision (Apr 26, 2012)

dumeril7 said:
			
		

> There are several ways to accomplish this.  First, the focus lock feature is made to do exactly what the OP asked for -- half-press the shutter button to auto-focus, lock it (either with that same half-shutter press, or with a dedicated focus lock button depending on how your camera works), then move the camera to re-compose.  The original focus object stays in focus but is placed in a different part of the frame.  Using a focus point located in the right area of the frame also works.  And so does manual focus.
> 
> D7



But that's still focus and recomposing.


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## dumeril7 (Apr 26, 2012)

MTVision said:


> dumeril7 said:
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Yes.  My interpretation is that the OP wanted to know if you could re-compose without losing the original plane of focus, which is exactly what the focus lock allows you to do.  Am I missing something here?

D7


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## Josh66 (Apr 26, 2012)

dumeril7 said:


> MTVision said:
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There is no way to recompose without moving the plane of focus.  Period.  It's not physically possible.  If you move the camera, you move the plane of focus too.

Now, that doesn't mean you can't recompose.  There are really only a few situations where recomposing would cause a noticeable shift in focus.  You do need to be aware of when those situations are though, so you don't focus-recompose when you are in that situation.  Those situations have been addressed a few times already in this thread.


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## MTVision (Apr 26, 2012)

dumeril7 said:
			
		

> Yes.  My interpretation is that the OP wanted to know if you could re-compose without losing the original plane of focus, which is exactly what the focus lock allows you to do.  Am I missing something here?
> 
> D7



Everytime you focus and recompose you have to lock focus to do it right? So you half press the shutter button to get the camera to focus. As long as you are in a non-continuous focus mode you can then recompose the photo because the focus is locked. But that can still lose the original plane of focus....right?

That's all I meant. Focus has to be locked to recompose otherwise you would lose focus. Maybe i'm wrong...


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## bhop (Apr 26, 2012)

I think a lot of people don't really know exactly what a plane of focus is.


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## Josh66 (Apr 26, 2012)

bhop said:


> I think a lot of people don't really know exactly what a plane of focus is.


This is probably my most posted link.  Hopefully it will enlighten a few people:

Why Focus-Recompose Sucks


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## dumeril7 (Apr 26, 2012)

bhop said:


> I think a lot of people don't really know exactly what a plane of focus is.



Actually, that's exactly the problem in my case, and what I was missing from this discussion.  I've always imagined that the plane of focus was not really a plane, but a sphere equidistant from the sensor.  This thread sent me to the books and now I understand why recomposing invariably changes the plane.

D7


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## Josh66 (Apr 26, 2012)

dumeril7 said:


> bhop said:
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> 
> > I think a lot of people don't really know exactly what a plane of focus is.
> ...


Read that link I just posted (right above your post that I'm quoting now) - it will explain a lot.  :thumbup:


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## dumeril7 (Apr 26, 2012)

O|||||||O said:


> Read that link I just posted (right above your post that I'm quoting now) - it will explain a lot.  :thumbup:



Just read it and it was very illuminating.  Thanks!

D7


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## bhop (Apr 26, 2012)

Here's a little diagram I just made..  red line is the focus, yellow is the lens field of view.  I think it's pretty self explanatory other than that.  Note the red dot at the end of the focus line, how it moves away from the focal plane after recomposing causing the subject to be slightly oof  but usually this isn't that noticeable unless you're shooting wide open.


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## Josh66 (Apr 26, 2012)

Nice diagram.  You should make the green line (plane of focus) move with (and stay perpendicular to) the red line in the second frame though, just for clarity.

edit
Or maybe do that, but add another line (different color) where the green line used to be.  Green line = actual plane of focus, other line = intended plane of focus.
Or keep the green line where it is and make the 'actual' plane a red line.  The colors don't matter - you know what I mean.  (I hope.)


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## dumeril7 (Apr 26, 2012)

O|||||||O said:


> Nice diagram.  You should make the green line (plane of focus) move with (and stay perpendicular to) the red line in the second frame though, just for clarity.



That seems like the critical part (at least relative to the way I was misunderstanding plane of focus).  If I understand this right, the re-composing would put the plane behind the original subject.

d7


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## bhop (Apr 26, 2012)

Ah.. I just put the green line there to make it easier to see how the focus spot moves away from the subject, didn't even think about that being the plane too.. heh, heh.. but i guess it'd be something like this.  I could make a cleaner version, but I closed that illustrator file already and didn't save it.. gotta get back to work anyway.


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## Josh66 (Apr 26, 2012)

That's better.  :thumbup:


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## usayit (Apr 26, 2012)

O|||||||O said:
			
		

> This is probably my most posted link.  Hopefully it will enlighten a few people:
> 
> Why Focus-Recompose Sucks



Its exactly the same notion as I calculated in the thread I linked earlier.... if you actually run through the calculations, the person who wrote that is making a bif deal out of nothing.


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## Josh66 (Apr 26, 2012)

usayit said:


> O|||||||O said:
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It all depends on the specific shooting circumstances...  Landscapes - yeah, you can pretty much ignore that.

Wide open, up close with a 50mm 1.4 - well, we get threads weekly from people wondering why their shots aren't in focus...


The link doesn't apply to every situation, but it does apply to some.  And I think it's a good idea that people are aware of it, for those times when they are in one of those situations where it actually matters.


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## usayit (Apr 26, 2012)

since no one seems to read links:

Here's what I posted several years ago:






Here's the calculations from the same thread:

=======

I'm no math wiz.. but.. something to consider. 

Assumptions: 
* 35mm frame
* recompose to 1/2 the Field of view at a specified focal length
* 10 meter subject distance
* Focal length of 24mm
* Aperture of f/1.4

Fov = 2*arctan(d/2*f)

Fov = Field of view
d = diagonal dimension (35mm for full frame)
f = focal length.

Let's just start at a 24mm focal length

Fov = 2 arctan(35/2*24)

Fov = 72.2 degrees @ 24mm. As per assumption, Frecompose = 72.2/2 = 36.1 degrees

Next... Focus plane error when camera is rotated during recomposition (Frecompose). 

E = a - a * COS (t)

E = focus error (behind subject)
a = distance to subject
t = degrees rotation.

Let's just take a subject distance of 10 meters with a rotation of 36.1 degrees

E = 10 - 10 * COS(36.1)
E = 1.9 meters


Now let's lookup the DOF for the same parameters (I use dofmaster.com online calculator)

DOF in front of subject = 4.2 meters
DOF behind subject = 27.7 meters

If my calculations are correct, 1.9 meter focus error should still be within DOF for a 24mm focal length at f/1.4 at a subject distance of 10meters.


I ran through the calculations for 50mm at f/1.4 as well (same 10meter subject distance)
E = 0.15 meters
DOF in front of subject = 1.44 meters

yup still ok... 

How about a super fast telephoto 200 f/1.4
E = 0.038 meters 
DOF in front of subject = 0.1meters

yup still ok...


Ok ... so 10 meters might be too far.. Lets try a subject distance of 5 meters

24mm Error = 0.95 meters
DOF in front of subject = 1.34 meters.

50mm Error = 0.28 meters
DOF in front of subject = 0.39 meters

200mm Error = 0.019 meters 
DOF in front of subject = 0.03 meters.


The way I see it, your are fine until you enter in to the subject distance of 3meters or less. Of course doing this on paper is different than real world use since there are so many other factors involved (CoC for different formats for example).

These are just numbers... whether or not it matters enough to worry about is a personal decision. (I'll keep mine to myself)


Then again.. I suck at math.. so if I made a mistake please point it out.

=========


IMO, making a big deal out of something pretty small.....    don't trust everything you read online... think things through.


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## usayit (Apr 26, 2012)

O|||||||O said:


> Wide open, up close with a 50mm 1.4 - well, we get threads weekly from people wondering why their shots aren't in focus...



As I said, I shoot with a 50mm f/1... I do see the shift but its so small I can easily correct it by leaning back.   The way I see it, the link takes a corner case and generalizes it without actually making any real experimentations or calculations to prove or disprove.  90% of the time, mis-focus is the person not the focus shift due to recomposition.  This is especially true with novice shooters with multi-area AF... the camera latched onto something and lighted the big green dot without the person realizing the "something" is not intended.


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## Josh66 (Apr 26, 2012)

10 meters is pretty far from "up close".


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## usayit (Apr 26, 2012)

O|||||||O said:


> 10 meters is pretty far from "up close".



fine.. how about 5 meters:

=====

Ok ... so 10 meters might be too far.. Lets try a subject distance of 5 meters

24mm Error = 0.95 meters
DOF in front of subject = 1.34 meters.

50mm Error = 0.28 meters
DOF in front of subject = 0.39 meters

200mm Error = 0.019 meters 
DOF in front of subject = 0.03 meters.

The way I see it, your are fine until you enter in to the subject distance of 3meters or less. Of course doing this on paper is different than real world use since there are so many other factors involved (CoC for different formats for example).

===



Or did you not read?????



Do the calculations.. determine yourself.. stop being argumentative for absolutely no reason... The math is right there.   I know at some point the DOF shrinks past shift error but at that point, its a corner case and not as big of deal as the link makes it out.


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## Josh66 (Apr 26, 2012)

usayit said:


> The way I see it, your are fine until you enter in to the subject distance of 3meters or less.


I didn't see this when I made my last post...

See, I'm not talking about 5 or 10 meters...  I'm talking about "up close" - at or near the minimum focusing distance.  3 meters or less.  Like I said - it depends on the circumstances.  All I'm saying is that you need to know when focus-recompose will cause problems, which (like I've said a few times already in this thread) is rare.


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## AperturePriority (Apr 26, 2012)

bhop said:


> Ah.. I just put the green line there to make it easier to see how the focus spot moves away from the subject, didn't even think about that being the plane too.. heh, heh.. but i guess it'd be something like this.  I could make a cleaner version, but I closed that illustrator file already and didn't save it.. gotta get back to work anyway.
> 
> View attachment 7112


I think what you bhop miss here is the the plane of focus is considered as flat but itn't. It's a sphere equidistant from the sensor. You can only consider it is a flat plane in cases of the long tele focal lenses which have narrow FoV and closing the apertures down.

For the landscape photos, mostly people use normal or wide-angle lens which have wide FoV. In such a case, the plane of focus is a REAL SPHERE EQUIDISTANT FROM THE SENSOR, it isn't a flat plane anymore. Specially, when one wide-opens a normal or a wide-angle lens, her/his photos only get sharp at the center but not at the edges nor the corners.

You may have a plane of focus is close to flat at wide-open aperture on some special lenses like 58mm 1.2 Noct or the long focal lenses, but not for the other normal or wide-angle lenses.


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## Josh66 (Apr 26, 2012)

AperturePriority said:


> bhop said:
> 
> 
> > Ah.. I just put the green line there to make it easier to see how the focus spot moves away from the subject, didn't even think about that being the plane too.. heh, heh.. but i guess it'd be something like this.  I could make a cleaner version, but I closed that illustrator file already and didn't save it.. gotta get back to work anyway.
> ...


Sorry, but you are wrong.  Especially the part you put in all caps.


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## usayit (Apr 26, 2012)

O|||||||O said:


> like I've said a few times already in this thread) is rare.



Do the calculations at 3 meters or less on a 50mm f/1.4 wide open.  You've got to be even closer than you think.   The key here is "rare" as you said.. something that most online articles about the topic fail to make obvious.  For most reasonable situations.... its not something to worry about too much.   In my case, I was wondering because I was shooting 50mm at f/1 at minimum distance the lens is capable of... 1m.    I would consider this a "corner case".   Here's the test photo I took along with the thread to show the razor thin DOF i was dealing with:






In my case, a simple lean back of the head and shoulders was enough to correct... something that I practiced since then.  btw.. those toy cars are about 1 inch in length each.


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## AperturePriority (Apr 26, 2012)

O|||||||O said:


> AperturePriority said:
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So, you don't know that the plane of focus is a REAL SPHERE EQUIDISTANT FROM THE SENSOR that makes optical engineer's headaches to make a lens that sharp from corner to corner and from edge to edge?
And, you don't know how expensive for those lenses?


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## Josh66 (Apr 26, 2012)

AperturePriority said:


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Please enlighten me.


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## bhop (Apr 26, 2012)

AperturePriority said:


> I think what you bhop miss here is the the plane of focus is considered as flat but itn't. It's a sphere equidistant from the sensor.



No it isn't..

actually, maybe it's possible depending on the lens, but generally speaking, it's flat.


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## AperturePriority (Apr 26, 2012)

O|||||||O said:


> AperturePriority said:
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What you don't get is your wide-angle lenses only get sharp at the center but not at the corners and the edges when wide-opened because of the plane of focus is a REAL SPHERE EQUIDISTANT FROM THE SENSOR. The plane of focus isn't flat.


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## usayit (Apr 26, 2012)

Its not flat.... but I think we are again... splitting hairs over something not really that big of a deal in real practical sense.


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## bhop (Apr 26, 2012)

AperturePriority said:


> What you don't get is your wide-angle lenses only get sharp at the center but not at the corners and the edges when wide-opened because of the plane of focus is a REAL SPHERE EQUIDISTANT FROM THE SENSOR. The plane of focus isn't flat.



I'm no lens engineer, but it seems like that's more of an issue with having to used curved glass to get a wide field of view rather than the plane of focus.  You know the definition of 'plane' is a flat, two dimensional surface right?



usayit said:


> Its not flat.... but I think we are again... splitting hairs over something not really that big of a deal in real practical sense.



True.. i'm over it.


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## Josh66 (Apr 26, 2012)

AperturePriority said:


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I don't have any wide angle lenses.


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## AperturePriority (Apr 26, 2012)

bhop said:


> AperturePriority said:
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> > I think what you bhop miss here is the the plane of focus is considered as flat but itn't. It's a sphere equidistant from the sensor.
> ...


As I said, it's considered flat in cases of the long focal lenses which have narrow FoV, not for the normal or wide-angle lenses what have wide FoV. And, you may say that it's flat in case of Nikkor 58mm f/1.2 Noct which cost you about 5 grands used but not an average lens.


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## AperturePriority (Apr 26, 2012)

usayit said:


> Its not flat.... but I think we are again... *splitting hairs over something not really that big of a deal in real practical sense*.


I think the sentence in blue bold is what you did in your "research" at post # 38?


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## bhop (Apr 26, 2012)

This is turning into wedding on an x100 part two.. sort of..  internet arguments suck.  

The only real answer worth anything here (IMO) is that it doesn't really matter unless you're shooting at f/1.0 or doing macro work.


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## usayit (Apr 26, 2012)

AperturePriority said:


> usayit said:
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> > Its not flat.... but I think we are again... *splitting hairs over something not really that big of a deal in real practical sense*.
> ...



I've been shooting for most of my life... I never considered it a big deal until I started to shoot a "corner case" situation.... again... 50mm f/1 at close approximate ranges.   I noticed that I was missing focused and decided to figure out how big of margin I was dealing with.     So in my case, it was a real problem.  BUT (please read) I've said many times in this thread... that in most cases it DOESN'T REALLY MATTER.... and most here (including most online blogs/articles) are making big deal out of nothing.   

I posted the calculations a few months later in response to another thread....  just to show how small it is.   Its one thing to say "It doesn't matter much".  Its another to say "It doesn't matter much... and here's why".   Researching a question is NEVER a waist of time...   Allowing a small margin of error impact how you go about your business is a waist of time.

 Read... understand the intent... it takes both.


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## AperturePriority (Apr 26, 2012)

usayit said:


> Its not flat.... but I think we are again... splitting hairs over something not really that big of a deal in real practical sense.


Because the focal plane isn't flat as you stated, now you can clearly say that *recompose doesn't suck at all*!


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## bhop (Apr 26, 2012)

AperturePriority said:


> usayit said:
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> > Its not flat.... but I think we are again... splitting hairs over something not really that big of a deal in real practical sense.
> ...



Why do you think the focal plane isn't flat?


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## AperturePriority (Apr 26, 2012)

usayit said:


> AperturePriority said:
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*I've been shooting for most of my life...*<<<<Quote from usayit>>>> Because of shooting for most of your life means you are right in your conclusion "*recompose suck!*"?


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## MTVision (Apr 26, 2012)

AperturePriority said:
			
		

> I've been shooting for most of my life...<<<<Quote from usayit>>>> Because of shooting for most of your life means you are right in your conclusion "recompose suck!"?



If I read correctly he never said recompose sucks.


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## bhop (Apr 26, 2012)

MTVision said:


> AperturePriority said:
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Yeah, I haven't read that either.. unless I missed a post.


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## Josh66 (Apr 26, 2012)

AperturePriority said:


> usayit said:
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Have you even been reading this thread?  I was the one that posted the 'focus-recompose sucks' article, and usayit 'called me out' on it.  You are attacking the wrong person, lol.


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## MTVision (Apr 26, 2012)

O|||||||O said:
			
		

> Have you even been reading this thread?  I was the one that posted the 'focus-recompose sucks' article, and usayit 'called me out' on it.  You are attacking the wrong person, lol.



I think it's called "selective reading".


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## Bitter Jeweler (Apr 26, 2012)

Happens 'round here a lot.


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## AperturePriority (Apr 26, 2012)

bhop said:


> AperturePriority said:
> 
> 
> > usayit said:
> ...


Still don't understand?
When you wide-opened a normal or a wide-angle lens, the center of image is sharp but the corners are not. It's telling you that the center is in-focus, but the corners are OOF. In other words, center point and the corners are not on the same plane of focus. That happens because the focal plane isn't flat but a REAL SPHERE EQUIDISTANT FROM THE SENSOR. Get it?

Understanding that, you will know why focus-recomposing isn't messing up the focal plane at all!


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## bhop (Apr 26, 2012)

AperturePriority said:


> bhop said:
> 
> 
> > AperturePriority said:
> ...



By your reasoning, you should be able to focus in the center and rotate the camera to recompose, with any lens, y'know, since the plane is not a plane, but a sphere, and the subject will stay focused along the walls of this sphere... but in reality that doesn't happen, because the focal plane is a plane, meaning flat.


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## Josh66 (Apr 26, 2012)

Assuming the same distance, the corners on a wide angle shot would be 'behind' the corners on a normal or tele shot.  That is why the corners aren't as sharp as the center.  The corners are not on the same plane that the focus point is on.

There is really only one distance in which the corners would lie directly on the plane of sharpest focus, and that would vary from lens to lens.  In all focus distances expect for that one, the plane of focus will never intersect the corners of the frame.


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## AperturePriority (Apr 26, 2012)

bhop said:


> AperturePriority said:
> 
> 
> > bhop said:
> ...


In reality, the focal plane is a name only. It doesn't mean you call it a plane, then it has to be flat.


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## AperturePriority (Apr 26, 2012)

O|||||||O said:


> Assuming the same distance, the corners on a wide angle shot would be 'behind' the corners on a normal or tele shot.  That is why the corners aren't as sharp as the center.  The corners are not on the same plane that the focus point is on.
> 
> There is really only one distance in which the corners would lie directly on the plane of sharpest focus, and that would vary from lens to lens.  In all focus distances expect for that one, the plane of focus will never intersect the corners of the frame.


This is why I said, in cases of the LONG focal tele lenses, focal plane is CONSIDERED as flat since the lenses have narrow FoV. But for short lenses which have wider FoV, focal plane cannot be considered as flat. 
Understanding that, one now can know why focus-recompose isn't messing up the focal plane. And it doesn't suck at all!


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## Josh66 (Apr 26, 2012)

No matter the FoV, the plane of focus is still flat.  Wider lenses just include less of that plane in the frame.  I never said that focus-recompose did anything to the focal plane other than move it from it's original position.


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## MTVision (Apr 26, 2012)

AperturePriority said:
			
		

> This is why I said, in cases of the LONG focal tele lenses, focal plane is CONSIDERED as flat since the lenses have narrow FoV. But for short lenses which have wider FoV, focal plane cannot be considered as flat.
> Understanding that, one now can know why focus-recompose isn't messing up the focal plane. And it doesn't suck at all!



At 5 feet away with a 50mm lens - shooting at f/1.4 - I can focus an recompose and it won't change anything?


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## bhop (Apr 26, 2012)

Dude...I think you're getting focal plane and dof mixed up..or you're letting dof mess with your understanding of focal plane..our something...


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## AperturePriority (Apr 26, 2012)

Now I know how good are the talents of this site.
Bye,


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## bhop (Apr 26, 2012)

LoL


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## Josh66 (Apr 26, 2012)

Bye.  We'll miss you.  Please let us know how it is "out there".


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## MTVision (Apr 26, 2012)

bhop said:


> Dude...I think you're getting focal plane and dof mixed up..or you're letting dof mess with your understanding of focal plane..our something...


LOL! It was a joke. I just wanted to see what he/she was going to say!


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## analog.universe (Apr 26, 2012)

AperturePriority said:


> O|||||||O said:
> 
> 
> > Assuming the same distance, the corners on a wide angle shot would be 'behind' the corners on a normal or tele shot.  That is why the corners aren't as sharp as the center.  The corners are not on the same plane that the focus point is on.
> ...



The focal plane is actually flat.  Some lenses don't manage to achieve it perfectly, sometimes the sharp edges are farther away than the flat plane, sometimes closer, but lens designs generally try to be flat.  (hence names like "planar", known for the flatness of it's plane)

You're likely never to find a lens where the area of sharp focus is equidistant from the sensor.  If such a spherical equidistant field was the norm, there would be no difference between recomposing on a wide lens and tele lens.  When thinking about focus and recompose, a flat plane of focus is the only geometry that allows you to make the proper compensations.  (unless of course you know exactly how not-flat a specific lens is at a specific focal length and focus distance)


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## bhop (Apr 26, 2012)

MTVision said:


> bhop said:
> 
> 
> > Dude...I think you're getting focal plane and dof mixed up..or you're letting dof mess with your understanding of focal plane..our something...
> ...



I meant to quote 'AperturePriority'


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## MTVision (Apr 26, 2012)

bhop said:
			
		

> I meant to quote 'AperturePriority'



Well it could've gone for what I wrote too. I almost said something about you calling me Dude since technically it'd be Dudette


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## dumeril7 (Apr 27, 2012)

analog.universe said:


> The focal plane is actually flat.  Some lenses don't manage to achieve it perfectly, sometimes the sharp edges are farther away than the flat plane, sometimes closer, but lens designs generally try to be flat.  (hence names like "planar", known for the flatness of it's plane)
> 
> You're likely never to find a lens where the area of sharp focus is equidistant from the sensor.  If such a spherical equidistant field was the norm, there would be no difference between recomposing on a wide lens and tele lens.  When thinking about focus and recompose, a flat plane of focus is the only geometry that allows you to make the proper compensations.  (unless of course you know exactly how not-flat a specific lens is at a specific focal length and focus distance)



Thinking about this last night, the flat plane notion makes the most sense.  Being one of the folks who was confused on plane of focus concept, please tell me if I have this right...

Hypothetically taking the design of the lens out of the equation, I would expect there to naturally be a sphere of focus.  But it also seems to me that a sphere of focus is far less pragmatic than a plane of focus, since we tend to arrange our world in straight lines rather than circles and curves.  So I would think that one important but difficult job of the lens is to try to flatten the focus space so we can take photos and have them be in focus for a majority of cases.  (I.e. Its a plane because the lens tries to make it so.)  And this is why it is considered a defect for a lens to have poor sharpness from corner to corner and why lenses that are sharp across the entire frame are expensive.

Do I have that right?

D7


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## EchoingWhisper (Apr 27, 2012)

Let me add - try using Dynamic Area AF. If your AF system is good enough, dynamic area AF may be able to keep the focal plane the same.


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## Demers18 (Apr 27, 2012)

O|||||||O said:


> bhop said:
> 
> 
> > I think a lot of people don't really know exactly what a plane of focus is.
> ...



That article has the quote of the year:

"the fact that depth of field at 50+ yards distance is usually at least a few feet; even an inch of error in that case is about as significant as a fart in a tornado."


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## analog.universe (Apr 27, 2012)

dumeril7 said:


> analog.universe said:
> 
> 
> > The focal plane is actually flat.  Some lenses don't manage to achieve it perfectly, sometimes the sharp edges are farther away than the flat plane, sometimes closer, but lens designs generally try to be flat.  (hence names like "planar", known for the flatness of it's plane)
> ...



It's actually more a question of symmetry as it turns out.  The focal plane is flat because the sensor is flat, that's just how the geometry plays out.  It is more useful in practice, I agree, but the math happened to make it flat before we got that far.  Sometimes it is the corrections for other issues (CA, nonrectilinearity, etc..) that causes the deviation from the flat field.

In large telescopes, where the aberrations in the optics are harder to correct for, often the sensor itself is a mild spherical section.  This way the optics can be allowed some field curvature, which is compensated for on the other side of the glass by the sensor shape.

In the most basic hypothetical scenario though, a single element (most camera lenses have 10-20 elements) mathematically ideal lens will focus a flat plane if the sensor its projecting on is also flat.


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