# Focus and Recompose Problem



## justinisfilming (Nov 13, 2011)

Hey guys! My name is Justin, I'm new here. I don't know if this is the right place for this question (if it's not, please tell me the right place), but I'm gonna give it a shot.

So I just got the Canon 50mm 1.8 for my T3i and it's wonderful. The large aperture is great. The only problem is, whenever I focus on the subject, then recompose, my subject becomes blurry. Is it because I'm recomposing my shot?  The shallow DOF is great but with a slight movement, your subject goes out of focus. Any tips? Should I change my AF points? I currently have it on the center. What I do is I light the center dot to the subject's eye then recompose. Should I use a different AF point? Or should I use a different AF mode? I have it on ONE SHOT right now so that the focus won't change when I recompose.

Any help would be greatly appreciated!! =))

Thanks!


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## usayit (Nov 13, 2011)

* Yes... there is an "error" when you focus on the subject in the middle of the frame and recompose.  Diagram below







Plug in some numbers to figure out how large or small the error really is.  Often it still falls within the DOF which is a function of focal length, aperture and distance to subject (and format)

* Yes... it would be nice if you can select a different AF point.  The T3i should be capable of this...  refer to manual.

* Often, lenses are their sharpest towards the center of frame.  This is especially true with fast lenses.  


I often shoot with a 50mm f/1 and try to compensate by leaning back a little for a shot.   Practice to get a feel for it.  Otherwise, use the nice feature of that camera and move the AF point.


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## SGorman (Nov 13, 2011)

If you are recomposing, it is entirely possible that you are moving just that much to affect the image. With a shallow DOF, you have a very narrow plane. If you move forward even a quarter inch, you push the focus off. You might not even realize you're doing it. In instances where you can, I'd switch to manual focus.

*edit -- ... and someone posts a friggin' hand-drawn formula. Whaddaya know ... those trig classes CAN be used for something!  I was an art major dammit!    :lmao:


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## cgipson1 (Nov 13, 2011)

When using a large aperture.. the DOF can be razor thin. Any movement on the part of the camera or the subject can throw it OOF. For shooting like this, use your focus points in the viewfinder... and specify which focus point you want the camera to use, rather than moving the camera.

I usually use single point focus (spot) and move the focus point where I want it to be. Don't know if your Canon will allow that or not... but it should do something workable. My advice would be to stop down a couple of stops... unless you just really need that razor thin DOF.


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## usayit (Nov 13, 2011)

SGorman said:


> *edit -- ... and someone posts a friggin' hand-drawn formula. Whaddaya know ... those trig classes CAN be used for something!  I was an art major dammit!    :lmao:



LOL...  came up for discussion a year or so ago.   I had numbers crunched shooting a 50mm f/1 at various subject distances to show how small and controllable the focus shift can be.    Personally, I wish I was more artistic.


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## unpopular (Nov 15, 2011)

Is there a 50/1 lens? Isn't it usually 50/0.9?


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## Forkie (Nov 15, 2011)

I hope this doesn't sound patronising, but have you got your camera set to continuous focus instead of "One Shot"?  I've done that a few times and missed shots by forgetting I have it on continuous and vice versa.


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## usayit (Nov 15, 2011)

unpopular said:


> Is there a 50/1 lens? Isn't it usually 50/0.9?



The max aperture of a lens is by design and not limited by some sort of physical law.  There are versions of my lens in f/1.2, f/1, and f/0.95.


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## Edsport (Nov 15, 2011)

When you focus on your subject by pressing the shutter button half way, you have to keep the button pressed half way while you are recomposing and take the shot by pressing the shutter button the other half.


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## usayit (Nov 15, 2011)

Edsport said:


> When you focus on your subject by pressing the shutter button half way, you have to keep the button pressed half way while you are recomposing and take the shot by pressing the shutter button the other half.




Half shutter button (as you describe) does not correct for focus error due to recomposing the photo.


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## MReid (Nov 15, 2011)

For those razor thin depth of field shots, use Continuous focus and choose your single point focus point so it can be right on your subjects eye area, where exactly depends on the subjects face angle in relation to your lens. With practice this is faster than recomposing.

Focus recompose will probably work outside of 10 feet or so but the closer you get to your subject the more it will fail.


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## Edsport (Nov 15, 2011)

usayit said:


> Edsport said:
> 
> 
> > When you focus on your subject by pressing the shutter button half way, you have to keep the button pressed half way while you are recomposing and take the shot by pressing the shutter button the other half.
> ...


True but i'm wondering if the OP was focusing then letting go of the shutter button and recomposing to take the shot...


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## usayit (Nov 15, 2011)

That is not how I read it....   either way...   focus error is still present.


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## Steve01 (Nov 15, 2011)

If you're shooting wide open you may want to enable all your focus points and use the one closest to your subject.
As already mentioned at f1.8 the DOF can be virtually non existent if your really close to the subject.


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## MTVision (Nov 15, 2011)

I shoot wide open fairly often due to low light but I never focus and recompose at 1.4/1.8. The DOF is just too shallow. The slightest focusing error will cause the photo to come out OOF and that's without recomposing. Is this only happening when you are shooting with a large aperture or all the time? Use another focus point or close the lens down to a smaller aperture.


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## usayit (Nov 15, 2011)

Found the thread with the rest of the discussion from which the diagram was originally a subject of.   Just reposting it here for some perspective...  


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.


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## Edsport (Nov 16, 2011)

usayit said:


> That is not how I read it.... either way... focus error is still present.


I think you misread mine as well and yes focus error can still be present lol. I think people get that part of it...


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