# Question that got stuck in my head...



## iskoos (Mar 16, 2010)

How is it possible that as you go from one of the the lowests f-stops (like 1.4) to all the way to f/16, f/22, f/32 or whatever, the image circle size doesn't change causing vignetting and/or FOV doesn't get smaller?
(Assume fixed focal length please...)

I just cannot come up with an explanation


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## iskoos (Mar 17, 2010)

He he... I wasn't expecting an immediate answer to this question.
Looks like I am not the only one that puzzled with it


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## Hardrock (Mar 17, 2010)

Im not a hundred percent sure but I will give it a try. Just because you change the F-stop doesn't mean you change the focal length. By changing focal length the fov changes but when changing the F-stop the entire image is still going through the lens and hitting the sensor.  The F-stop only changes the amount of light and Dof. I know my explanation sucks and may be wrong but that is the best I could come up with.


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## KmH (Mar 17, 2010)

Because the lenses bend the light.

The APS-C size image sensor is only 15.8mm high and 23.6mm wide (Nikon) so it's just under 1" wide. *1" = 25.4 mm*


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## iskoos (Mar 17, 2010)

> Im not a hundred percent sure but I will give it a try. Just because you change the F-stop doesn't mean you change the focal length. By changing focal length the fov changes but when changing the F-stop the entire image is still going through the lens and hitting the sensor. The F-stop only changes the amount of light and Dof. I know my explanation sucks and may be wrong but that is the best I could come up with.


 
All of the above saying is true but I didn't understand what it has (even remotely) anything to do with what I am asking
Plus I already said "Assume fixed focal length please...) so why chaging focal length. Sorry you tried but not even close...

KmH, what you are saying is true as well but it is not the answer to my question.

My question is not a cropped sensor body question. It is a generic questions that can apply to any SLR camera. 

Say you have a 50mm prime lens (CAN'T change the Focal Length!..)
at f/1.0 your aperture diameter is 50mm (a huge opening). You take a picture. Then you stop down to smallest opening say f/22 (or maybe smaller doesn't matter). At f/22, all you have is an opening that is less then 3mm. All other parameters same. You take another picture and you have the SAME FOV... (don't get into DOF, I am NOT even talking about it) you get no vignetting on the sensor. Same picture on both images (with different DOF of course) but same FOV!...
This I cannot explain. The only one explanation I can come up with is that if the image circle is large enough to cover the entire image sensor at smallest aperture (the highest f number ), then it is alright. If this is true then I know that at any f-stop, I will have an image circle that will cover the entire sensor.
But if not, then I cannot explain how we get the same FOV when chaging the opening that lets the light enter and reach the image sensor(or film).
I am hoping that I am able explain myself. If not, then I will have to sketch it up next time.

So it sounds complex but actually is not when you think about it. And knowing the answer to this will not make me a better photographer. But I will feel better.

As I think about it now, I feel that I should have posted this in "Beyond the basic" forum... Any moderator reading this


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## PhotoXopher (Mar 17, 2010)

Because it's not external to the lens, much like the pupil of your eye. When your pupil dilates your FOV doesn't change either. If the aperture blades were outside the lens it would be different.


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## iskoos (Mar 17, 2010)

PhotoXopher said:


> Because it's not external to the lens, much like the pupil of your eye. When your pupil dilates your FOV doesn't change either. If the aperture blades were outside the lens it would be different.


 
Good point!.. it seems it is not external to the lens but it sits inside the lens. The aperture blades sits in front of the rear glass-somewhere inside the lens barrel... So it definitely does NOT block the light that hits the front element, but it does block the light that hits the rear element(glass).


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## PhotoXopher (Mar 17, 2010)

I prefer the term controlling vs blocking - which is exactly what it's supposed to do.


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## KmH (Mar 17, 2010)

Ok.

Light has wave-like properties as well as particle-like properties. The famous duality of light.

Now, consider how light from way off on the sides, top, and bottom of a scene gets onto the image sensor *at any lens opening*.

With a wide angle I can make an image that shows a foreground 30 feet wide or with a telephoto a foreground 300 feet wide, all with a lens that is less than 6 inches wide.

Hummmmm!


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## JimmyO (Mar 17, 2010)

Not something im very well read on but you cant think of the  image traveling through the lens like a tube, it doesn't work like that.


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## iskoos (Mar 17, 2010)

I prefer the term controlling vs blocking - which is exactly what it's supposed to do. Today 04:37 PMI prefer the term controlling vs blocking - which is exactly what it's supposed to do. Today 04:37 PM





> I prefer the term controlling vs blocking - which is exactly what it's supposed to do.


 
That's okay. When I said blocking I really meant partial blocking. Term "Controlling" would make more sense. Agreed.

When I looked at the other comments, I feel that I couldn't explain myself well.
I will see if I can sketch this up on a paper. Actually if I had a lens that I can change the aperture manually, I could probably do a few test but I don't have any lens of that kind...


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## mdtusz (Mar 17, 2010)

Poke a hole through a sheet of paper then shine light through it and move the paper wider and farther away from a wall. The circle on the wall will get bigger and smaller. Although light travels in "straight" lines, it doesn't really, especially with cameras. Think of the scene as a bunch of little flashlight beams. They all hit the lens then bend different directions, ultimately, 'landing' on the image sensor upside down and backwards. If you were to cut this into cross sections (perpendicular to the lens axis) it would be a series of concentric circles of light, then eccentric circles of light until they are projected onto the sensor. Look at this picture.

http://dpnow.com/images/5339/lens-angles1.png

The aperture just so happens to lie close enough to the focal plane (intersection of the lines) that all the light passes through it.

I think.


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## Garbz (Mar 22, 2010)

Lenses bend light. I've done a quick google search to come up with a picture which shows how this happens. Basically the entire lens will render one point to another point.







Now in the above image there are three rays. Suppose we block the two outside rays with an iris like the camera aperture blades. That one point from the tip of the pencil will still pass through the middle of the lens and get rendered onto the same spot as originally. The flip side is we have blocked those rays now so the image is also darker. 

The entire lens element focuses each point on an image. This is why you can for instance stick a small piece of blutac on your lens element and have it not show up in your image. This is also why dust on lens elements simply doesn't matter most of the time.


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## iskoos (Mar 23, 2010)

Garbz said:


> Lenses bend light. I've done a quick google search to come up with a picture which shows how this happens. Basically the entire lens will render one point to another point.
> 
> 
> 
> ...


 
Thank you Garbz. When I looked at the image you posted first, I thought you were going to explain me how a lens bends the light; how a ray parallel to optical axis bends and goes thru the focal point etc kind of like lenses 101 as I remember from my physics classes 
Your 1st paragraph didn't do anything but the 2nd and the 3rd one did. I thought about it for a few minutes while looking at the image and yes!.., that was the answer I have been looking for!..
Now others also said "Lenses bend light" but never went ahead and gave me the direct explanation. So my thanks go to you first and then to others.
It was really it: there are hundreds, thousands, millions of light rays coming from the same point and they all intersect at the same point(for each point) on the image plane. Aperture blades may block majority of those rays when we stop down on the lens and this will indeed reduce the light thus the image brightness but we WILL have the same FOV indeed because there are still some rays coming from that specific point(which is the tip of the pencil in your image) and hitting the sensor... That was it...

Thank you; thank you; thank you


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## iskoos (Mar 23, 2010)

Ummm another quick thing. I tried to use one of my lenses to do some tests on a plain paper the other day but there is no way I can stop down the lens when it is off the camera.
I even tried to hold down the DOF preview button at f/16 while turning the camera off thinking that the lens would keep the aperture setting but as soon as I did turn it off, the lens stopped up all the way.

So is there any way to get Canon EOS lens to stop down while not on the camera?
(Without breaking it apart of course)


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## Garbz (Mar 24, 2010)

I think Canon have completely electronic control over their shutters. Do you have access to a friend with a Nikon? Their lenses have a mechanical shutter release triggered by a lever on the camera body. 

Not sure what you're looking for but if you want a quick example of what these look like: How understanding aperture can help improve your photography that's at f/2.8, roll your mouse over the picture to see it at f/22.
From the back it looks pretty much very damn similar to the front: http://upload.wikimedia.org/wikipedia/commons/0/0d/Aperture.jpg


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## iskoos (Mar 24, 2010)

Hey Garbz, thanks for the link. I clearly saw how the aperture blades work on my camera (especially with 50mm f/1.4 lens). Just like the one in the link you provided. What I really wanted was to do the same when the lens was not on the camera so I could see how the brightness of the image reduced as the aperture blades closed. This I couldn't do with canon EOS lenses unfortunately but I found another way to do that. I used the DOF preview button while looking thru the viewfinder. As I stopped down, brightness reduced obviously...

I know some people with Nikon but I am not very close friends with any of them so can't ask to mess with their equipment
Believe me if I get a chance to play with a Nikon gear one day, I will take all my time to do it...

P.S. You meant "aperture" in your above post when saying "shutter" right?


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## Garbz (Mar 25, 2010)

Yes, yes I did


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