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Thread: How come aperture doesn't affect angle of view and vice versa

  1. #1
    JK6065's Avatar
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    Jeroen

    How come aperture doesn't affect angle of view and vice versa

    Something I've been asking myself for quite some time is the following:
    Why is it that changing the focal length changes the angle of view and changing the aperture changes the angle of the incoming light rays that hit the sensor, but these two principles don't interact with each other?

    From the CiC tutorials I can't figure it out:

    How come aperture doesn't affect angle of view and vice versa

    vs

    How come aperture doesn't affect angle of view and vice versa

    The thing I thought of is that the above question explains why you have a much shallower DoF when shooting rather close to the lens, but it doesn't answer to why it doesn't work vice versa: Why changing aperture doesn't change the angle of view?
    Last edited by JK6065; 7th July 2011 at 05:36 PM. Reason: typo

  2. #2
    rob marshall

    Re: How come aperture doesn't affect angle of view and vice versa

    The aperture is just an opening in the lens. F/16 is a very small one, f/1.8 is a large one, but they both see the same FOV. It's the same with your eyes - when you go out on a bright day your pupils stop down to a small size to let in less light, but you still have the same FOV.

    The DOF thing is a different thing altogether. If subjects A,B, and C are 10,20, and 30ft from you and you shoot with f/16 it will probably resolve any DOF differences. But at f/1.8 it can't. But it still has the same FOV.

  3. #3

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    Remco

    Re: How come aperture doesn't affect angle of view and vice versa

    Quote Originally Posted by JK6065 View Post
    Something I've been asking myself for quite some time is the following:
    Why is it that changing the focal length changes the angle of view and changing the aperture changes the angle of the incoming light rays that hit the sensor, but these two principles don't interact with each other?
    To keep it simple: The lens in the 2nd image is placed at the point where the rays cross in the first image. That's also the spot where the diaphragm is located.

    The first image shows what you can see through the lens for a given image (sensor) size: from each point on the sensor you look through the lens at your object. What you can see through the lens doesn't depend on the size of the lens, only on its strength (focal length). So a lens with a small diameter will have the same field of view as a lens with a large diameter. The diaphragm reduces the effective diameter of your lens. As lens diameter doesn't influence the field of view, the diaphragm doesn't either.

    In the second image, you can see that a small aperture (high F number) allows less light through than a large aperture (low F number). The size of the opening doesn't change the angle at which the rays hit the sensor, it allows in a wider cone of rays (the image only shows the largest angle covered, all the smaller angles are covered as well. As the aperture gets smaller, the captured cone is narrower, so the spot on the sensor for out of focus parts (the 'circle of confusion' in image 2) gets narrower as well. As DoF depends directly on the size of this spot on the sensor, a smaller aperture means larger DoF. Try copying image 2 and draw in the rays for a lens with half the diameter (and perhaps add some points further away and closer to the lens). I think that will clarify the DoF issue.

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