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Thread: Aperture and focal length... a question.

  1. #1

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    Aperture and focal length... a question.

    Hey everyone, this is my very first post!

    I'm reasonably new to photography and something a friend of mine said is stuck in my head and really bothering me. It's maybe a bit of a technical question, but here goes. I hope someone can help me untangle this knot I've tied in my brain.

    His statement. "For any given aperture, there is a one focal length which creates a focused photograph." Now I think this is true because I believe I've read this or something equivalent to this elsewhere. But now I'm really confused. If this is in fact generally true, then how can I shoot at a given aperture at different magnifications? Is this statement just not true or am I missing something really basic here. I hope this isn't a dumb question.

    Thanks in advance!

  2. #2
    Rob Douglas's Avatar
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    Rob Douglas
    Welcome woty87 (not sure of your real name it's not in your profile, might want to fix that) I think what you are talking about is probably what most refer to as the "sweet spot" that would be the aperture at which a given lens focal length is its sharpest.

  3. #3
    herbert's Avatar
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    Re: Aperture and focal length... a question.

    Welcome to the forum.

    I think that statement makes no sense as it is. No wonder you are having problems trying to understand it.

    There are many things that effect the focus, the most important being the subject to focus on.

    However it is true that on most lenses when you change the aperture the focus distance moves. This is known as focus shift and can effect some lenses with a large maximum aperture enough that you can see it. This can be a problem since cameras focus with the lens at the widest aperture and then close the aperture when taking a photograph. If there is focus shift then the subject will be out of focus. This is annoying.

    However on most lens you will not see the shift (it is within the depth of field for the lens aperture) so you don't have to worry about it.

    Alex
    Last edited by herbert; 10th January 2012 at 01:15 PM.

  4. #4

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    Re: Aperture and focal length... a question.

    Thanks for the replies. I'm glad I found this forum as I try to better understand the fundamental concepts.

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