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Thread: Does aperture affect sharpness?

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    Senior Member GSte's Avatar
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    Does aperture affect sharpness?

    Just wondered as I took some shots using a very small aperture last week (f22 I think) and they seemed VERY hazy. The haziness could have been due to the weather or perhaps the shutter speeds were too slow to be hand-holding, but I just wondered if the aperture itself might affect the sharpness of a lens?

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    Senior Member r_j_k_p's Avatar
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    Re: Does aperture affect sharpness?

    i may be wrong but i think around f11 is the 'sharpest' point in a lens... although different lenses may vary.
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    Banhammer in peace PeterB kalniel's Avatar
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    Re: Does aperture affect sharpness?

    Yes. Physics dictates that diffraction will become a problem if you have too small an aperture, regardless of lens.

    Resolving power of an APS-C sized sensor at F22 is only 2 megapixels in yellow-green wavelengths. Any bigger than that is just blurring.

    But camera shake or other factors could contribute - quick test is to resize the image down to 2MP and see if it looks sharp.

    More info:
    http://www.luminous-landscape.com/tu...solution.shtml
    Last edited by kalniel; 19-05-2010 at 08:30 AM.

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    Re: Does aperture affect sharpness?

    Well in general there are, I guess "sweet spots" or best performing aperture setting for lenses, in relation to sharpness (providing other factors are good). Which is different lens to lens.

    I'd say maybe at that range (over f/10), it started to degrade.

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    Re: Does aperture affect sharpness?

    i've shot f/18 on my 20D and got sharp results, f/22 is pushing it a little (what was ur SS?)

    most lenses dont like being shot wide open..

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    Re: Does aperture affect sharpness?

    Take a look at this:

    http://www.dpreview.com/lensreviews/..._c16/page4.asp

    Drag the aperture slider all the way to the right and watch the sharpness first peak and then plummet. As Kalniel said, this is due to diffraction. When light passes a plane (like an aperture blade) it diffracts - it goes where it's not supposed to. When your aperture is really wide open, the percentage of light passing the aperture blades (diffracted light) compared to that going straight through the middle (non-diffracted light) is very small. When your aperture is closed down, the edges make a much greater proportion of the whole, so diffracted light makes up a larger percentage of the light finding its way onto the sensor. Larger proportion of diffracted light on sensor = soft images.

    Play around with the tools on DPReview to see how diffraction differs according to sensor size (take a 4/3rds, APS-C and FF camera) - this'll reveal why Ansel Adams founded the f64 club.

    You can also pick a lens of similar spec to the one you were using to figure out the sharpness curve of your kit. This is why learning about DoF ranges gets important when judging critical sharpness - it's not good enough just to say 'I've got a tripod and I want everything in focus, so f32 and be done with it'.

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    Re: Does aperture affect sharpness?

    Fantastic replies as always chaps, many thanks for the info.

    Somehow I'd never seen that tool on the dpreview lens reviews Brammers.... think I had just been skipping to the conclusion/samples page. Any idea what level of sharpness on that scale would give acceptable results?

    What's "SS" Bobster?

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    Banhammer in peace PeterB kalniel's Avatar
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    Re: Does aperture affect sharpness?

    Quote Originally Posted by GSte View Post
    Somehow I'd never seen that tool on the dpreview lens reviews Brammers.... think I had just been skipping to the conclusion/samples page. Any idea what level of sharpness on that scale would give acceptable results?
    Depends how picky you are, but green is basically very sharp. Explaination: http://www.dpreview.com/learn/?/Glos...duction_01.htm

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    Re: Does aperture affect sharpness?

    Quote Originally Posted by kalniel View Post
    Depends how picky you are, but green is basically very sharp. Explaination: http://www.dpreview.com/learn/?/Glos...duction_01.htm
    Probably depends more on how large you print. If you show uncropped shots on this forum, you'd have a tough time chosing between the sharpness of a Sony 18-70 plastic kit lens and a Zeiss 24-70 pro-spec zoom. If you start doing crops, big prints, or both together, then the Zeiss will start earning its keep.

    Look for other factors in lens performance too. AF speed is a biggie for me - helped by decent drive motors (SSM/HSM etc), lack of CA is hugely important if you're doing big prints, and bokkeh is something that's hard to measure but is also important. The Canon 50 1.8 for example is very sharp, but has terrible, terrible bokkeh. The 1.4 isn't much sharper, but the bokkeh is much better. Other factors in lens performance include colour, flare, vignetting... so much more to it than corner sharpness.
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