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In terms of quality of picture and overall detail of subject (e.g. the face of a child), would it be better to stand closer to shoot at 50mm or stand back at 85mm (assuming the quality of the lens was equal, and disregarding the difference in depth of field, and difference in perspective).

I have a Canon 24-105mm L and when I shoot at 85mm and stand back for a full body shot of my little girl, the overall detail in the face is poorer than if I stand closer at 50mm to get he same shot. (ignoring the change in perspective of course).

Is this just my imagination?

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3  
With a theoretically perfect lens, the tradeoff between DOF and image (not sensor) resolution will remain the same no matter what the focal length is, assuming the camera is always situated to give the same image size for the given subject. So the image quality will be exactly the same for ideal lenses. Real lenses are different. –  Dietrich Epp Jul 24 '13 at 22:50
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@StephanieWilliams - You seem to make two questions there. First you ask in theoretical point of view if focal length can affect level of detail, and then the same in practice naming a certain lens. –  Esa Paulasto Jul 24 '13 at 22:55

5 Answers 5

Peak sharpness will wax and wane with a zoom lens throughout the range. A common pattern is for sharpness to be highest at both ends and to drop in the middle. Not all lenses follow this pattern.

When it comes to prime lenses (and to some extent zooms) longer focal length lenses tend to have less field curvature and other aberrations and thus offer better average sharpness corner to corner. Some of the best lenses are in the super telephoto range, the Canon EF 300mm f/2.8L for example.

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The quality of the lens will determine which shot has higher image quality, it isn't an aspect of focal length. There is however no direct connection between quality and focal length. The Canon 70-200mm f/2.8 II will take far sharper images than a cheap standard zoom kit lens, but the 24-70 f/2.8 II will take sharper pictures than the 70-200.

You should choose your focal length and distance from subject based on the perspective you want and the quality of the lenses you have available to you.

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Yes, it's quite possible that the change in focal length and the change in distance would change the amount of detail. It's all a matter of how well the glass was manufactured and aligned and varies from manufacturer to manufacturer. Sometimes as the lens stretches defects in alignment become more apparent.

Note, though, that changing focal length is not generally the biggest culprit. But a change in focal length can necessitate a change in aperture, and changing the aperture can have a dramatic effect on the sharpness in an image. Most lenses are at their sharpest somewhere between f/2.8 and f/5.6, but again, it varies depending on the manufacturer and lens type. A prime lens, for example, is often sharper than a zoom lens of similar manufacturing quality because of it's simpler design.

If you would like a visual example of how this all works I would recommend looking at the lens reviews over at DPReview.com: It's really freaking cool besides being informative.

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Unfortunately, the EF 24-105mm f/4 L IS is not included in the lens selection for the DPReview widget, nor are any of the 85mm primes. –  Michael Clark Jul 24 '13 at 20:03
    
I only really have it there for illustration purposes. Take a look at the 50mm 1.8 prime vs. the 18-55 is from canon. They're both dirt cheap, and the difference is astounding. –  Jay Carr Jul 24 '13 at 20:19

This is normal and depends on the lens. Zoom lenses never perform perfectly the same way at all focal-length and probably not even at all focus-distances. Everything moves and maintaining image quality perfectly is excessively complex.

The best place to visualize this is at SLR Gear. They have tested the Canon 24-105mm F/4L and you can clearly see drop past 50mm when close to wide-open. Click on the image under Blur Index and you will get an interactive widget that lets you choose focal-length and aperture.

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Factoring out differences in lens built quality, the one factor that really changes with focal length is the amount of air between lens and subject. When the air is dusty of foggy getting closer to the subject will give a clearer image.

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