More detail close up at 50mm than further back with at 85mm?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

I find the EF 24-105mm f/4 very useful and have probably taken more shots below 105mm with it than any other lens. Not only is it versatile and covers many of the most often used focal lengths one might need in a 'walk around' lens, but it is built like a tank and solid as a rock. It can 'take a licking and keep on ticking' better than a Timex watch can!

But no lens is perfect for every role.

With your EF 24-105mm f/4 L lens you've discovered a common attribute of most zoom lenses: that they have varying optical performance at different focal lengths. For wide angle, wide angle-to-normal, and wide angle-to-short telephoto¹ zooms, the shorter end of the focal length range is usually the sharper end from one side of the frame to the other and the longer end usually demonstrates less geometric distortion and vignetting.

_{¹ The 24-105mm falls into this category}

That's one reason why many portraitists prefer to use prime lenses (a lens that doesn't zoom) - because it can be optimized for the single focal length it provides. You also can generally get better optical image quality at a lower price with a prime lens than with a zoom lens. Of course, what you give up is flexibility. By the time you buy a set of 24mm, 35mm, 50mm, 85mm, and 100mm prime lenses you've likely spent more than you would have for that 24-105mm zoom.

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).

Assuming the quality of the two lenses were equal and the shooting technique does not affect the longer focal length more than the shorter focal length, the overall detail would be the same.

But the quality of the two lenses is never equal. So in the end it comes down to which lens gives you the better quality at a price you can live with. Sometimes that might be the 50mm lens, sometimes that might be the 85mm lens.

Answer 7

When you are shooting at 85mm in the exact same situation you will be more susceptible to camera shake.. could that be what you are experiencing?

To really compare the results you should shoot the same test on a tripod, or in good enough light where the shutter speed is say 1/200th or better to be safe.

As an aside, when shooting faces and portraits the longer the focal length the more flattering the image. If you're using a crop sensor camera then 50mm is still in the range of "fine" but 85mm is better.