I recently acquired a full frame camera body, and I’m interested in understanding more fully how my lenses work with it. Issues that interest me include:

• How sharp is each lens on the full frame body?
• How is sharpness affected at various focal lengths on zoom lenses?
• What are the optimal apertures on each lens in terms of sharpness?
• How does vignetting change at different apertures and focal lengths?
• How is corner sharpness affected by aperture and focal length?
• Are there significant differences between the performance levels of two lenses that have the same focal length?

Today I conducted an informal test. I wasn’t trying to find out if I have “good copies” of my lenses. Rather, my goal was to understand the “personality” of each lens a bit better. I can’t say I answered all of these questions, but I did come up with some interesting results, some of which confirmed what I expected and some that surprised me a bit.

The Canon lenses I tested are the EF 50mm f/1.4, the EF 17-40mm f/4L, the 24-105mm f/4 IS L, and the 70-200 f/4 L.

The test was simple – and I can’t claim scientific accuracy. I went to a familiar location nearby that features a ridge covered with grass and oak trees a few hundred yards away with closer grass, oak trees, a trail, and a fence in the mid-ground, and new grass in the foreground. I put the camera on a tripod, used mirror lockup, and a remote switch. I used auto-focus to pre-focus on the ridge, and then I turned AF off before shooting. With each lens, I then made a series of manual exposures at apertures ranging from f/4 to f/16 (slightly wider in one case). With zooms I repeated the test at the wide and tele ends and at one or two points in between.

I’m not going to post all of the results here, but I will share a few interesting things that I confirmed or discovered.

• Overall, the lenses produce sharper images on the full frame body of my 12 MP Canon 5D. That said, the difference would probably not be noticeable unless you made a fairly large print.
• All of the lenses vignette noticeably at their widest apertures and generally at one stop smaller. Vignetting was reduced to inconsequential levels in all cases by f/8 and in some cases sooner. (In any case, vignetting is not always a bad thing – just something to understand.)
• I was surprised to see how well the lenses did at f/16. The common wisdom regarding diffraction (or the “circle of confusion”) is that lenses will be sharpest around f/8 or even a bit larger aperture, and that sharpness will degrade noticeably by f/16. Frankly, I just didn’t see that. There might be a little bit of degradation at f/16, but I have to look very hard for it in 100% crops. I’m more inclined to go ahead an use f/16 now in situations where I might previously have avoided it.
• Not surprisingly, corner sharpness improves at smaller apertures. My 17-40mm has been soft enough on occasion that I’ve been compelled to crop off the edges of images. However, even this lens does quite well almost all the way to the corners at f/16 and it is better at f/11 than at f/8. An important factor is that I did not see any significant (as in “noticeable”) degradation of center sharpness when using the smaller apertures to get better corner sharpness. This is an important discovery – I think I’ll be much more likely to shoot the 17-40 at f/11 or even f/16 now.
• I need to clean my sensor… :-) I had previously noticed a couple of spots that required post-processing removal in Photoshop when I shot at f/8. Such spots and even slightly smaller ones become more visible at the smaller apertures.
• Good zooms are very sharp. While my 50mm f/1.4 prime is very good corner to corner at apertures like f/5.6 or f/8, the performance of the zooms is very close in almost all cases.

Some people get carried away with testing their lenses and trying to find the absolute perfect lens. That wasn’t my point. My goal was to understand the effect of my lens choices and to understand how to get the best image quality that these lenses can produce in a variety of situations. In that regard, I think this was a very valuable exercise.

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