Tag Archives: Commentary

Corner Sharpness of the Canon 17-40mm f/4 L Lens on Full Frame

Since the question of how the Canon EF 17-40 f/4 L performs across the frame for landscape photography comes up periodically, I have posted an older test photo I made last year (2007) – updated here to include a comparison corner and center sharpness.

Canon5D17_40f16CornerVsCenter.jpg

Technical data: Canon 5D. Canon EF 17-40mm f/4 L lens. Focal length: 17mm. Aperture: f/16. Shutter speed: 1/60 second. Shot on the tripod with MLU and remote release. If the full image were reproduced at this resolution the print would be about five feet wide. (Not that I’d do that – it is a really boring photograph! :-) In a more typical size print the corners would like very good, indeed.

A 100% crop would not be expected to be “razor sharp” – and we see typical results here. It is impressive to see how well the corner image quality holds up – despite the fact that grass is one of the most challenging subjects for a digital sensor and the fact that this part of the scene was much closer to the camera than the focus point in the center of the scene – i.e. the corner section showing the grass is only a few feet from the camera, and the camera is focused hundreds of feet away on the objects in the center of the frame. (On that subject, I’m convinced that a good number of the reports of “poor corner performance” in ultra wide lenses are actually due to the subjects in the corner being much closer to the camera position than the subjects in the center of the frame, especially when the “tests” are done by shooting actual landscape subjects.)

BOTTOM LINE: What does this tell us, how do we view this in the context of reports of soft corners on the EF 17-40mm f/4 lens, and what does this mean for anyone trying to choose a wide (or ultra-wide in the case of full-frame cameras) Canon zoom lens?

While this lens is soft in the corners when shot wide open, the lens is not particularly soft in the corners when stopped down. If your primary use for such a lens is, for example, shooting very low light handheld wide angle photographs the 17-40 is perhaps not your best choice. (The EF 16-35mm f/2.8 on full frame or the EFS 17-55mm f/2.8 IS on a cropped sensor body could be more appropriate zooms.) On the other hand, if you are primarily interested in subjects that are usually shot at smaller apertures (urban/wild landscapes, architecture, etc.) then the 17-40 can be an outstanding lens – though this is more true on a full frame body than on a crop body, given that you are unlikely to use the smaller apertures on a crop sensor body given the diffraction blur issues there. So, to state it very succinctly…

… the Canon EF 17-40mm f/4 lens is an excellent lens for shooting deep DOF small-aperture photography on a full-frame camera. (It is OK but not necessarily ideal for use with cropped sensor bodies, where I would prefer the EFS 17-55mm f/2.8 IS.)

G Dan Mitchell is a California photographer whose subjects include the Pacific coast, redwood forests, central California oak/grasslands, the Sierra Nevada, California deserts, urban landscapes, night photography, and more.
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Text, photographs, and other media are © Copyright G Dan Mitchell (or others when indicated) and are not in the public domain and may not be used on websites, blogs, or in other media without advance permission from G Dan Mitchell.


Sharpness and Aperture Selection on Full-Frame DSLRs

(Note: This article was originally posted in 2007 and I should probably update the test using newer gear – though the point of the test and the post remains.)

Last month I wrote about a set of tests (“Full Frame Lens Test“) that I conducted with my lenses and my Canon 5D body. My object was simply to better understand how the camera/lens combinations would behave so that I could make better decisions about appropriate lenses and apertures while making photographs.

One discovery was that, compared to using a crop sensor DSLR, I can get excellent results when I shoot at smaller apertures with good lenses on the full frame body. I tended to avoid apertures smaller than about f/8 on the crop sensor camera, but there seems to be little or no real liability in using f/11 or even f/16 on full frame.

To illustrate I put together the following composite image. (The image appears in reduced form on this page. Click the link to see the full size version.)

Diffraction Blur Test Image
A sequence of text photographs illustrating diffraction blur at several apertures on a full frame camera

The example includes five versions of a small section from near the center of a photograph taken with the Canon EOS 5D using the EF 24-105mm L IS lens at a 50mm focal length. The camera was on a tripod, MLU and a remote release were used, and the AF was turned off. The images are 100% crops – in other words, actual pixel size is displayed in these tiny excepts from the much larger original images. (You would virtually never view a print at this magnification. These are equivalent to tiny sections from a print that might be about 5 feet wide!) The images have been slightly sharpened in post-processing, but are otherwise unaltered.

I shot at apertures of f/4, f/5.6, f/8, f/11, and f/16. In terms of the sharpness of this portion of the image, I am quite certain that all five examples are plenty sharp for making prints. That said, there are some differences. To my eye:

  • The f/4 and, to some extent, the f/5.6 versions are slightly but noticeably softer at this magnification.
  • The f/8 and f/11 versions seem to me to have approximately equal sharpness. Some parts of the f/8 image seem slightly sharper, but other parts of the f/11 image seem sharper. In the end they are pretty darn equivalent, though I’d maybe give the f/11 a very slight edge overall.
  • The f/16 image may be slightly less sharp than the f/8 and f/11 versions, but the difference would not be noticeable in a print, even a rather large one. In any case, f/16 appears sharper than either f/4 or f/5.6.

After doing this test I no longer hesitate to shoot at f/11 or f/16. Not only does this give me the possibility of getting greater depth of field when I need it, but it also means that I can compensate for corner softness on some lenses (e.g. the 17-40mm) by using a smaller aperture without fear of losing center sharpness.

(Addition: 4/23/07 – Other Canon L lenses seem to give similar results, including my 17-40mm f/4 L and my 70-200mm f/4 L.)

Added 2/23/08:

In response to a question in a photo forum I put together a sample image showing corner sharpness from the same original images used in the example above. (The earlier example shows 100% crops from near the center of the frame.)

(image temporarily unavailable) Canon 24-105mm f/4 L IS lens corner sharpness test

Technical info: Shot using a Canon 5D with the Canon EF 24-105mm f/4 L IS lens at a 50mm focal length. In aperture priority mode, the aperture was changed manually between shots. Initial focus was with AF, which was then switched off before shooting the series. Camera was on a tripod and MLU and remote release were used. Shots were converted from RAW with ACR and no additional post-processing applied. Print made at this resolution would be approximately five feet wide. The crop is from the far lower left corner of the frame.

In addition to noting the softer image in the corner at f/4, also note that the image is a bit darker due to the expected increase in corner light fall-off (“vignetting”) at the largest aperture. Sharpest version in this series shot with a FF body seems to be at f/11 as in the center crop example above. But note that f/8, f/11, and f/16 are not very different in overall sharpness – and in the end any of these apertures would produce a very sharp print.

In response to another forum discussion, I have added another example, this time using the Canon EF 50mm f/1.4 lens and showing performance at f/5.6, f/8, f/11, and f/16. In other respects the test is the same as described already in this post. This image is seen to the right and below.

(image temporarily unavailable)

Conclusions: Based on this set of images and other taken with different lenses under similar conditions, I have come to some conclusions that work for me with my Canon lenses and full-frame Canon 5D body.

  • In general the sharpest whole aperture seems to be around f/11.
  • It is very difficult to distinguish any resolution differences at f/8 or f/16 – there are subtle differences when viewing the test images at 100% magnification on my monitor but these are essentially invisible in prints.
  • f/5.6 or f/22 will tend to be a bit less sharp, though perhaps not for the same reasons. At f/5.6 I begin to notice a bit more of the diminished sharpness as a lens is opened up – more on some lenses than on others. At f/22 the effects of diffraction become just a bit more noticeable. However, if the shot demands it I do not hesitate (much) to use either of these apertures as the very slight decrease in sharpness is quite tiny if visible at all in a print and both provide some other advantages in certain situations. (I’ll even use the largest f/4 aperture on the test lens when isolating the subject is important or when low light demands it – and the results will typically be just fine.)
  • At larger apertures the performance becomes more tied to the particular lens so it is more difficult to make any generalizations beyond the fact that vignetting increases and sharpness will be less optimal.
  • The smaller apertures decrease any corner light fall-off (“vignetting”) or softness, generally to a point where both are insignificant.
  • With all of this in mind, unless I have a reason to select some other aperture I typically use f/11 as my general starting point when shooting with my full-frame DSLR body.

G Dan Mitchell is a California photographer whose subjects include the Pacific coast, redwood forests, central California oak/grasslands, the Sierra Nevada, California deserts, urban landscapes, night photography, and more.
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Text, photographs, and other media are © Copyright G Dan Mitchell (or others when indicated) and are not in the public domain and may not be used on websites, blogs, or in other media without advance permission from G Dan Mitchell.