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Category Archives: Tests
Canon EF 17-40mm f/4 L: Testing a Claim About Sharpness and Focal Length
Today I read a claim in a photography forum thread that “some 17-40’s are soft on the long end.” Having not noticed that on my copy I went back to some test shots I did earlier this year to compare sharpness at 40mm and 24mm with this lens.
The two images below are 100% crops – in other words, they are very small sections of a much larger print that would be something like 5 feet wide if printed at this resolution! The crop came from an area just below the center of the frame. Both were shot on a Canon 5D a 1/125 second and f/11. The camera was on a tripod and I used mirror lockup and a remote release. Both were converted from RAW using ACR with no adjustments to the original settings. The same sharpening process was applied to both.
| 24mm: | 40mm: |
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My verdict? There doesn’t seem to be a lot of difference in terms of sharpness. And if there is a difference, it would hardly be significant in even a pretty decent sized print. I don’t think a difference would be noticeable in a 12″ x 18″ inch print, and frankly I doubt that anyone would notice even at a larger print size.
What do you think?
There is one other possibility. The person who posted the original message claimed that “some” copies of this lens exhibit this problem. There is no way for me to rule that out with only one sample.
One other related idea. Any zoom lens is going to perform differently at different focal lengths, and there will likely be some focal length or focal length range in which the “quality” may be measurably “best.” But this doesn’t mean that the other focal lengths are not good – in some cases so good as to provide virtually the same quality.
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A Test: Correcting Lens Distortion in Post-Processing
Earlier this weekend I read a forum thread about the Canon EF 50mm f/1.4 lens, of which I have a copy. The thread lamented the barrel distortion of this lens (which I don’t really find to be that big of an issue) and continued with posts suggesting alternatives including software correction in the post-processing phase. One response to this proposal was that software correction would degrade the image and would therefore be unacceptable.
That theory seemed sound and I have believed this to be true in the past, but I decided to test this idea for myself. Using an old photograph taken with this lens, I cropped a small section from one of the far corners – the worst part of the frame and, according to some, subject to a lot of softness and distortion on full-frame bodies like my Canon 5D. To make things a bit more challenging I used a photograph that included a bunch of dried california grasses – full of very fine details and high contrast.
I converted the original RAW file using ACR (Adobe Camera RAW) with no sharpening. In Photoshop I cropped to a section of reasonable size for web presentation, using that section from one of the corners of the image. Then I made a duplicate of the cropped section of the image.
In one of the two versions of the crop I used the LensfixCI plugin to correct for the slight barrel distortion of the EF 50mm f/1.4 lens. This $29 plugin* includes a database of many lenses, and also keeps a smaller databases of your lenses. It uses EXIF data to identify the lens (and focal length with zooms) used to take the photo and automatically applies optimum distortion corrections from its database. It takes me about 10 seconds to select the plugin and apply its changes.
* (I have left the reference to this plugin that I used when I did the test several years ago, even though I no longer use it. Today I simply use the built-in correction in Lightroom or ACR, where I apply lens- based corrections by default in virtually all cases.)
Next I used my normal sharpening methods on both images, inspecting the results and making adjustments as I applied them. In the end, as would typically be the case, I used slightly different sharpening settings for the two images – but that reflects the normal way of operating. Finally, I took the two images and placed them side by side in the single high quality jpg file that follows.
(NOTE: The version shown above on this page may have been downsized for formatting purposes, which limits the amount of detail that is visible. Click the image to view it at its original size, or follow this direct link to the original image.)
I have a darned hard time seeing any difference in sharpness, contrast, or color that might have been introduced by the correction process. If a difference is visible a) it is almost impossible to say which version is better, and b) the difference is almost certainly completely insignificant in an actual print. (Keep in mind that these are 100% crops of the worst part of the frame in the far corner – and that the area shown here would be a very tiny section of a full print that would be something like five feet/60″ wide.)
After doing this test, I’m not really concerned at all about any negative effects of using this method of correcting lens distortions, and today I simply allow ACR or Lightroom to automatically correct for such lens characteristics by default. And whatever the tiny negative effect on sharpness we might imagine to produce, it is far outweighed by the ability to straighten lines and so forth when necessary.
(Anyone care to guess which half contains the “corrected” version of the crop? Feel free to post a comment and an explanation of what you (think you) see… ;-)
(This 2007 post was slightly updated on 1/5/2013)
G Dan Mitchell is a California photographer and visual opportunist. His book, “California’s Fall Color: A Photographer’s Guide to Autumn in the Sierra” is available from Heyday Books and Amazon.
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About Vignetting
In photography forums I often read posts that dismiss a lens because “it vignettes.” The assumption seems to be that really good lenses don’t vignette. I’m afraid that searching for lenses that don’t vignette is a hopeless quest.
Vignetting (or “fall-off”) in the corners of the frame is a fact of life with essentially all lenses. As I understand it, this is simply a condition that can’t be completely eliminated in the design of normal camera lenses – though there are ways to reduce it. So the question really is not “does my lens vignette?” It does. More useful questions are: “How much does the lens vignette?” and “How apparent is vignetting at different apertures?” and “How do the vignetting characteristics of this lens affect my photography?”
Canon EF 50mm f/1.4 lens at f/1.4 and f/4
I have four excellent Canon lenses: 17-40mm f/4 L, 24-105mm f/4 IS L, 70-200mm f/4 L, and 50mm f/1.4 prime. I looked at all of them when I did my recent Full Frame Lens Test. I confirmed that all four can produce really excellent results… and that all four produce noticeable vignetting in certain circumstances, especially in test conditions. (Results posted at various lens test sites on the web will confirm this.)
But is this a problem? Generally, no – though it is important to understand the vignetting characteristics of your lenses if you want to take advantage of this characteristic when appropriate and minimize it when you don’t. The body you use also makes a big difference. Fall off from a given lens will be more visible on a full frame sensor than on a crop sensor.
Stopping down greatly reduces the effect with all of my lenses. On some of them (e.g. – the 50mm prime and the 70-200mm zoom) light fall off becomes very minor when stopped down just a bit. It does not diminish quite as quickly on the 24-105, and the 17-40 can show the effect at apertures as small as f/8, depending upon subject.
Given that vignetting is a fact of life, how can we deal with it? There are actually quite a few approachs that can be used individually or together in different situations.
- Vignetting can often be effectively reduced or even eliminated in post processing. When necessary I use adjustments in ACR or Photoshop to reduce the effect.
- Shooting at smaller apertures can minimize or virtually eliminate the effect. Since I often shoot from a tripod I tend to use slower shutter speeds at smaller apertures. Fortunately, full frame DSLR cameras can use smaller apertures than crop sensor cameras; at f/11 or f/16 vignetting effects are rarely visible.
- In many situations vignetting is actually a pleasing effect. My theory is that vignetting often simulates the way we see with our eyes – we are more aware of objects in the center of our field of vision and less aware of those in peripheral areas. Vignetting can de-emphasize the peripheral area and draw attention to the center. (In some images I intentionally add a bit of vignetting for this purpose.)
- Vignetting that is cleary visible when the subject is a perfectly uniform test target is often completely invisible in photographs of detailed subjects. In other words, the lens vignettes but you can’t see it. (This is a good illustration of the principle that what happens in photographs is more important than what happens in tests.)
So, vignetting is a fact of photographic life – for better or worse. Searching for a lens that does not vignette is hopeless. It makes a lot more sense to understand vignetting and learn how to work with the vignetting characteristics of your lenses.
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