I have updated my Sharpness and Aperture Selection of Full-Frame DSLRs post to include the corner crops from the same photos used for the center crops that were already there. Here’s the image I added – see the link for full information.
Tag Archives: aperture
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.
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.
DSLR Sensor Cleaning – My Approach
Update: June 14, 2023 — I just returned to this article after not reading it for quite some time, perhaps years. I still think it contains useful information, and the steps I describe still represent more or less what I would do today. However, things really have improved when it comes to the dust issue. Back when I had a Canon 5D, dealing with sensor dust was a major issue that required frequent cleanings (often with wet cleaners) AND a lot of spot removal in post. But today that is hardly ever the case. It is actually pretty rare to encounter visible spots, and when they do show up the cameras dust reduction system often takes care of them and they disappear a few frames later. I still firmly believe in tolerating a tiny bit of dust and dealing with it when necessary by using the least intrusive methods first.
When I got my first DSLR (1) I was very upset if I got any sensor dust in a shot. I was also very paranoid about cleaning the sensor (2), having read too many posts about how one can damage the sensor during cleaning. Now that I’ve used digital cameras for quite some time I’ve gotten over it and life is much, much easier.
Here is a summary of my approach (3) to dealing with sensor junk…
Rule #1: Modern cameras typically include dust-reduction systems that vibrate the sensor to dislodge dust particles. I set mine to operate automatically each time the camera is turned on or off. In addition to ensuring that the process runs regularly, this automatically runs it after every lens change, the time when you are most likely to pick up dust. You can also manually trigger a dust-reduction system cycle from the control menus, and I do this if I notice a dust spot while shooting. (If you have ever shot for a day or a few days without checking, only to realize that you had picked up a big dust bunny on day one and that it appears in all of your several hundred or more photographs, you will adopt this practice!)
Rule #2: I don’t worry too much about a small amount of dust in my images. I rarely can get a sensor clean enough to get perfect, dust-free shots at smaller apertures, and when I do the dust will soon return. Rather than obsess about dust-free perfection I quickly fix most small spots in post-processing. I can usually deal with most dust spots in a matter of a few seconds in Adobe Camera Raw (ACR) or Photoshop. (My preference is to do this in ACR during the raw conversion process. This fits better with my workflow which relies on the use of smart layers in Photoshop.)
Rule #3: When the dust gets to the point that dealing with it in post is no longer efficient, I try the easiest thing first. I use a blower to try to clean out the worst of the stuff. More often than not this is enough and I can go back to relying on rule #2. Point the tip of a good blower bulb into the chamber but keep the tip itself just outside. As you blow a few dozen puffs into the chamber and toward the sensor, change the angle of the bulb to ensure that you get full coverage. It is probably best to hold the camera with the open chamber facing down. (Don’t overdo it, since the blower can move some dust onto the focus screen of some cameras, leaving annoying bits of dust that do no real harm and will not affect your photographs but which are very difficult to remove.)
Rule #4: Sometimes rules #1-#3 aren’t enough and a more direct cleaning of the sensor itself becomes necessary. At this point I used to try a static charged sensor brush, being very careful to avoid letting the brush touch anything but the sensor* glass itself. Yes, the brush can pick up other stuff in the chamber, and I have learned from experience to avoid this. I still occasionally use the brush, but with the availability of the sensor gel products (4), these days I’m more likely to try them before I try a brush, though the brush can still be useful sometimes for stuff caught right up against the edges/corners of the sensor.
Rule #5: On rare occasions rule #4 fails, too. If the contamination is adhering too firmly to the sensor surface I resort to wet cleaning with Eclipse fluid and PecPads. I can rarely get it right in one attempt, so I plan on having to work at this a bit, but eventually I get a reasonably clean sensor with no streaks. Be very cautious to not use pressure, to not “scrub” the sensor glass, and to not use too much liquid. Let the fluid loosen and/or dissolve the material and gently wipe it off with the pad attached to the “spatula” tool. Read the instructions for this cleaning method very carefully before attempting it. It isn’t terribly difficult but there are a few ways you could go wrong including: pressing too hard and damaging the coating on the sensor glass, transferring lubricants to the sensor from other parts of the camera chamber, leaving streaks on the sensor.
Rule #6: On very rare occasions a combination of methods is required. Often the wet cleaning works well for me but leaves a few spots of dust on the sensor. For this reason I frequently follow the wet cleaning with a quick once-over with the static charged brush and/or the sensor gel stick.
From all of this, it might sound like I’m sensor-obsessed. I’m not. Remember rule #1 is the one I follow most. I usually go many months between real sensor cleaning sessions, and it is very rare for me to have to resort to a wet cleaning.
In my opinion, it is not necessary to fear the sensor cleaning operation as long as you are reasonably careful. Once you do it a few times it becomes quite quick and easy. Taking your camera to the shop or sending it to the repair facility is going to cost you a significant sum, take considerable time, and probably not result in a cleaner sensor in the end.
However, one photographer pointed out that he has a service plan that includes six free sensor cleanings per year. A few years ago, I would have counseled against relying on this – since early cameras without sensor cleaning systems often needed to be cleaned frequently and on short notice. However, the newer cameras rarely need a serious sensor cleaning – and in this case I can see how simply sending the darn thing in (while you keep shooting with your backup camera) could make sense for some people. (I still feel that you should be able to clean the thing in the field if necessary.)
Notes
(1) Interchangeable lens mirrorless cameras have become widely available since I first wrote this article about sensor cleaning. I use one along with my DSLR system, and both need occasional sensor cleaning — to the techniques described here are also broadly applicable to mirrorless cameras, too.
(2) I’ll anticipate that someone might feel obligated to write, “You aren’t really cleaning the SENSOR! You are cleaning the glass cover over the sensor, you nitwit!” Yes, I know that. It is just easier to refer to the whole assembly as “the sensor.” :-)
(3) Disclaimer: This report describes what I do, but I am not any sort of certified expert on these things – as I wrote, this is “my approach.” I strongly urge you to seek out and learn from other official sources of information on sensor cleaning and related issues. The inside of your camera contains fragile and sensitive electronic and mechanical components and it is possible to cause damage while working there. You should read and carefully consider warnings from the manufacturer of your camera and any accessories and tools you use on it. If you are not convinced that you are competent to do this work on your camera, you can always take it to a professional. I do not claim that my methods are the best or most appropriate, nor that they meet the standards of the manufacturers of the camera equipment nor do I recommend that you use my methods in place of manufacturers’ official recommendations.
(4) The Sensor Gel product (available here) is a cube of a sticky gel attached to the end of a plastic “wand.” The gel cube is placed in contact with the surface of sensor’s glass cover, and the dust adheres to it. Since the cube is smaller than the sensor, this process is repeated across the sensor surface. Press the cube against a supplied sheet of “sticky paper,” to clean it. I tried the Sensor Gel Stick partially out of desperation, as my aging Canon 5DII had picked up a ridiculous number of dust specks that were resistant to other methods of cleaning. After the first cleaning there were no noticeable spots left on the sensor. I continued to use it during a four-day shoot in Death Valley, a location known for dust—and, again, the result was impressive. The product is not cheap, costing about $50 for the gel stick and the sticky papers. Frankly, I think it is overpriced—but because it works so well I was willing to pay the price. I have recently seen similar products online from other distributors at lower prices, though I cannot vouch for their quality. I understand that two versions are currently available. One is the “blue” version that I have. The other “pink” version is apparently designed for certain camera brands, particularly for some Sony cameras. Check the product descriptions and make sure you get the right version for your camera.
(Most recent update: June 2023)
Articles in the “reader questions” series:
- Concerned About Image Theft
- How to Add Borders to Online Photographs
- One Lens for Landscape and Wildflowers on Hikes
- Yosemite in October?
- DSLR Sensor Cleaning
- About Sharpness and Detail
- Camera Stability and Long Lenses
- Photographing in the Rain
- Landscape Lenses
- About Depth of Field
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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All media © Copyright G Dan Mitchell and others as indicated. Any use requires 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.)
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)
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.
Blog | About | Flickr | Twitter | Facebook | Google+ | 500px.com | LinkedIn | Email
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.