Is this my lens or my sensor? Canon Rebel XT

Discussion in 'Canon EOS' started by nathan_wolfson|1, Nov 5, 2011.

  1. I've got a Canon Rebel XT which has been a trooper, and a "Canon EF-S 17-85mm f/4-5.6 Image Stabilized USM SLR Lens for EOS Digital SLR's" which I have used since day one, ie, about six years ago.
    Recently, I have noticed that there is some interesting (polite word) blurring, color shift, and blow out in the highlights of some daytime shots. This doesn't appear to be related to exposure settings, or white balance setting -- but I may be wrong.
    I wonder if there is something wrong with my sensor or with the lens or something?
    00ZYst-412607584.jpg
     
  2. One more example.
    00ZYt1-412607684.jpg
     
  3. Both are taken wide open at slow shutter speeds. In the first one, the focus point appears to be on the column on the left side, so yes everything beyond that is out of focus and blurry.
    Wide open is not the best setting for sharpness on any lens, excepting a few multi thousands of dollars super telephotos.
     
  4. I think there are three separate issues, all related. The first is that as Bob says, most lenses are not at their best shot wide open. The second is shallower depth of field as you open the aperture more. This has nothing to do with the quality of the lens. The third is where your focus point is. In the first image, as Bob says, it is way up front, which is not where you presumably want it. Keep in mind that if you have all AF points turned on, the camera does not know which one is most important to you, and also there may be a lot of contrast at the wrong AF point, causing the camera to focus where you don't want it.
    In any case, this does not look like anything to do with the sensor. To test whether the lens has gone south, do some controlled shots, e.g., of brick walls.
     
  5. It's the lens.
    That particular lens suffers from a variety of long term maladies, mostly related to optical problems. It was among Canon's first EF-S lens (came out w/ the 300D if I recall correctly -- that or the 20D), and it shows in the lens's behavior, and durability.
     
  6. I'd really like to see some links supporting the assertion that the 17-85mm lens has those problems.
    I have one, use it all the time, and follow references to it when I see them. All I have ever seen were complaints about its optical quality (sharpness, distortion, etc., usually disagreed with by those of us who actually use the lens) not "problems" with its operation or "maladies" like the ones described in the OP as developing over time.
     
  7. Well... without condemning a whole line of lenses, then. I see strong chromatic aberration in both shots around the blown out sky. Few lenses will be completely clear of CA in those circumstances. I don't know if it's more than expected for this grade of lens. Internal flare and attendant loss of contrast is also evident, possibly more so than expected. It might respond to a good cleaning of both front and rear elements. I was not able to find any distinctly sharp detail in either shot. This might be due to motion, poor selection of auto-focus point, or indicate a problem with the lens.
    I would test the system by mounting the camera securely on a tripod, and try to shoot at least one clear, sharp, contrasty shot, bracketing focus manually if necessary. If I were still unable to get one good shot, I would try the same with a known good second lens, to eliminate possible problems with the camera itself. The outcome will let me know what next steps are reasonable.
     
  8. I agree that the focus and zoom is be part of what makes those shots poor. But it's really the bad rendering and halo of the sky in the
    rock shot, and the complete lack of definition and wholly inaccurate color of the sky in the other, that no amount of work on my part
    seems able to correct.

    Interested to hear if this is the kind of problem others have seen. Now I really wish I had another lens to test with.

    But since so far the diagnosis is it's user error or the lens I am thinking I might take it as likely the body and sensor are okay and
    invest in a new lens. Open to recommendations.
     
  9. I'm thinking a zoom again but i'm less in need of the telephoto end than of a good wide angle end. I'll bet there are
    ten great threads that exist already on that :)
     
  10. Well you can't get definition in the sky in those two shots when it is very out of focus and rather overexposed. The flare in the rock shot is expected in that situation, just emphasized because of the aperture and you are at the wide end of what I think may not be a great lens to start with. Both the flare and apparent CA would be affected if your lens's internals are wearing out or damaged.
     
  11. I'd really like to see some links supporting the assertion that the 17-85mm lens has those problems.
    I have one, use it all the time, and follow references to it when I see them. All I have ever seen were complaints about its optical quality (sharpness, distortion, etc., usually disagreed with by those of us who actually use the lens) not "problems" with its operation or "maladies" like the ones described in the OP as developing over time.​
    JDM - next to you I am probably the greatest living fan of the 17-85. Occasionally I lust after the newer 15-85, but I simply can't justify the expense, especially since a change to the 15-85 would mean that my 10-22 sees even less use. In addition, I really can't say I find the results I get from the 17-85 lacking.
    That said, I recently noticed when searching 'canon 17-85' on Fleabay that quite a number of sellers are offering diaphragm flex cables for this lens. I figure if people are offering that part, there has to be a demand for it, and presumably it's a weak spot of the lens. However, seeing that some folks sell the part for $3.99 including shipping, I am not loosing much sleep over it.
     
  12. Interesting. Do my photos look like that part has gone faulty in my lens?
     
  13. I don't even know what that part is but I would have to say it is doubtful. Take in to consideration what everyone has posted thus far as everything seems rather accurate. The issues with those photos can primarily just be attributed what has already be mentioned.

    I've got a 17-85mm as well. While I haven't been blown away by it, I have never had an issue with it. For whatever faults it may have I find it to be a very useful lens which has never gotten in the way of doing what I want it to.

    Edit: The bad rendering of the sky really just seems to be a metering deal. The location of the rocks seems to be a pretty low-light area. The camera metered for the rocks, and when the sun or sky poked through at the top it was blown out because it was just significantly brighter than the rocks.

    In landscape photography this is a common issue dealt with because the sky is just about always brighter than the landscape. This is one of the benefits of a polarizer, or graduated neutral density filter in that they will darken up the sky a little bit to get more detail.
     
  14. I should do some more testing, and put the polarizer on. Unfortunately, the UV filter won't unscrew, it's been on there so long, so I gotten get creative with a method to remove it, before I can put the polarizer on it.
     
  15. Nathan - I forgot to ad, any problems in your photos to me look like a combination of a difficult lighting situation (insufficient exposure latitude plus flare) combined with focus problems (shallow DOF and focus on the least desirable part of the frame). None of this has anything to do with the lens. Just my 2c.
     
  16. One option if you think it's just on really tight is a basic filter wrench:
    http://www.bhphotovideo.com/c/product/70726-REG/General_Brand__Filter_Wrench_48_58mm_Set.html
    (That's just one option in a general brand)
     
  17. @frank

    Thanks. User error is the cheapest thing to fix :)
     
  18. Nathan,
    I've owned two of the 17-85s, and both suffered from the kind of problem you seem to be having now. Even though your image choice is poor for evaluative purposes, the flaws of the 17-85 seem to shine through --especially in the rock picture. (particularly in the lower 1/3). Unfortunately this is very common for that lens. especially at focal lengths wider than ~40mm. The distortion and "smearing" is almost to be expected. I recall shooting a cinder block wall to determine just how bad it was. It was bad - on both lenses. A shame really, because the range was ideal, and the USM/FTM helped make up for it's slow speed. I wanted to like it so badly, but unfortunately just couldn't - the IQ was just too poor.
    That said, replacing it with an optically superior lens is not only easy, but rather inexpensive these days. The biggest thing you'll loose is the full time manual focus and USM (go hand in hand and on a lens this slow is really a necesity), but replacing it with a lens that actually is sharp is something you may find rewarding.
    Some lenses to look at are the:
    Tamron 17-50/2.8 (both VC and non VC versions)
    Sigma 17-50/2.8 OS
    Canon 18-55/3.5-5.6 IS (yes, the new 'cheap' kit lens optically outperforms the 17-85)
    If you have the budget for it, the Canon EF-S 15-85 is supposed to be a stunningly good lens (and a bit wider).
     
  19. Don't worry about your lens.
    The first picture as already stated the camera guessed on the focus point and guessed wrong. In this picture the sky is blown out becaause the forground is dark. The camera set the exposure for the dark forground and overexposed the sky.
    In the second the sky was even more overexposed than in the first picture resulting in some flair around the sky. Th flair is un avoidable in that situation and can be made worse if your lens is dirty or if you have cheep or dirty UV filter on. If you see flair and have a UV filter remove it and take another shot. whenever you have a dark forground and sky in the picture the range of light levels will almost always exceed the capabilities of the camera. You eather get the exposure of the sky right with a very dark forground or you get the forground right the sky is overexposed.
    That said, I recently noticed when searching 'canon 17-85' on Fleabay that quite a number of sellers are offeringdiaphragm flex cables for this lens. I figure if people are offering that part, there has to be a demand for it, and presumably it's a weak spot of the lens.​
    If the flex cable fails you will get an error code and the camera will not take the shot. As to the optics of a lens, they are made of glass and generally held in place very securely. The glass of the lenses won't degrade in any way over time and very rarely do lenses shift position. What you need to doe it to learn how to meter and expose an image and then learn how to take control of focusing. If you know those two things you would have probably recognized the focus issue and fixed that and taken a second shot.. As to the exposure there are generally very few easy things you can do other than waiting for light levels to change, or re frame the image to reduce or eliminate the sky in the image.
     
  20. @ Marcus: Thanks for the perspective. I will take some controlled low contrast shots of a brick chimney when the light and weather permit and post what I see. That may help determine whether there is a mechanical problem, or whether as several have suggested, it's user error.
    I really appreciate your lens suggestions. If a little controlled testing indicates that my lens has gone south, you have given me some good ideas about what to consider for replacement.
    @ Steven: Thanks for the suggestions. Actually I did have a cheap UV filter (single coat) on there, which probably made things worse. I actually studied and used the zone system years ago, but with digital and auto exposure and focusing I have gotten lazy. I need to take more control and not just leave everything on Auto.
    I picked up a multicoat polarizing filter today and will use that when shooting in bright daylight, which, along with paying attention to what I am doing, may help.
     
  21. Spearhead

    Spearhead Moderator

    I've owned two of the 17-85s, and both suffered from the kind of problem you seem to be having now.​
    That's anecdotal evidence, hardly scientific or indicative of any kind of problem. If there is a problem, it needs to be documented through statistical data. Otherwise, it's just the web...
     
  22. Consider a 50mm 1.8 II for the heck of it. I just bought one for its low light capabilities. The image quality is markedly better than the 17-85mm. I'm tempted to sell mine right now (though I won't).
     
  23. Nathan -
    The 18-135 will probably perform a fair bit better on the wide side (which you seem to have indicated is more important to you) as far as sharpness (even in the corners), and CA goes, but expect to have to stop it down a bit to get decent performance out of it on the tele end. Also, even though it's equipped w/ a micro USM, it doesn't have FTM focusing, so instant focal overide is out of the question. I've only shot with it a couple times, so those impressions are less than in-depth.
    To be honest though, If you want a lens that's vastly superior to the 17-85, and a bit wider (effectively a 24mm FOV vs. 29mm FOV), the 15-85 is going to be the best bet. the other lenses I suggested (and even the 18-135) are considerably better optically than the 17-85, but won't give you any capability to go wider. Of course it's also the latest/greatest, and it's cost is significant, darn near twice that of the 18-135...
     
  24. That's anecdotal evidence, hardly scientific or indicative of any kind of problem.​
    Sorry, you misunderstood my point (maybe I didn't make it abundantly clear ;-) ). My point was that the edge (really more than just the edge) performance @ <~35-40mm was so bad as to make me think that my copies were bad, or misaligned, or something was actually wrong with them. The second of the two lenses was new. When I went online to compare images others had taken, I found that at least half of images posted (I saw) had the same issue (to a lesser or greater degree - it varied, obviously) - the same outer area smearing and blurring and CA. This, apparently, was largely independant of relation to the focal plane, but decreased by stopping it down. I concluded that this was not a specific 'problem' that I was suffering, but an optical design weakness in the lens, which perhaps affected many (but obviously not all -- maybe not even most) of the lenses to a varying degree.
    So the point I was trying to make was that what I assumed was something actually wrong with my lenses, really seemed to be a weakness in optical design that varyingly affects individual units - on a large scale -- aka a marginal design. The images the OP posted seem to fit right in, demonstrating that same weakness.
     
  25. Failed to remember to take a photo of a neutral brick wall during the daytime/no work time. Here's a random photo, same lens, but after putting a polarizing filter on it. I suppose I could reduce the exposure time to control the blooming around the trees....
    00ZZaw-413455684.jpg
     

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