Is there something wrong with my autofocus?

Discussion in 'Canon EOS' started by ying_li, Jul 29, 2009.

  1. I've been taking extremely soft photos since I got this camera (Canon Digital Rebel XT), and I've been thinking that perhaps I was doing something wrong, since my photos with the 17-40 L lens AND my 50mm lens have all been soft. But a couple of times lately I've just been using the program mode or full auto, and the photos STILL come out blurry. Then I tried autofocusing without recomposing, and they are still a little blurry. So now I'm wondering if perhaps there may be something wrong with the autofocus on my camera. I'd appreciate any thoughts on this matter.
    Here are some test shots.
    These were taken with a Canon 50mm f/1.4 with an ISO of 100. The camera was on a table, but I did not use a shutter release. I am using center point focus, and the dot was aimed at the exact center of the left side of the bar code. These were all taken in Av mode.
    I know that with the much smaller depth of field on the f/1.4 photo, more of the photo should be blurry. But at least part of the bar code should be in focus, and it's not. I also know that photos will be softer when the lens is wide open, but it seems excessively soft to me.
    f/1.4 (larger version here ): [​IMG]
    f/2.0 (larger version here ):
    f/2.8 (larger version here ):
    Now, I don't think it's the lens, because my 17-40 L also seemed to take soft photos. I can't submit any test shots with that lens yet though, because I just moved and it's still packed somewhere. =/
  2. The 50/1.4 @ 1.4 at such a close focus distance is soft; @1.4 you have to be CARFUL of the DOF. Many times my shots are out of focus @1.4 because I've move back an inch.
  3. Hm... so the level of softness above is normal? I did not move the camera at all, except for whatever movement was made while pressing the button. The camera was sitting near the edge of the table, so I wasn't holding it. I looked through the viewfinder, pressed the button half-way to autofocus, and took the picture.
  4. As you say, the first image is fuzzy.
    I downloaded one of the larger images, opened it in ZoomBrowser, and tried to display the focus points. That information isn't in the jpeg file that your link points to. Because the pill bottle is curved, I would expect part of the bar code to be sharp, and the rest to be fuzzy. I would like to see if the focus point is ON the bar code, or just to the left, in the white area.
    Have you tried manual focus? If that works better, it would prove that your camera or your lens needs adjustment.
  5. yeah... you should repeat this while focusing manually. However, you should remember that neither of your lenses is going to perform optimally at such a close focus distance.
  6. Ying,
    This is not a good test because we can't see if objects in front of or behind the intended focus plane were sharper and thus if your camera/lens need AF calibration, download this short tutorial, follow up the instructions carefully to see if there is any back or front focus problem.
  7. Well I heard that those lens, especially the Canon L serie, cannot really work properly on other camera than Canon 40D, but especially on Canon 5D and above!
    And I read that you have the Canon Digital Rebel XT!
    I have a similar problem with my new lens Canon 85mm 1.8, and especially with low light condition, the focus becomes very soft!
    Then some college told me that my the center focus point on my Canon 30D is not so good, and that the best is the Canon 40D to be used with those lenses!
    But, I know for certain, that this wonderful lens 50mm 1.4 is working magnificently on the Canon 5D.
    So, to conclude, this is not a lens problem, but a camera problem! you need to have a better camera for those Lens L serie! but maybe I heard wrong ?! So, is this correct?!
  8. i had an xt and never was really pleased with the focus. a lot of photos seemed soft to me.
    my 50mm f1.8 pretty much was useless except at f8-11 or 18-22( the opposite of all reviews i read) my 70-200 f4 was bad over f5.6 or so, unusably soft over f8. my other lenses were hit or miss but none really good ( the two i mentioned are known for sharpness though)
    i think the focus system on the my xt was either off or the focus system in general just isn't that great. the lenses work much better with my 40d. the 50mm in particular is now great through out. at 1.8 is maybe even a little to sharp for subjects like flowers etc. i dropped the 70 just before i upgraded the body so can't really say how much is the camera and how much is it getting repaired but it' s good through out also now.
    since i doubt my technique improved that much over a week or so i figure it had to be the camera
  9. [[Well I heard that those lens, especially the Canon L serie, cannot really work properly on other camera than Canon 40D, but especially on Canon 5D and above!]]
    Nearly everything you've said is completely wrong.
    [[I know that with the much smaller depth of field on the f/1.4 photo, more of the photo should be blurry. But at least part of the bar code should be in focus, and it's not.]]
    You can do the DoF calculations yourself:
  10. The 50/1.4 takes practice when used at f1.4.
  11. You should check that your lens is not back or front focussing. There's a downloadable test chart for this, at the bottom of this page:
    No lens is perfect, but this will help verify if it's off, by how much.
    You focus on the single bit of detail at the center of the chart (inclined at 45 deg), and then check the sharpness of the incremental scales running up the peripherals.
    Also, sharpness is going to fall-off at f1.4 with this lens. You could compare here (and play around with the f stops):
  12. Wow, thanks for all the info everyone!
    When I googled for "focus problem xt" and variations thereof, most of what came up was arguments as to whether or not the xsi had focus problems - I never found that chart, which is incredibly useful. As is the image comparisions and DOF calculations.
    @Craig Meddaugh : I tried and everything came out as blurry or worse, but that's probably more an indication that I am rubbish at manual focusing than anything else :)
    So photos at f/1.4 are definitely softer than I expected, and I will definitely need much more practice (I don't think I've ever taken a good photo at f/1.4). But it looks like my lens is almost spot on (it looks like the focus may be very minutely too forward) and I just need far more practice:
    f/1.4 (larger version here ):
    f/2.0 (larger version here ):
    And it doesn't seem to be my camera, because when I finally dug out my 17-40 and did the same test, it seemed very slightly back-focused:
    30mm f/4.0 (larger version here ):
    One thing I noticed though. Even though my camera was on a tripod pointing at the same spot, some of the pictures I took seemed out of focus. My camera was set on autofocus, so I guess it refocused for every shot, but for instance this one on my 50mm taken at f/1.6 seemed out of focus, especially when you compare it with the others. And my camera didn't move (larger version here ):
    I sort of expected the autofocus system to not have changed, since it was pointed at the same spot and lighting conditions did not change. Can anyone tell me why or give me any tips/suggestions for how to keep this from happening with autofocus in the future?
    Thanks so much! Everyone has been so helpful. :)
  13. Canon's autofocus system uses sensors that are larger than the focus boxes in your viewfinder, so it won't necessarily focus on what's in the center of the focus box. Also, since it re-focuses each time, it's possible that it might focus on a slightly different area of high contrast than in a previous shot, even though the camera and subject have not been moved. I don't know how likely that is, but I think it's possible. One final point is that the system will complete focus within a certain tolerance range, so a second shot with exactly the same settings as the first may be slightly differently focused than the first while still being within the set tolerance range.
  14. Yang,
    Now we are in business :) From your tests I can see no systematic front or back focus issue, everything is within tolerance. The main issue is 50 f/1.4 is not going to be razor sharp at f/1.4, this is normal and a design compromise, most lenses are not sharp in wide open setting, you need to stop down to f/2.8 to get acceptable sharpness. Also keep in mind DOF is very narrow at f/1.4 so some objects maybe out of focus. But your gear is fine.
  15. @Rob Bernhard - Thank you for that information! I will tell that to my fellows photographers! They told me so, and they are wrong! well thank you for this truth :)
  16. Thank you for your answers all! :)

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