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I have just shot a long outdoor trip using the Sigma 17-50mm F2.8 EX DC OS HSM on a Canon 70D. I've read excellent reviews of the Sigma lens but the photos didn't turn out that great. Neither the image quality nor the sharpness is great. I mostly shoot in aperture priority mode.

I am not an expert photographer but I have been using a DSLR for a couple of years. Previously, I used the Nikon D3200 mostly with a 35mm f/1.8G lens and was quite happy with the pictures I got.

I think something is wrong with the 70D, or the Sigma lens, or the combination. I bought the lens used. I also get quite nice pictures with the Canon EF 50mm f/1.8 STM.

A landscape jpeg straight out of camera can be found here. Single point AF on the central pillar.

enter image description here

Exif: 20 mm, 1/40 sec, f/16, ISO 100, -0.33 EV

In response to the issue of diffraction, I post another image. Single point AF on the red flowers in front.

enter image description here

Exif: 18 mm, 1/80 sec, f/10, ISO 100

Can anyone suggest how to check the camera and the lens combo?

  • f/16 : already diffraction's limit ? – Olivier Sep 11 '16 at 17:11
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The first sample image in the question is focused well in front of the central pillar. The second is focused well behind the flowers.

You have told us that you are using single point AF but you haven't told us which AF mode you are using: One Shot, AI Servo, or AI Focus? If you're trying to focus and recompose using AI Servo the camera will refocus when you move the camera to point at a different spot. If you use AI Focus the camera will initially hold focus as it would in One Shot mode, but if you recompose and hold the camera too long in the new position the camera will sense that your selected AF point is no longer in focus and will switch over to AI Servo.

There appear to be other issues at work that might also be contributing to your results:

Diffraction The 70D is a Canon APS-C camera with 4.1µm pixel pitch. The Diffraction Limited Aperture (DLA) of the 70D is f/6.6. This is the point at which the effects of diffraction begin when viewed at the pixel level. As apertures are narrowed beyond the DLA the results get more and more noticeable at normal viewing sizes. The best way to avoid this is to shoot at around f/8 or wider and at f/6.3 or wider if possible.

Camera movement Not everyone can hold a camera steady enough to use the 1/focal length rule-of-thumb, even when viewing at the standard 8x10 sizes for which it applies. You may get useable results for viewing at smaller sizes, but nowhere near the equivalent viewing size of looking at part of an image at 100% on your monitor. If you have an HD (1920x1080 pixels) monitor that measures 23" diagonally you are viewing images at 96 ppi. That means an 18MP image viewed at 100% is being magnified at the equivalent of 54x36 inches! That's 5X the magnification of the standard 8x10 print.

The optical limits of your lens I'd like to know where you read excellent reviews of this lens. I've never seen any critical reviews from reputable reviewers written about it that impressed me very much. Before you can blame AF you need to be sure that something else isn't causing your images to be blurred. To do that you need to eliminate as many of the other possible causes as you can.

  • Mount your camera on a stable tripod and use a cable release or the self timer to release the shutter. This will help eliminate camera movement as the source of your problem.
  • Shoot under bright enough constant lighting that your shutter speed at ISO 100 can be 1/100 second or faster. Use the fullest spectrum lights available to you. This will further help to eliminate camera movement including vibrations caused by the movement of the camera's mirror. Properly exposing using low ISO will also help eliminate poor image quality caused by a low signal-to-noise ratio and the resulting noise reduction.
  • Use a flat target that is lined up parallel with your camera's image sensor and perpendicular to the optical axis of the lens. An easy way to do this is to aim your camera at a flat, stable mirror. Center the viewfinder on the center of reflection of the lens in the mirror. Then tape your focus target onto the mirror being careful not to move the mirror.
  • Use careful manual focus with magnified Live View. Take several samples while refocusing manually between each sample.
  • Repeat the test shots using One Shot AF mode with the single center focus point selected. Move the lens to infinity or minimum focus between each test shot. Use a half shutter press with your cable release to allow the AF to confirm focus before taking the photo.
  • Compare the best of the manually focused shots to the best of the AF shots.

If there is a significant difference then you have an AF issue. If there is not a significant difference then your problem lies elsewhere.

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    Learnt a lot. DLA is new stuff! Where to find it for different cameras? I will try out the steps. I wanted to put another interesting image but doesn't allow me. – SM3 Sep 13 '16 at 7:42
  • For Canon you can find the DLA for a specific model at the-digital-picture.com/Site-Index – Michael C Sep 13 '16 at 7:44
  • Other cameras with the same pixel pitch as a specific Canon model should also have the same DLA. There's also a good article at that site that explains what DLA is and what it is not: the-digital-picture.com/Canon-Cameras/… – Michael C Sep 13 '16 at 7:46
  • Thanks! Even my upvote doesnt show because of my reputation. – SM3 Sep 13 '16 at 7:54
  • I still doubt that there is a focus issue. I have been using Single shot AF in almost all pictures. And shooting at 4-5 times of 1/focal length at ISO less than 800 I doubt the hardware. Of course its difficult to doubt on ones own abilities! :) – SM3 Sep 13 '16 at 9:31
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My 2 cents :

  1. Shutter time : Shooting at 1/40 can be problematic if you have trouble keeping your camera steady or if it accidently moves while shooting (even with lens stabilization). To be certain, try to take a photo with a tripod or with your camera on a still support (maybe a delayed shutter and no stabilisation to be on the safe side). You can also try to shoot faster than 1/40.

  2. Diffraction limit (What is a "diffraction limit"?) : Have you already shoot at f/16 with this lens ? You can read the following on the review of the Sigma 17-50 f/2.8 by The Digital Picture (http://www.the-digital-picture.com/Reviews/Sigma-17-50mm-f-2.8-EX-DC-OS-HSM-Lens-Review.aspx) :

    At f/11, diffraction begins to show - reducing image sharpness over most of the frame (though corners continue to improve).

    Illustration from http://www.lenstip.com/256.4-Lens_review-Sigma_17-50_mm_f_2.8_EX_DC_OS_HSM_Image_resolution.html : Lenstip diffraction chart

    At f/16, you can see a significant lowering of the lens resolution, but it shouldn't be as dramatic as what your camera produce. Try at f/8 and see if you notice a difference.

With this aperture, almost everything in your photo should be in focus (=sharp), even if you have front/back focus problems.

  • I have posted another picture at f/10 and 1/80. The image is quality also bad for larger apertures like f/4. Really disappointing compared to canon 50/1.8 STM. – SM3 Sep 11 '16 at 18:29
  • Can you make one with lens stabilization off ? – Olivier Sep 11 '16 at 21:05
  • I will try to find one or shoot one. Though I have couple of tripod shots with OS off but at higher ISO and low light. – SM3 Sep 12 '16 at 10:02
  • Does not let me add a 3rd link as I have score less than 10. – SM3 Sep 13 '16 at 7:39
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Some of these Sigma lenses have reported focus issues. I have one, and I had to get it re-calibrated to solve the issue - when I investigated the problem initially, there were several reports online about similar issues others had had.

Looking at the images, certainly the second one looks like a focus issue, and the first could be explained by a focus problem (although Olivier's answer covers some of the other issues with it).

If you do have a focus issue. The good news is that should be fixable. Find a Sigma service centre and they should be able to calibrate it for you. This should be quite cheap.

To identify if you have a focus issue:

Buy (or print off) a focus test chart, then set up a shot with your camera on a tripod, focus on the appropriate point on the chart, and then take some shots - if you have a focus issue, then you'll see that the target will be out of focus, but another area of the chart will be in focus. Try this at several different focal lengths and several different distances.

Fine calibration on your camera:

Your camera may have a focus fine-tuning option to fine tune the focus of individual lenses (certainly all my DSLRs do) - this may be sufficient to compensate for the effect.

  • "...if you have a focus issue, then you'll see that the target will be out of focus, but another area of the chart will be in focus." If one part of a chart is in focus and the other side isn't then it isn't an AF issue, it is either a lens alignment issue or a test chart alignment issue (because it means the test chart isn't properly aligned parallel to the sensor/perpendicular to the optical axis). – Michael C Sep 12 '16 at 5:07
  • When you say "focus on the appropriate point on the chart" is it viewfinder or live view or manual focus? Also is back/front focus noticeable when doing manual focus? – SM3 Sep 13 '16 at 9:35
  • @SM3 AF using any mechanism you prefer. When doing manual focus you may instinctively correct for the error. – Harry Harrison Sep 13 '16 at 16:31

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