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It's obviously my fault for living in the UK and expecting to want to enjoy my bird photography in grey January afternoons, but, I would appreciate some opinions on the following.

I took 150 shots with my 1100D and 400mm 5.6L yesterday; all of Great Crested Grebes in drab winter colours, on a drab river, reflecting the drab winter grey sky (it was sunny when I set off but turned cloudy and as usual I just hit the shutter button and hoped). The camera had centre AF point selected, and was in AI Servo mode. Every shot was very poorly focused despite the camera quickly snapping to an autofocus decision. Nothing surprising here, but my question is this......

In poor light and contrast what is it focussing on? Wouldn't I just expect it to hunt for focus and give up? Why a quick focus and then NOTHING in the shot in focus? I could imagine it missing focus and grabbing a splash of water in front or behind the bird — but _nothing sharp(-ish) at all?

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What autofocus mode were you using? What focus points were enabled? Also, there is no way whatsoever to give any sort of opinion without seeing the photos. Maybe the camera focused on the man in the boat, or the zebra swimming on the foreground. :) –  Iucounu Jan 15 '13 at 9:14
    
were you half-depressing the shutter and waiting for it to lock? or fully depressing and allowing it to focus and shoot? I have found that in the later case, the camera's priority is to just focus on ANYTHING as fast as it can and shoot, as opposed to actually putting some effort into focusing. my guess is that it cant find a focus point so just shoots anyway. –  Darkcat Studios Jan 15 '13 at 9:14
    
Are you sure the blur isn't due to camera shake? Otherwise, if autofocus was snappy, something ought to be in focus - do you know which autofocus point was set? –  MikeW Jan 15 '13 at 9:48
    
Thank you for the suggestions. I believe that I was doing most things correctly; good tripod, centre focussing point, reasonable settings (ISO 400, f8, shutter between 100th and 250th, AI Servo AF, careful shutter releasing, no wind, stable ground, even tried a few on self timer to elinate all hand vibrations). –  Roy J Jan 15 '13 at 12:49
    
And despite all that, I end up with such soft images that any magnification at all shows grainy out of focus mush. I put it down to very low contrast just causing the autofocus to give up. But my reason for asking the question is that I just don't understand what the AF system is actually registering upon. –  Roy J Jan 15 '13 at 12:53
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3 Answers 3

There are many reasons for the shot to end up blurry. The 1100D only has the center focus point as a cross-type AF

  • First of all, you have to know what focus points were used by the camera. If you shot RAW, you can see this easily in DPP. If you let the camera decide everything for itself, it is likely that the camera "found" some part of the image where it could work out contrast better and focused on that instead of your Grebes. You say you let the camera focus and snapped. When you do that, the focus points it uses are lit in red in the viewfinder. So you could have seen from the start on what the camera was focusing.
  • Second, in order to avoid the issue in the future: Manually select the AF point, preferably the center one (you can always crop and reframe in post).
  • Third, if you are going to take pictures of moving targets, you may want to use the "AI Servo" AF mode, so that the camera would "lock" on the bird and follow it. If not, you may want to make sure you are not using "AI Servo" AF mode.
  • Another thing you may want to check, since the 400mm F5.6L does not have IS, is that your shutter speed is high enough that camera shake would not be an issue. Especially since the lens itself is heavy. A tripod may help, but might not work if you are taking pictures of birds in motion.

As was mentioned in the comments, some sample pictures (along with the EXIF) might help in confirming or infirming the diagnosis.

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Ok, I left out some detail. It was centre AF point selected, AI Servo and on a big tripod. The birds were moving only slightly. That's my point - on a sunny day all would be good. But in low contrast the whole image is poor. –  Roy J Jan 15 '13 at 10:19
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@RoyJ well that's the thing, if you have bad contrast on your picture, and in AI Servo for a non-moving target, you may end up with the camera struggling and refocusing constantly, actually losing focus constantly. For a non-moving target (or only slightly moving), you'd have better focusing results using "One Shot" AF instead of "AI Servo". I've experienced some troubles with the 1100D in AI Servo for still subjects, and use it only for moving targets now... –  matt.nguyen Jan 15 '13 at 10:35
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"AI Servo mode" is the clue to the answer. In general across cameras I've used, the single shot focus mode will "give up" and refuse to take a picture — but in continuous or "servo" mode, the camera does its best to take an image of something when you hit the shutter, even if focus isn't right.

You say "Why a quick focus and then NOTHING in the shot in focus?", and the answer is that the camera thinks you've asked it to prioritize capturing a frame over accurate focus.

On some cameras, this behavior is adjustable, but I don't think it is on yours.

That means you should change to AF One Shot (AF-S on Nikon or Pentax). The camera might be slower and you might miss the exact moment, but at least it will be in focus.

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I believe you shot the pics of Grebes with a "long focal" lens...400 mm Right?

So, the Canon 1100D has an APS-C sensor, and this means the equivalent focal lens would be something like 540mm. Therefore, the slowest shutter speed you should use to avoid blur from camera shake should be "1/540" sec.

You had the 1100D on a heavy tripod, so you would expect to avoid camera shake - and normally this works fine. But with such a long lens, the effect of the vibrations due to the mirrors flipping up is drastically magnified and it is certainly possible that it introduces enough vibration to cause too much blur.

In your case, I would try to shoot at a faster shutter speed. Increase ISO (3200?) and try 1/1000 if the aperture allows it.

This is the first thing to try in order to eliminate blur from camera shake.

But your problem could also have come from the AF accuracy. On a bright sunny day you don't have any problem with AF accuracy. Good news! So the AF works fine. But in hazy/foggy conditions, the AF doesn't work as well. It hunts and misses focus - not surprising because the AF system is generally based on detection of contrast.

Of course I don't exactly know how the AF system of the 1100D operates but I imagine it is probably about the same used by a lot of modern cameras... When the contrast has a sufficient level, the AF gives a good "sharp setting"...and therefore when the contrast's level is a bit to low, it can give a "wrong" setting.

In your case, when the light is a little bit dim and when the air is "hazy" (= low contrast) you should perhaps try to focus manually.

And the last tip I could suggest: change the setting of contrast. Your camera offers certainly a menu dedicated to "custom image settings" (or something like this). In this menu, I will find an option named "contrast". Go there and increase the contrast, then shoot the same scene in the same light conditions. Perhaps this will help the "AF" to find the accurate "sharp point"? If it works, the result in the pic' will PROBABLY not be what You expect! The white parts of the image will look like burned and the shadows will look too dark...but the aim of this new contrast's setting is to check how to get sharp focus.

If this works...then You know how to get a right focus. - Do it. - Now You have a sharp focus. Fine. - Disable the "AF" (= set "manual") and don't touch anymore the focus ring! - Then carefully set the contrast function back to the previous contrast setting - Don't forget to crank up the ISO and the shutter speed (minimum = 1/(focal length * 1.6) or 1/1000 if there is still enough light)

Now the 1100D is set with accurate focus and contrast for "hazy" ambiance. Release the shutter via the self timer.

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