Not Your Everyday Banana

by Bart Arondson

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I purchased a Canon EOS 650D and EF 50mm f/1.8 II lens.

Unfortunately, the auto-focus seems to miss focus at f/1.8. I've tried setting it to single-point auto-focus: it is better, but still not good enough.

This is my first DSLR, so is this likely a result my lack of skill, or perhaps is the lens at fault?

I made some tests on a target. Here is the result: test target

The focus is at 1cm in front of target.

Shooting parameters:

Aperture f/1.8, ISO 250m, Exposure time 1/250

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I seem to recall that Canon's DPP software can show you the focus point that was triggered and where on the subject. Not sure if that's for all Canon cameras, being a Nikon user myself. –  John Cavan Apr 11 '13 at 11:04
    
It only shows which focus point was selected. If you focus and recompose it doesn't reflect that. Also keep in mind that for many DSLRs including most current Canons, the areas in the viewfinder covered by each focus point are much larger than the squares that mark their center in the viewfinder. –  Michael Clark Apr 12 '13 at 0:43
    
Did you initially notice this in taking test photos, or did you notice in real photographs first and did the test to confirm? –  mattdm Apr 12 '13 at 13:14
    
@mattdm I noticed this on usual photos, that I had taken. Today I provided test on the target and saw that it is not my imagination. –  Alex Apr 12 '13 at 13:39
    
Another question: did you try multiple times, defocusing the lens and then starting again? How consistent were the results? I notice that (unless it's cropped) the target is quite close; did you try at further distances, and how do the results compare? –  mattdm Apr 12 '13 at 14:57
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4 Answers 4

up vote 8 down vote accepted
  1. At f/1.8 the depth of field is very small - any movement (of subject or camera) between focusing and taking the shot can push the subject out of focus - so you want to take the photo as quickly as possible after focusing and don't use the "focus and recompose" technique - not at f/1.8

  2. The auto-focus on the 50mm f/1.8 is very slow, if the camera is set up to take the photo even if focus isn't locked it's possible you take pictures before the lens finished focusing - when the camera focuses it will beep and a small circle will light up in the view finder.

  3. And last - and definitely least probable - your camera or lens can be miss-calibrated (at least for each other), there are questions on this site about focus testing (like this). I believe the 650D does not have any way for you to calibrate focus and you need to take the camera and lens to a service center (or at least a good camera shop).

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I agree with 1 and 3, but regarding to the 2 I always wait for the indication (sound and visual). Anyway, thanks. –  Alex Apr 11 '13 at 6:44
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@Nir this might be what you are looking for: photo.stackexchange.com/questions/12215/… –  Regmi Apr 11 '13 at 6:47
    
@Regmi - Thanks –  Nir Apr 11 '13 at 11:18
    
Regarding #1 use a tripod for AF testing --- #2, I believe most cameras (650D included) in single-shot mode will not take the photo until focus is achieved. –  drfrogsplat Jul 24 '13 at 3:41
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As much as everyone loves to talk about how sharp the "nifty fifty" is, if you look at the ISO 12233 charts here you see that it is much sharper at f/2.8 than at f/1.8 or f/2. The lens is manually focused using Live View for the test charts.

You may not be missing focus as much as you think. Is there an area behind or in front of your target that is in focus? If not, then you are experiencing the loss of sharpness shooting wide open that is characteristic of many lenses.

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I agree with this, I think it's sharpest is always a little soft at f1.8. One way to test this theory would be to zoom in as much as possible in Live View, and focus manually then compare the images –  laurencemadill Apr 24 at 12:54
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I think it would be the best if you uploaded a few shots to show us what you were shooting and how, I mean distance, angle etc. I had a little problem with Nifty Fifty at the very beginning, then I read a few articles on the Internet. After using this lens for over 3 years now, I can surely say that this lens needs a more special approach than let's say Kit lens.

Canon 50mm 1.8 is a very small and light lens, you really need to hold it tight when taking a photo and don't forget to hold your breath while pressing the shutter, it helps to not to miss focus at f1.8. These are only small tips, more could be suggested after seeing your shots.

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As I've tested (took 10 series of shots) the 50mm f1.8, I've come to this result: If the lens starts focusing from the nearest focus point (inner barrel is fully out of the outer barrel), then the image will be very-very front focused. If you set the focus ponint to infinity, and then start autofocusing, then the image will be as I say 98% spot on focused. In live view it's not a question, it's always correctly focuses. See attached image: Focus accuracy ("kép" means "image" in english)

Pictures have taken with f1.8, ISO 100, from a massive tripod with relatively fast shutter speed (1/120) with a 600D (t3i). The focus point was set to the "C" letter in the "FOCUS" text.

EDIT: So I've disassembled the lens, and I've soldered some points (as described in this video: DIY 50mm f1.8 calibration video) These are the results:enter image description here

I think it's working great now, there arent any BIG misfocus now, and it's much more reliable than before.

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This potentially shows a problem with your camera / lens combination, but is there any evidence this is a general problem with the 50mm f/1.8 which is likely to apply to the original poster as well? –  Philip Kendall Apr 24 at 12:25
    
With my friends 60D the problem is the same with his 50mm f1.8 too, so I think it's a general fault. –  Asycid Apr 24 at 16:12
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