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Old 11th Jan 2013, 11:55   #1
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Poor AF on 50mm f1.8?

Santa brought me a Nifty Fifty for xmas, and while it's a nice bit of kit I can't seem to get a decent AF out of it on my 1100D, is it just me expecting too much from it?

Even going to manual I find it extremely difficult to get sharp snaps out of it, and the tiny viewfinder makes focussing quite hard work as it is. I'll have to post a sample up once I'm back home, maybe it's just me expecting too much sharpness out of it.

Anyway, my question is, has anyone come across a lens which didn't quite focus right? I'm presuming it's a lens problem, as the others I've tried have all given decent sharpness and AF performance.
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Old 11th Jan 2013, 12:12   #2
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I have the same lens and although the af is noisy and a bit clunky it does the important bit ok (focusing).

Google around for ways of measurong front or back focus. You wont be able to correct it (only possible on a pro level body with micro adjustment feature) but at least if you identify an issue you can send it back for a replacement.
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Old 11th Jan 2013, 13:21   #3
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Same with mine, clunky and cheap but it'll take sharp images even under less than perfect lighting with my 50D.

You might have just got a bad lens, if you've got a local camera shop (Jessops) it might be worth popping in and trying their display model to compare, you'll instantly see if you've got a duff lens then.
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Old 11th Jan 2013, 13:40   #4
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Depends on a lot of things - which AF points your camera is using (cross-type are best); the subject's "focusability" - some things are just a proper b*tch for the camera to focus on properly; lighting; aperture; subject distance from lens; camera movement.

If a lens doesn't focus properly it's known as front focus and back focus, but it's not something you normally find on a nifty fifty... generally speaking, if you get an f/1.8 lens it is really difficult to get your subject correctly focused simply because the DOF is so shallow, not because the lens isn't focusing correctly.

Took me a fair bit of practice (and a damned good AF system) to get along with my Sigma 50mm f/1.4, but when it does focus correctly, you realise just how shallow the DOF is when the aperture is wide open. Stop your lens down to f/2.8 or f/5.6, and your focus woes will disappear.

And forget about trying to focus a wide-aperture lens manually if you're using a 1100D...

Focusing is actually a bit of a skill in and of itself - when you frame your picture, you need to know what specific part of the frame camera's AF system will manage to latch onto. A skill often used by wedding and reportage photographers is "focus and recompose," which allows you to alter the area of focus but still use the center AF point, which is usually the strongest in cheaper dSLRs.
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Old 11th Jan 2013, 13:49   #5
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Interesting thoughts, I've tried as many AF modes as I can easily get to in the menu, including using a single rather than multiple points, and it always seems to focus as quickly as with other lenses, the results just aren't as sharp.

I've also tried different shooting situations - from portraits to macro and they're all a bit fuzzy, in fact I think I've only got 2 or 3 vaguely sharp photos out of 100+.
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Old 11th Jan 2013, 13:57   #6
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You can try the quick-and-easy 30cm ruler test if you have time - place a 30cm ruler at 45 degrees to the plane of focus and, crucially, set it up so that the 15cm mark is bang in the middle of the frame on the center AF point.

Make sure your lens is at f/1.8 and once you've taken a snap, review the image at 100% and it will certainly give you a rough idea whether or not your lens is focusing where it should be.

EDIT here's an example of a focus test I did a couple of years ago with a 40cm ruler focusing on the 20.

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Old 11th Jan 2013, 14:16   #7
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I think it may require calibration. the focus test above will make it crystal clear.

as said, at f1.8, you really got to nail the focus. set one shot focus method, single centre point and try on a stationary, flat object.

I also have the same lens, thrown in when I bought my 5D2 and 24-105L. it only gets really sharp at f4 and after, which IMHO makes it pointless compared to the zoom lens. Since then, I've bought 35/2 and 85/1.8, both are sharp at large apertures and much better build quality. (no need to worry about mount breaking when picking up the camera by the lens)
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Old 11th Jan 2013, 14:31   #8
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Thanks, I'll try it out later, I might even get my cut-and-stick on for some of the more elaborate test-card type dealies. Unfortunately, as I've just had a google and found, the 1100D doesn't have any AF Microadjustment, so if it is out, do I have grounds to return it?
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Old 12th Jan 2013, 00:50   #9
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Originally Posted by Krikkit View Post
Thanks, I'll try it out later, I might even get my cut-and-stick on for some of the more elaborate test-card type dealies. Unfortunately, as I've just had a google and found, the 1100D doesn't have any AF Microadjustment, so if it is out, do I have grounds to return it?
Definitely, actually successfully returning it could be hard, depending on where it was bought from.
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Old 12th Jan 2013, 01:49   #10
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Amazon, who are usually pretty good about returns in my experience (drop in the ocean and all that). Let's see if it's me or the lens that's useless, maybe it's the former.
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Old 12th Jan 2013, 01:50   #11
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Have the same lens for my XSi, and though I've never used the AF (seriously, ever), I must say it is an exceptionally clear lens - for the price. Rather, most reviews on it that I have read state that it's AF performance is about the worst you'll find, but otherwise it is a difficult lens to beat for performance, and absolutely blows away the kit lens for clarity and bookeh.

As others have echoed above: DOF is massively thin when this lens is wide open, so try it narrower and see if you get better results. I cannot give you specifics as to where the DOF starts to widen, as once again: I've only used this lens wide open It was a very specific purchase for me, you might say...
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Old 12th Jan 2013, 01:59   #12
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I found mine just took a bit of getting used to, as with any lens really. The fastest lens I'd had till my nifty fifty was F4 so F1.8 was a whole new world and it took a while to get decent shots.
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Old 12th Jan 2013, 09:25   #13
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You guys should try f/1.2 and f/1.4 on a full frame camera... THAT is what "shallow depth of field" means. LOL

As for stopping down, even if you go to f/2.8 you'll notice a big difference, and by f/4 it should be markedly different. The trade-off here however is that you lose the low light performance, because f/4 just isn't "fast".
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Old 12th Jan 2013, 22:32   #14
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Sounds like it's not in focus, and as you've discovered, microadjustment can't be done on the 1100D.

I had to stick a -1 adjustment on my 50 f/1.2L & 5D2 combo as that was slightly off as well.
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