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01-01-2012, 02:10 AM   #1
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Intermittent AF front focus problem on FA 50mm

Hi guys,

So I noticed on my FA 50mm f1.7 that it was MASSIVELY soft at any aperture greater than about f3.5. I knew from reviews and from previous experience using this lens, that results should be at least useable at f1.7, and quite sharp at f2.0 and slower. I tried manual focussing and the problem went away... now initially I thought it was the camera because I recently switched from a K-x to a K-r. But I also noticed that my other lenses (in this case the Sigma 105mm macro) and the Tamron 28-75mm did not have an AF issue on my K-r.
Then I got a bit scared when I took the FA 50mm off the K-r and put it back on the K-x and AF was fine, even at f1.7... obviously this made me think it was the camera (the K-r's problem).

So I did a test with a tripod - the K-r with the 50mm failed dismally, focussing a good 3cm forward from about 70cm away (by the way, bright clear conditions with a heavily contrasted object).

I put the lens on the K-x and initially got an absolutely hopeless result... then a PERFECT one (focus could not have been sharper) and then a hopeless one again. I put the 50 back on the K-r and got a PERFECT result. So it seems to be intermittent, and it often seems to go between being terrible and being perfect when it's been taken off one camera and put on another, which makes me think possibly it's in the contacts for the AF or something, and when I move it, it either goes from being right to being wrong, or vice versa?

Edit:
These are suggestions you may have that I have already tried:
- Am I focussing on the right thing? - Yes, I have both cameras set on centre focus only, and I'm making sure the entire centre is taken up with my subject, which as I said is well lit and is a contrasty item. The camera is definitely focussing on the right thing.
- On the K-r have I got fine AF adjustment on? - I fiddled around with this but these tests were done with it off. The focus problem, when it's there, is so bad that neither +10 or -10 is strong enough to correct it. I THINK it is a front focus problem, though one of my test shots (on the K-x) is actually back focussed. When the K-r was having the problem, I tried -10 focus (back focus) and the problem was LESS but still obvious.

I guess I have three questions:
1. I don't know exactly how AF works, but the lens is telling the camera that it's focussed when it isn't - is this possibly just a problem with the contacts on the lens?
2. Should cleaning it with some contact cleaner (like you use to clean circuit boards etc) do the trick?
3. If not then what should I try?

Thanks for any suggestions.


Last edited by Quazimoto; 01-01-2012 at 02:20 AM.
01-01-2012, 04:29 AM   #2
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Cleaning the contacts won't hurt at all, so why not try it! I don't know that that's the problem though. It would help if you posted your test shot set up.

There seem to be so many things which can affect AF that it can be quite difficult to pin down the issue. The AF sensors are a bit larger than you might think from the red dots, and in some cases aren't perfectly aligned. AF sometimes just goes madly out of focus for no apparent reason - as if there is some sort of computational error going on inside the camera. I'm always ready to intervene with my camera's autofocusing! The fact that the AF behaves better when you switch lenses *might* purely be a coincidence. Good luck!
01-01-2012, 06:55 AM   #3
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You are waiting for the SR to lock aren't you?

Only clean the contacts with a dry lint-free cloth. Liquids could seep in and do damage.
01-01-2012, 08:24 AM   #4
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You need to post test shots or there is no way anyone can guess what the problem is - could be with the camera, with the lens, or with the testing methodology.

01-01-2012, 02:24 PM   #5
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Original Poster
QuoteOriginally posted by ihasa Quote
There seem to be so many things which can affect AF that it can be quite difficult to pin down the issue. The AF sensors are a bit larger than you might think from the red dots, and in some cases aren't perfectly aligned. AF sometimes just goes madly out of focus for no apparent reason - as if there is some sort of computational error going on inside the camera. I'm always ready to intervene with my camera's autofocusing! The fact that the AF behaves better when you switch lenses *might* purely be a coincidence. Good luck!
I'm happy to manual focus when I'm doing stills on a tripod but I didn't pay $300 for an FA to have manual focus... I already had the K f1.4 which has heaps better MF and nicer bokeh, so yeah I do want it sorted out.

QuoteOriginally posted by JohnX Quote
You are waiting for the SR to lock aren't you?
No, tests are on a tripod so SR is turned off entirely - but when I first noticed the problem, I was going handheld with SR on and using speeds that wouldn't show any significant shake anyway, like 1/80 and it was still an obvious problem.
In terms of whether it is fully focussing before I take the photo, I have the camera set to 'focus priority' so it has to wait until it locks before taking the picture.

I will post the test shots when I get home from work. I'll also clean the contacts.
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