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05-20-2008, 07:03 PM   #1
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Manual focus discrepancy

I have but one manual focus lens, a Vivitar 70-210 f/4.5 (the one with the flange that's too big). I've only had it about two weeks, and I've played with it a little, but this evening was my first real opportunity to use it at the first T-ball practice of the year. I figured I could hone my skills while the kids worked on theirs.

Anyhow, if I didn't know any better, I'd say I had a back-focusing MF lens. It seems as though to get a sharp picture, I have to focus a tad in front of my subject. I popped on the kit lens and it seemed to be the same.

Now, I seem to recall a link someone posted (though it may have been on that other forum) that showed pictures of a sewing needle in and out of focus, but I can't find it now. I think it was talking about a discrepancy between what's in focus in the viewfinder versus the photo. Is this what I'm experiencing? Everybody else see the same thing?

Just to calm myself, I did my first D70 focus test, and with the FA50, it looks like I might have a 0.5mm front focus, but that's narrow enough to attribute to normal AF margin of error, right?

Thanks, guys.


EDIT: Duh, I forgot the most important thing... While I had the test chart set up, I popped on my MF lens and checked it, and it seemed dead accurate.

Last edited by joefru; 05-20-2008 at 07:33 PM. Reason: Addendum
05-20-2008, 08:20 PM   #2
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It is remotely possible that the focus screen is not perfectly positioned. I left out a shim once, and the focus was waaayyyy off. Perhaps if you gently reseat the focus screen, the problem will disappear.
05-20-2008, 09:24 PM   #3
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I've never touched the focus screen. And the AF has always seemed sharp on the screen, even at f/1.4.

I'd like to get a split-prism one of these days...
05-20-2008, 09:33 PM   #4
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I recall the article you are talking about. But I can't find it either. I think it was posted on DPR and involved an Olympus. If I recall, the jist of it had something to do with the combination of a modern "bright" focusing screen, and a fast lens led to inacurate focus.

Hopefully someone can find that link again. I would like to re-read it.

Last edited by KungPOW; 05-20-2008 at 09:34 PM. Reason: spelling.
05-20-2008, 09:50 PM   #5
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My Sigma 135 - 400 is like that, but making it even weirder it's only like that between something like f11 and f18
So if you try playing around in different f-stops you might be able to work around it

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