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07-23-2013, 09:52 AM   #1
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Question about focus lock indicator on manual focus lens

Hey everyone, I've been shooting with my K-r for some time now and I've found the auto-focus to be pretty good most of the time with AF lenses. But I've been seeing some strange behavior (to me anyway) when using my SMC Pentax-A 50mm F1.7 which is a manual focus lens.

Firstly it seems that in lower light the camera will do a good job with an auto focus lens and will focus for me, but with the manual focus lens it often will not give me a focus lock indicator at all in the same low light, or it will be way out to lunch.

Secondly, in good light it seems that the focus lock indicator is often way off. If I'm shooting anything where I have to get the shot right the first time then I can't go anywhere near the fast side of this lens or I'm guaranteed to have a focus problem even if I get focus confirmation from the camera. To make things more interesting, the camera will indicate focus lock at a little different position depending on which direction I'm turning the MF ring. This lens is SHARP and the results are spectacular if I happen to nail the focus, but the indicator in the camera only seems useful to get me in the ballpark (which I could have done by eye anyway), and then it's a pure numbers game firing off enough shots in multiple setups hoping that one of the setups will yield a proper focal point.

I've done a few events, will be shooting my first wedding this weekend, and am getting enquiries for another wedding. Wondering if it might be time to step up to a K5ii with a better AF system...

One more comment, when I fire up the camera with the 50mm prime on the body it asks for the focal length. Should I be telling it that it's 50mm or 75mm? (APS-C equivalent with a FF lens on a smaller sensor). I've experimented with both settings but it doesn't seem to make any noticeable difference.

Thanks in advance for any advice.

07-23-2013, 06:01 PM   #2
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Af confirmation with manual lenses has a fairly big tolerance, so I'd recommend stopping the lens down to make up for it with the dof.

You should be setting the fl to 50mm.

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07-23-2013, 11:18 PM   #3
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Thanks for the reply Adam, confirms some things for me. I wonder if I might get more consistent results with an AF 50mm prime, letting the AF do it's job then recomposing the shot. The AF / MF switch probably wouldn't like that treatment though.
07-23-2013, 11:42 PM   #4
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QuoteOriginally posted by jostafew Quote
To make things more interesting, the camera will indicate focus lock at a little different position depending on which direction I'm turning the MF ring.
With a bit of practice, you can hit the middle of the indicated "acceptable" range.
I often go back and forth with the focus, exploring the end points of the "acceptable" range and then try to hit the middle point. This often works well for me.

I'm not sure whether your K-r supports "catch in focus" but this once worked well for me. Another time, with another lens, it didn't but I haven't tried to get to the bottom of this.

Finally, if feasible, using a high magnification factor in live view can yield excellent results.

07-24-2013, 05:16 AM   #5
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I also have the K-A 50, f/1.7 lens. Frankly, 90% of the time I ignore the focus indicator and just use my own eyes; and then I select an aperture at least a couple stops down from wide open.
07-24-2013, 09:30 AM   #6
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All good advice, thanks everyone.
07-24-2013, 10:50 AM   #7
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QuoteOriginally posted by jostafew Quote
All good advice, thanks everyone.
i have the same issue. same lens same k-r. i use live view and the focus locks perfectly.
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