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11-16-2012, 11:46 PM   #1
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Consistent back focus with fast primes and low light? Is it me or the camera?

I'm asking this in the lens forum because this seems more lens than camera related. Maybe its a bit of both. I find that using focus lock confirmation with MF in low light (0-2EV) is not working well with both the A50 1.7 and M50 1.4 I have. They seem to consistently back focus. This is with the center point locked on a rather large target, like someone's face. I find the 18-55 locks well usually (when it get get enough light to the AF module in bad lighting that is). As does my tamron zoom with marginal light. I have a k-7 and upgraded to a k-5. The k-5 AF seems a bit faster to me, but nothing mind blowingly better. It does handle low light a bit better it seems esp around 0-1 EV. Still its so frustrating to go back and find the wall behind your subject perfectly in focus when you know it confirmed lock on their face or their jacket. I understand that its only a f5,6 sensor array, I get that. Thing is with a 1.4 lens it should be much easier for the lens to pick up focus. I don't get it. Can someone explain to me why fast primes want to lock on focus incorrectly? Is it the size of the focus point? I mean they are pretty big.

Usually what I do is back focus off and then refocus and do this a couple of times until it seems perfectly sure. Shoot. Chimp. Grumble. In street you need the damn shot. You don't have time to miss. So with MF, I use catch in focus and hold the AF button if I need to recompose. I'm typically shooting around f2-f2.8 these days in low light and when I want a fairly shallow DOF, so missing doesn't leave you with much to take home with. I hate that its always the wall immediately behind someone. Has anyone else noticed this? In good light these lenses are usually tack sharp when in focus and give reliable focus locks. I've never felt the need to adjust any of my cameras for focus, thought I probably should see if I could tune up my 12-24, but its already pretty sharp.

Was just hoping I'm not the only one that has had this problem.

ps forgive any typos or bad sentences if there are. I keep falling asleep (twice now lol) while writing this and am far too tired to proofread. Its been a long day and some good pictures were taken. Thanks pentaxians.

11-17-2012, 12:06 AM   #2
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I find that the focus indicator for focus confirmation with mf lenses is unreliable in the extreme. Actually, it's at its best with my 50mm lenses, and it's hopeless with (for example) 28mm and 135mm lenses. This rules out catch-in focus as well, of course. My camera is a K-m (aka K2000), but I'm not surprised to hear that other (more advanced) cameras behave similarly.

I don't have an easy explanation: at the wide end, I find the focus indication too "sloppy", which is bound to be problematic, but for longer lenses there's very precise indication - it's just that there will usually be very significant front or back focus, depending on the actual lens in use.

I don't think there's an easy solution - mf lenses, for all their attractive qualities, are at a great disadvantage compared with modern autofocus lenses, in my opinion.
11-17-2012, 01:09 AM   #3
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The focus confirmation has a range in which it will think the subject is in focus. Try this: pick a subject and then use CIF from infinity and then from the MFD. You'll probably find that in the first case it's slightly back-focused and vice-versa.
11-17-2012, 02:00 AM   #4
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QuoteOriginally posted by zosxavius Quote
Still its so frustrating to go back and find the wall behind your subject perfectly in focus when you know it confirmed lock on their face or their jacket.
Just to rule out other potential factors,
are you sure that the focus "point" area
wasn't large enough to include a nice, contrasty piece of wall?

11-17-2012, 02:11 AM   #5
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QuoteOriginally posted by m42man Quote
I find that the focus indicator for focus confirmation with mf lenses is unreliable in the extreme.

I don't think there's an easy solution - mf lenses, for all their attractive qualities, are at a great disadvantage compared with modern autofocus lenses, in my opinion.
I've found it helpful (with lots of MF lenses on the K-x)
to mimic the way the AF works when you're focussing manually.

In other words, move the focus back and forth to confirm the focus
(or move yourself relative to the subject),
rather than just shooting as soon as the green hexagon appears.

Often, the green hexagon will disappear when you start to do this,
and will only reappear definitively at a different setting.
11-17-2012, 02:15 AM   #6
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QuoteOriginally posted by zosxavius Quote
In street you need the damn shot. You don't have time to miss. So with MF, I use catch in focus and hold the AF button if I need to recompose. I'm typically shooting around f2-f2.8 these days in low light and when I want a fairly shallow DOF, so missing doesn't leave you with much to take home with.
In those situations, focus bracketing may be the best technique,
but of course it won't work with CIF.
11-17-2012, 02:23 AM - 1 Like   #7
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QuoteOriginally posted by zosxavius Quote
I hate that its always the wall immediately behind someone. Has anyone else noticed this? In good light these lenses are usually tack sharp when in focus and give reliable focus locks.
Low light = low contrast = any AF is going to be challenging (except for the K-5II).

I get the most reliable low-light AF using centre-spot focus only. And as some have suggested, the AF can usually be 'trained' to a degree when the lighting gets bad to work more effectively.

Also note this from Nikon:


Also - shooting street at almost wide-open
What happened to the old adage 'f8 and get close' ?
11-17-2012, 02:34 AM - 1 Like   #8
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QuoteOriginally posted by rawr Quote
Also - shooting street at almost wide-open
What happened to the old adage 'f8 and get close' ?
OP did say "low light."

11-17-2012, 08:41 AM   #9
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Here are a couple of points to consider: first the center AF point is rather larger, and depending on your distance may include part of the wall. It is not really a 'point' at all but an area within which the camera will find something of high contrast to lock onto. If that is the wall behind the subject then you get a bad image.

Second, as noted above the CIF or focus confirmation does have a little slop in it. I find that on some lenses it is better to work from MFD out and on others from infinity back to get best results. Try that in a test situation.

Third, have you tried fine tuning the AF? With the k-5 you can set multiple lenses, though they need to be modern enough to have ID chip them. Not 100% sure how it handles older lenses but you could adjust the global setting at least temporarily to run some tests.
11-17-2012, 09:13 AM   #10
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Thanks everyone. I don't think I need a global tuning on the k-5. Lenses lock easily in most lighting situations. I've found in extremely low light its kind of best to hold my position (moving back and forth moves focus) shoot, chk focus, readjust and work from there. Agreed that the center point is more of an area from experience. I think that was my original conclusion, but I found it odd that faster lenses seemed to have more slop. I do understand that focus lock in MF is kind of a hack in a way, and yeah I do try to rack focus like the AF would usually starting near and moving to far. I know its calibrated properly because the walls are nice and sharp.

I probably need to move up to an FA50 1.4. With AF you at least get the speed to have it lock quickly and take a few quick snaps. Also you gain 10 more focus points to work from, MF really slows you down too much for street without a good split prism focusing screen and even that is as only accurate as your eyes can see in a tiny window. With liveview I can easily see the plane of focus and have actually used that in really challenging situations at least to give me an idea of where focus is. I can see the allure of leica for this type of photography. Being able to focus on a patch would highly useful. Its been making me want an x-pro, but I understand they don't have shake reduction, manual focus on native lenses or focus peaking of any sort for manual lenses. So that's kind of a deal breaker. I keep looking at the Q and wanting it, but honestly its not anywhere near DSLR quality and my days with point and shoots are long over.

I'm going to try to refine my technique some more in the mean time. I agree that F8 and get close is good advice. That's not helping me much when the sun starts to go down, which it is doing very early here now. I'm shooting primarily at 1600 in these conditions. I don't like to use 3200 unless I really have to. 1600 with a fast lens gives you plenty of shutter speed unless your subject is lit by a single bulb or something. Even then I can work within 1/20 or so and watch for my subject to stop momentarily.
12-03-2012, 07:12 PM   #11
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Ok. Dunno if anyone cares but I've learned to rely on my eye more for visual confirmation. For really tight situations with a flash being used, I'm not above using live view as I can see everything much better and nail it right away. I did a recent shoot in bad lighting and was missing a lot, so I'm going to start using some head mounted lights to lock in focus when I have to. Obviously I can't do this all the time or look like a total retard, but I kind of wish you could trigger the focus assist light somehow. It only kicks in on af, but would be really useful. Anyways an AF lens would probably give me more consistent if at least predictable results I think. The k-5 certainly seems better in low light. Oddly the k-7 seems to struggle less with tungsten. I wonder why that is. Their focusing systems are pretty darn similar.
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