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11-13-2011, 09:15 AM   #1
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K5 with A lens Focus Issue

I've been using my K5 with a newly acquired A f1.2 lens and found couldn't rely on the AF confirmation (red light and beep) for focus accuracy. Anyone please comment if to trust your eye or AF confirmation? Also, has anyone tried Focus Trap/Catch-in Focus on K5, I couldn't seem to able to get the camera to fire automatically once focus is in.


Last edited by mxlinn; 11-14-2011 at 08:27 PM.
11-13-2011, 09:45 AM   #2
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QuoteQuote:
Inherent focus accuracy is not high in general for fast f/1.4 lenses because contrast is quite low wide open, and it's contrast which helps the autofocus system achieve accurate focus.
diglloyd - Ask Lloyd Chambers - Focus calibration for ?fast? lenses
11-13-2011, 10:11 AM   #3
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Focus confirmation is really nothing more than an approximation when focusing manually. Your best bet is to get a split-screen focusing screen to make it easy to tell when stuff is in focus, IMO.

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11-13-2011, 11:00 AM   #4
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QuoteOriginally posted by mxlinn Quote
I've been using my K5 with a newly acquired A f1.2 lens and found couldn't rely on the AF confirmation (red light and beep) for focus accuracy. Anyone please comment if to trust your eye or AF confirmation? Also, has anyone tried Focus Tarp/Catch-in Focus on K5, I couldn't seem to able to get the camera to fire automatically once focus is in.
I've found that I had to dial in some global AF adjustment to get CIF working with some of my lenses. Of course, when I go back to an AF lens, I have to remove that adjustment. Usually though, for something critical I rely on the screen and DOF Preview (stop the lens down to about f4) for greatest accuracy.

11-13-2011, 11:04 AM   #5
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QuoteOriginally posted by mxlinn Quote
Also, has anyone tried Focus Tarp/Catch-in Focus on K5, I couldn't seem to able to get the camera to fire automatically once focus is in.
Yes, but so far only with a manual lens. I don't know if it works the same way with an A lens. Anyway, make sure you're set on AF-S, and have it's mode set to shutter priority. (I think...)
11-13-2011, 01:45 PM   #6
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QuoteOriginally posted by pentup Quote
Yes, but so far only with a manual lens. I don't know if it works the same way with an A lens. Anyway, make sure you're set on AF-S, and have it's mode set to shutter priority. (I think...)
CIF will work in any mode but has to be on AF-S. The only thing you're doing with CIF is replacing the auto focus drive with your hand or subject motion. I use it all the time with macro lenses when the wind is blowing. Get it close, use a high shutter speed, and let the wind blow the subject (usually a plant) into focus.

11-13-2011, 03:06 PM   #7
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QuoteOriginally posted by JeffJS Quote
CIF will work in any mode but has to be on AF-S.
Just to clarify, I wasn't referring to modes on the mode dial; I meant the menu options for AF-S needed to be set on focus priority, not release priority. (I mistakenly referred to Focus-priority as "shutter priority.")
Anyway, it's C menu, tab 3 item 15, option 1
11-13-2011, 03:49 PM   #8
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The K-5 has a faulty AF module or algorithms period. Wait until you get a fast lens and want to crop heavily in fading light. Other than that a great camera.

11-14-2011, 01:13 PM   #9
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I discovered the same thing with my "A" 35 f2 today. I have no AF lenses and only the 25 and Kiron 105 f2 with the "A" designation. The focus indicator is not accurate at all as it beeps after I've gone past the focus point in either direction. If I knew this was an issue, I would've stayed with my LX and not spent the money on a K-5(always wanted a camera w'focus confirmation as the eyes were getting older).

Last edited by tabl10s; 11-14-2011 at 01:27 PM.
11-14-2011, 01:35 PM   #10
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AF microadjustment ('apply all') might be worth trying.
11-14-2011, 04:50 PM   #11
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QuoteOriginally posted by tabl10s Quote
The focus indicator is not accurate at all as it beeps after I've gone past the focus point in either direction.
Even if it beeped at precisely the right instant, would anyone be able to guarantee stopping fast enough to stay in focus at that point? I think it's best to turn the beep off, and only view the green hexagon as being a sort of stop sign, and adjust from there. It's a given that it's only approximate; the camera can't account for every user's reaction time.
11-14-2011, 07:14 PM   #12
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Thank you all for your responses so far...
11-14-2011, 08:02 PM   #13
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This is not an issue with the camera (K5) as much as it is an issue with using AF with fast lenses. Simply put, the AF system, regardless of brand, is not up to the task when DOF is very thin. This includes catch-in-focus as well as focus confirm. Consider:
  • AF precision is on the order of +/- a few millimeters. This is true regardless of how well-calibrated the camera is.
  • DOF at portrait distances with a 50mm lens at f/1.2 at a reasonable final image size is also a matter of a few millimeters of acceptably sharp focus
  • Actual desired focus point is difficult for the AF system to intuit. Translation...the camera has no way of knowing that you want the leading surface of the model's cheek to be the point of focus. Heck! The target zone for focus often covers several potentially good points for focus. The camera will choose the area that snaps to highest contrast first. So what if it is the trailing line of the lower eyelid!
The bottom line is that consistent fine focus with fast lenses is not a reasonable expectation from an AF system. The alternatives are a high quality aftermarket focus screen with focus aids or focus from live-view on the LCD using a magnifying loupe.

All that being said, I can reliably predict a chorus of examples from people who use catch-in-focus/focus confirm on a regular basis with fast lenses and/or for macro photography. A lot depends on the subject, the intended point of focus, the degree of enlargement, and the photographer's tolerance for missed focus. A good example would be doing catch-in-focus at 1:1 macro of an approaching insect, chances are that the AF system will do a credible job of firing just after the leading edge of the bug face passes through the plain of focus. The subject is high contrast and the point of focus is acceptable. Try the same thing panning in on a collection of white marbles in strong diffuse light.


Steve

(BTW...I regularly shoot my FA 77/1.8 wide open using AF. Works great as long as I don't push the envelope and am willing to accept the camera's judgement...)
11-15-2011, 12:53 AM   #14
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QuoteOriginally posted by pentup Quote
Just to clarify, I wasn't referring to modes on the mode dial; I meant the menu options for AF-S needed to be set on focus priority, not release priority. (I mistakenly referred to Focus-priority as "shutter priority.")
Anyway, it's C menu, tab 3 item 15, option 1
I knew that was an option with AF-C but had no idea it was with AF-S as well

11-18-2011, 10:34 AM   #15
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Check this thread

I had the same problem with my A*85mm f1.4. I read this thread, got the new screen, and... taramtaram... problem solved!

https://www.pentaxforums.com/forums/pentax-k-5-forum/142241-best-screen-ever-...anon-ee-s.html

It's an extensive thread, but the one that brought new life to my fast lenses like the 85mm, the FA 50mm f1.4 and the FA 35mm f2.

Kjell
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