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01-05-2012, 01:32 PM   #1
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Catch in focus accuracy? [Solved]

Got my pentax-M 50mm f1.7 lens today and did some test shot.

While catch in focus is a really great and convenience way to MF, I experience some trouble with it.

Here is a simple scene set up:



The focus point is in the middle, the number "2.5"
f1.7
iso 400
1/200s

situation 1: viewfinder, CIF disable, green dot confirmation is on, take the shot;
situation 2: viewfinder, CIF enable, slowly turn focus ring, green dot is on and shutter auto fired;
situation 3: live view, enlarge resolution 2x by pressing "info" button twice, focus and take the shot.

100% crop:


Obviously the shot using live view is in focus and a lot sharper.

The 2 shots from situation 1 & 2 are soft throughout the image (or saying none of the subject is in focus).
So I suspect the camera is slightly front focus?
Or is it other issue?


Last edited by telly0050; 01-05-2012 at 05:41 PM.
01-05-2012, 01:38 PM   #2
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Hmm what kind of light source?

I have a had some frontfocus with my K-5 and an old naked lightbulb shining over my head in which case LV or MF was much better. otherwise not much at all compared to the K-r I had.

If that is not it maybe you need to dial in some AF adjust if you are using the AF lights for focus confirmation?
01-05-2012, 01:38 PM   #3
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We are discussing this issue:
https://www.pentaxforums.com/forums/pentax-k-5-forum/169407-pentax-k-5-auto-focus-accuracy.html

I found my k5 is worse than my k20.
01-05-2012, 01:39 PM   #4
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Interesting test.

Focus to the right on the word "ingredients" (or maybe even further than that like "papas") and test again. If the word "ingredients" is out of focus, but the 2.5 is in focus, then you have a slight front focus issue.

01-05-2012, 01:44 PM   #5
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just did some more test with 4 batteries.

Confirmed that the camera seems slightly front focused....
01-05-2012, 01:52 PM   #6
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Unfortunately, CIF will never be totally accurate due to the movement. That doesn't change the fact that the camera AF can also be out of focus. Though I do think it compounds the issue further.

I ran some tests with our K-5's and K20's about a year ago on this and found the results to be quite difficult to substantiate. And so I chalked-up CIF as a less than ideal but still helpful for tough shooting conditions with MF lenses.

PS. the wider the aperture, the sharper the accuracy falloff.

Last edited by JohnBee; 01-06-2012 at 07:56 AM.
01-05-2012, 02:53 PM   #7
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telly... I've observed the same performance with CIF on my K-r. I've also noticed that the focus-lock confirmation light (which I only keep on center) often appears when I wouldn't expect it. I'm not sure if this is a front-focus issue or if the focus criteria are just "loose". While CIF is a great feature, especially given the rather limited viewfinder, I find myself using your method #3--zoomed LiveView--whenever using my M lens.
01-05-2012, 05:40 PM - 1 Like   #8
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ofer4 i just solved my problem, maybe you can follow what i did too.

inspired by lensalign (they sell a few pieces of printed cardboard for $80?? ridiculous...), i created my own.

here is the set up:



a box that i DIY for simple and small object shots,
a ruler,
a binder clip,
and a random business card with good contrast.

actually.. any white background will do, and anything that gives good contrast for the camera to focus is also good.

then place the camera on a tripod, straight and level to the business card, and start taking shots.



after messing on AF fine adjustment for a bit, a -6 value finally gives me the best result of this lens.







a few shots after the adjustment... all taken with CIF.
i would say the focus is pretty dead on now, since it gives the same sharpness as using live view focus.

also make sure you turn the focus ring very slowly when it gets near focus distance.
the slower the shutter speed is, the slower your have to turn it.


Last edited by telly0050; 01-05-2012 at 05:46 PM.
01-05-2012, 05:59 PM   #9
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I came in here to see the solution. I adore your setup. I have stolen it now.
01-05-2012, 06:26 PM   #10
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I'll be trying this on my k-r later. Thanks for the solution. And is that a hood for your DA 35?
01-05-2012, 06:30 PM   #11
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Looks like a rubber expandable hood - same as what I use.
01-05-2012, 06:32 PM   #12
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feel free to steal the setup idea, i hope it can help anyone that suffers ff/bf issue.

and yes, that is a rubber hood for my 35mm.
01-05-2012, 06:42 PM   #13
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Can I get those at amazon? a link would be awesome, and thanks (And sorry for jacking your tread)
01-05-2012, 06:46 PM   #14
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i got it from adorama:

RLH49 Adorama 49mm Rubber Lens Hood

same thing can be found on ebay for cheaper:

Rubber Lens Hood 49mm 52mm 55mm 58mm 62mm 67mm 72mm - Your choice - | eBay

35mm uses 49mm filter, so just search "49mm rubber hood" and you will get many result.
01-06-2012, 01:31 AM   #15
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AF fine adj all AF lenses on all bodies. note the parameters, cheers
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