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06-24-2010, 07:46 AM   #1
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Front Focus issue, a wierd one. Only with manual lenses.

Ok....bare with me on this! I have an Cosina 55 1,2 wich i have used and been happy with! Just recenty i got myself an Pentax A-50 1,2.

Curious as i am i wanted to check out the differences in sharpnes, bokeh, rendition etc etc, between the two of them. So i did something i never do with manual lenses. I hooked the camera to an tripod, used various items such as flowers, keys and also an focus test chart.

Sidenote: I'd never thought about doing this before, because i trusted manual lenses to focus correctly. And to be honest....is hard to nail focus handheld @ f1,2

But now i did test it, and to my surprise i had front focus with both Pentax A-50 and Cosina 55. I could'nt believe it. I have perfect eyesight. I can clearly se how i get focus in the VF, on the "focus" line on the chart, and the camera approves with the hex.symbol.

Yes. i've done everything by the book with the tripod and camera.

Strange thing 1: If i switch over to LV, no problem!
Strange thing 2: I have no problems with my other lenses, 16-45, 55-300, D-FA 100 macro. If i manually focus them, i get the focus where i see it in the VF.

So....whats going on? Has my camera a front focus issue with manual lenses only? I just cant see what could cause this. Why front focus with manual lenses, and not with the others?

06-24-2010, 08:37 AM   #2
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It probably has to do with the speed, all your other lenses are F4 etc.
The viewfinders of AF camera's ,and APS-C dslr's even more, suck, even with a better (microprism etc) screen.
These fast lenses suffer probably from spherical aberation, which means a focus shift as function of aperture.
06-24-2010, 08:37 AM   #3
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Are you using a split focus screen ?
06-24-2010, 09:01 AM   #4
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QuoteOriginally posted by glasbak Quote
It probably has to do with the speed, all your other lenses are F4 etc.
The viewfinders of AF camera's ,and APS-C dslr's even more, suck, even with a better (microprism etc) screen.
Had no idea about that
QuoteOriginally posted by robbiec Quote
Are you using a split focus screen ?
Nope.

As it is now, when i use the Pentax A50 1,2 (or the Cosina for that matter) lens for "boke shots" (my word for closeups with distant background) i often miss focus.

I will try to explain:
When i clearly see that i'm in focus in the VF and i get the confirmtion from the camera. If i then turn the focus ring just a litle more towards infinity, and precisely when i lose focus and the confirmation light turns of: THEN the picture will come out a "in focus".


Last edited by the swede; 06-24-2010 at 09:09 AM.
06-24-2010, 09:10 AM   #5
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Ok, so that rules one possible variable out
Glasbak is correct in that your other lenses will not be so critical with focusing due to their slower aperture compared to the f1.2s.
Have you tried the focus adjustment option?

Previous thread on this:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/84286-other-le...stable-k7.html

Nice set of glass by the way!

Last edited by robbiec; 06-24-2010 at 09:20 AM.
06-24-2010, 09:32 AM   #6
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QuoteOriginally posted by robbiec Quote
Have you tried the focus adjustment option?
Nice set of glass by the way!
Yes i tried the AF adjusment, but i didnt sem to make any difference

Thanks! I love my glass. Cudos for the price vs quality of 16-45 and 55-300!
No doubt the best combo around, and might as well cost some more imo.

Edit: I tested the AF adjustment once again. I can get the hexagon to move to the right place with an adjusment of -10. That really helps, but means that i'll have to set all my lenses manually to "0" in AF adjustment, wich isnt a biggie. Could as well adjst for the A50 when i put it on.

Last edited by the swede; 06-24-2010 at 10:52 AM.
06-24-2010, 04:14 PM   #7
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If you're going to use tripod shots you might as well take advantage of the live-view. It's more useful than it seems at first and you wont miss the focus that easily.
06-24-2010, 05:02 PM   #8
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Swede did you try adjusting the diopter slightly?

06-25-2010, 01:15 AM   #9
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I noticed that my M50 f/1.7 on my K20d suffers from back focus. I am using a split focus screen, albeit one of the cheap ones. My theory is that the focus screen isn't perfectly aligned. The mount for the focus screen strikes me as a bit fiddly. I'm not sure but I'd think that even very slight variations in the distance between lens/mirror and focus screen cause variations in what appears to be in focus and what not. I've often wondered why the mount for the focus screen isn't more solid or offers better assurance the screen is correctly installed.

IMHO, focus confirmation is not accurate enough for f/1.7, let alone f/1.2.

Also, correct me if I'm wrong, but I'd think AF adjustment has little to do with what you see in the VF and whether or not it seems to be in focus.
06-25-2010, 02:33 AM   #10
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QuoteOriginally posted by summonbaka Quote
If you're going to use tripod shots you might as well take advantage of the live-view. It's more useful than it seems at first and you wont miss the focus that easily.
Yes, i agree with you on that. I was testing with tripod to be sure of consistency.
QuoteOriginally posted by Rico Quote
Swede did you try adjusting the diopter slightly?
Diopter is fine, i set mine 1 step from right.
QuoteOriginally posted by Scrib Quote
IMHO, focus confirmation is not accurate enough for f/1.7, let alone f/1.2.
Also, correct me if I'm wrong, but I'd think AF adjustment has little to do with what you see in the VF and whether or not it seems to be in focus.
Yes, but setting the AF adjustent so that it will confirm the correct focus (regardless of what you see in the VF) can help to "nail" the shot faster.

However after fiddling around and finding that AF adjustent helps, i have turned it off, and trying to anticipate focus seems easier.

I now know that when the camera thinks im in focus i turn a tad more and just as i "loose" focus and the hexagon turns off, im spot on!

Its easier when close to the object, but further away its all about luck anyway
06-25-2010, 05:15 AM   #11
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I have a similar problem. My A 50mm f/1.2 front focuses with focus confirmation on my k7 consistently and reliably.

This does not occur with my FA 50mm f/1.4

Furthermore the FA 50mm focus accuracy is the same for contrast focusing and phase detection.

However, when you put the A 50mm on the camera, the only way to get a clear shot is to use liveview and zoom in a bit.

Doesn't make a bit of sense to me.
06-25-2010, 05:17 AM   #12
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QuoteOriginally posted by Scrib Quote

IMHO, focus confirmation is not accurate enough for f/1.7, let alone f/1.2.


My K10D could use confirmation focus just fine at f/1.2
06-25-2010, 08:33 AM - 1 Like   #13
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This issue is because the focusing screen isn't sitting precisely and you'll need to replace the shim holding the screen. You don't see the problem when using an AF lens because the AF system doesn't rely on the focusing screen to determine focus. The AF module sits at the bottom of the mirror box. Now when you're using manual focus, you're using your eyes to eyeball focus and not relying on the AF confirmation readout, so if the focusing screen is either sitting a little higher or lower than it should, you'll mis-focus for sure.
06-25-2010, 10:11 AM   #14
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Creampuff nailed it. Until I bought my Cosina and really started playing with manual focus, things with my (slower) AF lenses were "good enough". I had to trial-and-error different focus screen shims, but now my AF and MF correlate perfectly. And I can use the Cosina very well with Catch-In-Focus now too.

P.S. I'm also using a split-screen focusing screen.
06-25-2010, 12:35 PM   #15
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I'll agree with NeverSatisfied, creampuff, and Scrib. My keeper rate increased DRAMATICALLY after sitting down at my desk for a couple of hours, to shim and re-shim and adjust and re-adjust the focusing screen until it was EXACTLY right with my Cosina 55/1.2.

I used layers of blue (painter's) masking tape along the side edges of the focus screen -- I think it ended up being 3 or 4 layers. I made very thin strips by putting layers of the tape down on wax paper, cutting very narrow (1mm maybe) strips through the entire stack with a razor blade, then peeling off the wax paper backing to attach the tape to the TOP (prism-facing) side of the screen. The tape is outside the field of view in the viewfinder -- totally invisible.

It was kind of a frustrating project, because doing this lets dust into the focus screen area, and every time you handle the screen you risk scratching it or getting fingerprints on it. (But with an inexpensive eBay split-prism screen, that's not a huge deal).

But it was SO worth it. I'm very pleased now.

I don't think too much effort goes into aligning the focus screen at the factory, because (a) the autofocus system doesn't need it to be aligned, and (b) the prismless screen with microlenses doesn't show critical focus very well anyway.
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