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04-20-2009, 08:09 AM   #1
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Front focus/ back focus on Manual lens. Possible?

Hi guys,
I would like to pose this question because I missed quite a number of shots using manual focus. I thought they looked correctly focussed on the viewfinder but upon viewing the image it back focussed.

Most likely its my lack of skill in manual focussing but could there be another reason?

04-20-2009, 08:33 AM   #2
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If you posted the picture here it would make it easier to form an opinion. Were you using the focus lock indicator when manually focusing?
04-20-2009, 09:00 AM   #3
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What you are describing is possible if the focus screen is not properly positioned.

However, before anyone jumps to conclusions, it would be good to get some additional information:
  • What lens?
  • Stock focus screen?
  • How are you assessing focus?
It should be noted that focus with the stock Pentax screen (LF-80) is not particularly precise. The apparent DOF in the viewfinder is never less than what you would get at about f/4 regardless of the lens used. As a result, fine focus control is often difficult to attain.

Steve

Last edited by stevebrot; 04-20-2009 at 01:01 PM.
04-20-2009, 11:18 AM   #4
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To expland on what Steve wrote:

It is indeed possible if the focus screen is misaligned, and if that turns out to be the problem, it is possible to correct backfocus using shims between the screen and prism.

However, it is very important to note that the DOF shown in the viewfinder will be a lie for apertures larger than around f/3.5 or so. If you are trying to focus your 50/1.4 manually, DOF is *much* narrow than what appears in the viewfinder. That's easy enough to test by looking at a sheet of printer matter, noticing how many lines of text look in focus in the viewfinder, than snapping the picture and seeing how many are actually in focus. That's just a side effect of the design of most modern focus screens. What this means is that there will *always* portions of the image that look in focus in the viewfinder but aren't in the image. As long as areas don't show up in focus in the image that were *not* in focus in the viewfinder, there is no "problem" - it's just the DOF issue.

Aside from replacing the screen with one like the Katz Eye that shows the actual focus center of the focus zone more accurately, all you can really do is practice anticipating *which* areas that appear in focus in the viewfinder will end up not in focus in the actul picture. To some extent the focus confirmation indicator can help with this, but I've become better at just eyeballing it - knowing the area toward the *back* of the apparently-in-focus zone is the area I'm going to lose on my camera. So I try to position my subject as far forward withint the in-focus zone as I can, or else I may find my shot front-focused. Sounds like it might be the other way around for you, but that's why you test for this and practice working with it.

04-20-2009, 03:25 PM   #5
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QuoteOriginally posted by Marc Sabatella Quote
If you are trying to focus your 50/1.4 manually, DOF is *much* narrow than what appears in the viewfinder. That's easy enough to test by looking at a sheet of printer matter, noticing how many lines of text look in focus in the viewfinder, than snapping the picture and seeing how many are actually in focus.
...or even easier, set your f/1.4 lens to f/2 and then look through the viewfinder as you press the green or AE-L button to stop the lens down. Note that the viewfinder doesn't get any dimmer and the out-of-focus areas don't change their appearance.
04-20-2009, 07:41 PM   #6
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Thanks for the replies guys, I never knew that the DOF shown in the viewfinder is not accurate bigger than f/3.5-4.
  • What lens?
    The lens that I found hardest to get the right focus is the SMC Tak 50/1.4
  • Stock focus screen?
    Using Stock focus screen
  • How are you assessing focus?
    I just observe in the viewfinder, did not use the focus indicator.

Oh, one more thing. I adjusted the diopter near the viewfinder area to correct my shortsightedness by shifting it to the right. Does this affect the focus?
So sorry, I can't show any pictures cause I'm in office now. Most of them are deleted anyway.




QuoteOriginally posted by stevebrot Quote
What you are describing is possible if the focus screen is not properly positioned.

However, before anyone jumps to conclusions, it would be good to get some additional information:
  • What lens?
  • Stock focus screen?
  • How are you assessing focus?
It should be noted that focus with the stock Pentax screen (LF-80) is not particularly precise. The apparent DOF in the viewfinder is never less than what you would get at about f/4 regardless of the lens used. As a result, fine focus control is often difficult to attain.

Steve
04-20-2009, 07:50 PM   #7
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QuoteOriginally posted by WangJianWei Quote
......

Oh, one more thing. I adjusted the diopter near the viewfinder area to correct my shortsightedness by shifting it to the right. Does this affect the focus?....
Yes, the diopter setting can affect your perception of focus. The diopter adjustment is correct when the writing you see in the viewfinder is sharpest.

Manually focusing with the original focusing screen is difficult. I find it preferable to rely on the focus indicator or to purchase a split-prism focusing screen.

Iowa Dave
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