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04-03-2011, 02:43 PM   #1
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Focus Adjustments on a K20 with manual lenses

Last Thanksgiving, I had an opportunity to acquire a Contax Carl Zeiss 28mm/f2.8 Distagon *T lens (a C/Y mount) at a pretty good price, in mint condition. I purchased the K mount conversion kit from Leitax (in Spain) - and it was caught in US Customs for "awhile" during the holidays. [David at Leitax has tracking numbers on everything he sends - wonderful service]. It finally arrived, and then I sent it off to Eric to be modified. I guess I could have done it myself, but I considered it too nice a lens for me to botch the job on. Eric did a great job, and now I have it back.

Normally, it would not be a real fair comparison, but I pulled out the 31 Ltd and shot several comparison images. The difference in focal length is much more noticeable than I expected. I did a set of distance shots and then a set of close in shots, into the center of a spider cactus plant.

I have some back focusing on the 28 @ f2.8. I do have to say the 31 is absolutely perfect - and comparing anything with it, you see the differences immediately. I found and printed the target along with the focus adjustment instruction page from the K20 manual. This afternoon in the kitchen under the skylight (natural sunlight), I set everything up and it looks like I have about 5mm of backfocus - that is, when I get everything in focus. I do have to say, that this is more difficult than an eye examination - I really miss my split prism from my old Spotmatic. [Note - I think that a Katzeye is in my future.]

I have a couple of questions - since I have never had to apply a focus adjustment before.
  • Since this is a manual lens (K lens equivalent), the focus adjustment I specify to this one lens, will it be applied to all manual lenses or just to all 28mm manual lenses? (I also have a K 28/3.5 Shift and an A50/1.7.)
  • Is it any better to use the Pentax Remote Assistant Tether utility in this effort to get (and verify) the right focus? Or would Live View be better?
  • I am finding that it is pretty difficult not to move, bump, jiggle or jostle anything - so as to ensure the ability to get repetitive results.
My intent for the lens is low ambient light landscapes, and stitching panos with it. Thus, I am going to have some latitude with it in terms of focusing in actual use (to some degree). Also, I was very interested in seeing how one of these lenses (Zeiss) compared to the 31 in terms of sharpness, color, contrast, shadows and overall IQ. I haven't come to any conclusions as of yet - other than its a very nice lens - substantially smaller than the 31, with the same build quality - all metal and glass.



04-03-2011, 02:49 PM   #2
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QuoteOriginally posted by interested_observer Quote
Last Thanksgiving, I had an opportunity to acquire a Contax Carl Zeiss 28mm/f2.8 Distagon *T lens (a C/Y mount) at a pretty good price, in mint condition. I purchased the K mount conversion kit from Leitax (in Spain) - and it was caught in US Customs for "awhile" during the holidays. [David at Leitax has tracking numbers on everything he sends - wonderful service]. It finally arrived, and then I sent it off to Eric to be modified. I guess I could have done it myself, but I considered it too nice a lens for me to botch the job on. Eric did a great job, and now I have it back.

Normally, it would not be a real fair comparison, but I pulled out the 31 Ltd and shot several comparison images. The difference in focal length is much more noticeable than I expected. I did a set of distance shots and then a set of close in shots, into the center of a spider cactus plant.

I have some back focusing on the 28 @ f2.8. I do have to say the 31 is absolutely perfect - and comparing anything with it, you see the differences immediately. I found and printed the target along with the focus adjustment instruction page from the K20 manual. This afternoon in the kitchen under the skylight (natural sunlight), I set everything up and it looks like I have about 5mm of backfocus - that is, when I get everything in focus. I do have to say, that this is more difficult than an eye examination - I really miss my split prism from my old Spotmatic. [Note - I think that a Katzeye is in my future.]

I have a couple of questions - since I have never had to apply a focus adjustment before.
  • Since this is a manual lens (K lens equivalent), the focus adjustment I specify to this one lens, will it be applied to all manual lenses or just to all 28mm manual lenses? (I also have a K 28/3.5 Shift and an A50/1.7.)
  • Is it any better to use the Pentax Remote Assistant Tether utility in this effort to get (and verify) the right focus? Or would Live View be better?
  • I am finding that it is pretty difficult not to move, bump, jiggle or jostle anything - so as to ensure the ability to get repetitive results.
My intent for the lens is low ambient light landscapes, and stitching panos with it. Thus, I am going to have some latitude with it in terms of focusing in actual use (to some degree). Also, I was very interested in seeing how one of these lenses (Zeiss) compared to the 31 in terms of sharpness, color, contrast, shadows and overall IQ. I haven't come to any conclusions as of yet - other than its a very nice lens - substantially smaller than the 31, with the same build quality - all metal and glass.

I'm having trouble telling what you mean. There is no focus adjustment for manual lenses. The AF Adjustments are meant for fine tuning inaccuracies in autofocus for autofocus lenses.

As far as I can tell you may be suffering from one of two problems:

1) you are using the focus indicator beep or light through the viewfinder to tell you when your lens is in focus. The reality with this is it's not very precise, and has a bit of a margin on either side of actual focus.
2) your viewfinder screen is poorly aligned and what appears to be in focus through your viewfinder is not when the shot is actually taken. If this is the case you may need shunts for the screen to adjust its alignment.
04-03-2011, 02:59 PM   #3
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Check your diopter as well. BTW, the difference in focal length between a 31mm and a 28mm is marginal- only a few degrees FOV.

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04-03-2011, 03:34 PM   #4
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QuoteOriginally posted by v5planet Quote
I'm having trouble telling what you mean. There is no focus adjustment for manual lenses. The AF Adjustments are meant for fine tuning inaccuracies in autofocus for autofocus lenses.

As far as I can tell you may be suffering from one of two problems:

1) you are using the focus indicator beep or light through the viewfinder to tell you when your lens is in focus. The reality with this is it's not very precise, and has a bit of a margin on either side of actual focus.
2) your viewfinder screen is poorly aligned and what appears to be in focus through your viewfinder is not when the shot is actually taken. If this is the case you may need shunts for the screen to adjust its alignment.
Thanks for the correction and suggestions. Yes, I was using the first as an assist. The second is certainly a possibility. The third was - I am sooo far sighted, that I did not realize my glasses were smudged and dirty. ... Obviously, I was thinking AF and FF/BF on the manual focusing lens. The bottom line is that the 31 auto focuses much better (more accurately and consistently) than I can manual focus the 28. Re-examining my results again on the 28, when I focus it, it is exceptionally sharp, when its not in focus - its not. At f11, f16 and f22 depth of field is wonderful, and even the salt crystals my wife left on the kitchen counter, were photographed well. All in all it was an interesting exercise.

QuoteOriginally posted by Adam Quote
Check your diopter as well. BTW, the difference in focal length between a 31mm and a 28mm is marginal- only a few degrees FOV.
I was using the side by side viewing option in Bibble Pro, so even the couple of degrees difference was noticeable. That said, my diopter was not where I usually have it set. Thanks!

My wife wanted me to move my camera out of the kitchen anyway and to get to the market for some shopping, so she would not have to go out in the heat....



04-03-2011, 03:40 PM   #5
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QuoteOriginally posted by interested_observer Quote
Thanks for the correction and suggestions. Yes, I was using the first as an assist. The second is certainly a possibility. The third was - I am sooo far sighted, that I did not realize my glasses were smudged and dirty. ... Obviously, I was thinking AF and FF/BF on the manual focusing lens. The bottom line is that the 31 auto focuses much better (more accurately and consistently) than I can manual focus the 28. Re-examining my results again on the 28, when I focus it, it is exceptionally sharp, when its not in focus - its not. At f11, f16 and f22 depth of field is wonderful, and even the salt crystals my wife left on the kitchen counter, were photographed well. All in all it was an interesting exercise.



I was using the side by side viewing option in Bibble Pro, so even the couple of degrees difference was noticeable. That said, my diopter was not where I usually have it set. Thanks!

My wife wanted me to move my camera out of the kitchen anyway and to get to the market for some shopping, so she would not have to go out in the heat....

I recently had my K-7 sent in for backfocus problems -- I first noticed it with one of my AF lenses, but the focus indicator when I was manually focusing my Takumars also seemed to consistently be pretty far behind the real focus point. This resolved after the body returned to me and even though the indicators have some wiggle room on either side of razor focus, it's not off in one direction nearly as much as it used to be. I was going to suggest that as another possibility for yoru camera, but if your 31 is AF'ing fine with no adjustments, it's likely just a technique thing, possibly mixed with some slight misalignments either of the focusing screen or the diopter.

As a test, why not try manually focusing your 31 and see how it compares to manually focusing your 28?
04-03-2011, 03:48 PM   #6
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Actually yes, MF'ing the 31 could be a very good test to see if this problem is real, since it has a considerably wider max aperture and slightly longer FL than your 28. In other words, the depth of field will be considerably narrower and therefore MUCH more readily show focusing inaccuracies.
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