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12-30-2011, 07:22 PM   #1
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Bad focus problem with Kr + Super Takumar 1:1.4 50mm

Hi,

First, I don't know if "bad" means "front" or "back" focus problem (according to their definition, my MF lens wouldn't be case.) After many tests, I may say to myself that my combination "Kr + Super Takumar 1:1.4 50mm" has a "front-focus" problem. My lens was disassembled/assembled as you can see in this thread https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/167889-how-use...-1-4-50mm.html .

My tests are as below

Test 1:
  1. Mount the Super Takumar 1:1.4 50mm on the K-r
  2. Turn off the AF fine adjustment
  3. Take a picture at the maximum aperture (1:1.4)
Test 2:
  1. Mount the Super Takumar 1:1.4 50mm on the K-r
  2. Turn on the AF fine adjustment, select the value -7 (I get this value after many tests in some previous days)
  3. Take another picture at the aperture 1:1.4
Results: In both cases, the red signal is active to report right focus, and (the most important) I can see the clear focus from the viewfinder (I use a stock focus screen.) All images in Test 1 are *soft* (or wrong focus) (from any angle), and all images in Test 2 are sharp or very sharp as expect.

I think that my repair made something wrong to the lens (before I dissambled the lens, there's only infinity focus problem.) I'd like to know the *name* (and if possible, the cause) of this problem before moving to next fix

Please note that the problem doesn't happen with another lens (Helios 44m-4, at the maximum aperture 1:2).

Thank you for reading and helps.

Regards,

12-30-2011, 07:36 PM   #2
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If it's just the one lens, it's probably the lens, but what kind of light are you testing in?
12-30-2011, 07:39 PM   #3
icy
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QuoteOriginally posted by Philoslothical Quote
If it's just the one lens, it's probably the lens, but what kind of light are you testing in?
I did my tests in the morning light (very good), and I don't think the problem relates to known problem of Kr.

What makes me suprised is that "AF Fine Adjustment" can correct the focus in this case...
12-30-2011, 07:48 PM   #4
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Well, if it's just off by a bit, then the AF adjustment should work. The AF doesn't care if it's a manual or AF lens, if you're invoking it with the AF button or shutter release.

Hopefully someone else can offer you info leading to a fix, but in the meantime, how often do you generally switch lenses? Perhaps you could get by with setting the adjustment for now.

12-30-2011, 08:07 PM   #5
icy
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QuoteOriginally posted by Philoslothical Quote
Well, if it's just off by a bit, then the AF adjustment should work. The AF doesn't care if it's a manual or AF lens, if you're invoking it with the AF button or shutter release.
So may we call this problem "front-focus"? (I know it's a *front* problem when testing with some low angles.)

QuoteQuote:
Hopefully someone else can offer you info leading to a fix, but in the meantime, how often do you generally switch lenses? Perhaps you could get by with setting the adjustment for now.
I don't switch lens quickly, so setting the adjustment won't be a problem. The only (annoyed) problem is that sometimes I may forget to reset the adjustment for other lens ^^ I will use a red tick on the Takumar to alert myself when I need to mount/unmount it
12-30-2011, 10:41 PM   #6
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In my experience the focus confirmation at f1.4 is always qoing to be a bit unreliable because the depth of field is so thin and the camera doesn't know which part of the image you want to be in focus.

If you're planning on using a lot of manual focus lenses I suggest you get yourself a split prism focusing screen. That way you're in control of what is in focus when you release the shutter.
12-31-2011, 12:22 AM   #7
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QuoteOriginally posted by icy Quote
and (the most important) I can see the clear focus from the viewfinder (I use a stock focus screen.)
I don't know the K-r, but on the K-x,
apparently "clear focus" on the viewfinder
is not necessarily a good indication
of sharp focus with a fast lens.
Magnified live view may be more reliable.
12-31-2011, 09:35 AM - 1 Like   #8
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The red light doesn't indicate that focus has been achieved; just that the focus point is actie. You need he green hexagon to confirm when focus is achieved. But even so, that just tells you *something* within range of the focus sensor is in focus - perhaps not the exact spot underneat the red light in the viewfinder (the focus sensors are much broader). And the viewfinder always shows way too much in focus, due to its design. More will be in focus in the viewfinder than in the picture, as is easily verified by shooting some text on an angle and comparing which lines seem to be in focus. So it's perfectly normal, unfortunately, that some things will appear in focus in the viewfinder but turn put not to be.

Once tou fully grasp these attributes of the focus system - the role of the green hexagon, the large size of the focus sensors making it hard to pinpoint focus using confirmatiom, and the too-large DOF of the focus screen, it becomes possible to practice compensating for these and reach a point where you can focus effectively. But it *does* take practice.

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