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09-09-2013, 01:32 PM   #1
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Focus problem on pentax ME super

Hello,

I've acquierd an pentax ME Super recently in good working order, but after my first roll, i've discovered a back focus problem.
The focus screen is in the correct way, and there are 3 setscrews to change the focus screen position, which is good.

The only thing is i don't know how to check if the focus screen is correctly placed (with digital it's easy, trial and error).
Anyone that has some tips how to tackle this one?

Regards,

Adam

09-09-2013, 02:13 PM   #2
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You can test by focusing your lens to infinity and looking through the finder for a light post or a tower that is far away
Caould it have a diopter adjustment to it?

PS. test the lens in a DSLR or use a lens that you are sure focus correctly.... it maybe the lens
09-09-2013, 02:33 PM   #3
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My ME Super doesn't have a diopter adjustment, not an easily adjustable one like we see on many cameras anyway. They might have interchangeable viewfinder lenses though?
09-09-2013, 03:40 PM - 1 Like   #4
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This has nothing to do with the viewfinder diopter. The problem the OP is experiencing is that the optical distance between the lens mount and the viewfinder screen is not equal to the distance between the lens mount and the film plane. Not sure which way results in backfocus.

I haven't any experience with the setscrews to make a more helpful comment.

I would also check to see that the mirror is properly seated in the normal down position, since that would also change the optical path while focusing.

I would also check to see that the spring-loaded panel on the film back, which presses the film gently against the film rails, is doing its job. (Not sure if they are really called film rails, but anyhow they are the shiny bits that guide the film horizontally when you look into the back of any film camera.) If the film does not press against the film rails, the distance between the film and lens mount will be vary, of course.

09-09-2013, 05:03 PM - 1 Like   #5
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QuoteOriginally posted by crabe919 Quote
The focus screen is in the correct way, and there are 3 setscrews to change the focus screen position, which is good.
Huh???

No, No, No!!! You do not do this.

How did you determine your back-focus? Did you put a ground-glass at the film plane and compare it to the split image in the viewfinder? That is what a repair tech would do.

A true focus accuracy issue (back-focus is an AF term) would have three potential causes:
  1. Focus screen was improperly reassembled into the viewfinder optical path during service
  2. Camera has been damaged resulting in non-alignment of the mirror box (fatal)
  3. Mirror angle is wrong
...and would affect all lenses mounted to the camera. As Tom noted above, this only happens when the distance from focus screen to subject is different than the distance from film plane to subject. A weak or missing film pressure plate is another possibility.



Steve

Last edited by stevebrot; 09-09-2013 at 11:44 PM.
09-09-2013, 09:08 PM   #6
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With those older cameras a weakened pressure plate can cause most of the issues, and thankfully it is one of the easiest problems to fix. The focusing screen on the ME super is held in pretty tight* and it should be in correct position ( within tolerances) unless someone has tampered with it. Check the pressure plate and make sure it is delivering enough force to keep the film smashed flat against the rails.

* if it is moving around then there is a chance someone has tampered with it. On my ME super I did have to shim my focus screen it 0.2mm so the infinity point was accurate, but it was a task and a half to get the blasted thing out in the first place.

Last edited by Digitalis; 09-09-2013 at 09:14 PM.
09-10-2013, 04:17 AM   #7
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Thanks guy's for all the response!

Certainly not the lens, this is the lens i use on all my camera's to test the focus accuracy (50 1.7)!
Onto the first roll of film inside a new camera a always make the 4 same pictures to see if all goes ok, this time focus was of by a big margin.

I've checked the pressure plate, and it's as onto all my other camera's so this will not be the cause.

I've tried to resolve the problem as follow:

Used 2 different camera's and focus from one point onto an object a couple meters away (with a vertical line) and checked the distance scale on the lens.
Both where spot on identical. The me super was off by about a half meter. I've adjusted the 3 set screws that position the focus screen equally by a 1/4 turn till the distance scale matches the other 2 camera's.

Time to test run the camera again with some film to check if the problem is solved.

btw, thanks for the ground glass plate tip, will try it out if i can find some ground glass!
09-10-2013, 08:23 AM   #8
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Instead of ground glass you can use a piece of clear plastic (overhead projector sheets) covered with magic scotch tape

09-10-2013, 11:29 AM   #9
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will try that one if the camera still isn't focusing correct!
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