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Sleepytomcat
Tinkerer
Username: Sleepytomcat
Post Number: 1
Registered: 10-2011
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, October 01, 2011 - 05:13 pm: | 
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Update: sorry for my mistake, NOT 800mm, just 80mm lens.
Hi everybody,
I'm quite new to medium format film photography with my "new" Mamiya M645 Pro with Sekor 80 mm lens.
I've used my first film rolls this week, and looking at the results I feel uneasiness. Could you please advise me on the following issues:
1) when focusing on a very distant subject (maybe some miles away) the lens shows "infinity" (and stops to rotate further), however the focusing screen shows "no focus yet";
2) when focusing on close subjects I mostly get out of focus (backfocus) pictures (portraits);
3) test: focusing on a clearly visible close subject and checking the actual distance (2 m 25 cm) against lens readings (3 m) - I note the difference about 25%. (Hovewer I havent conducted a real phototest with real pictures in controllable environment.)
Please advise me on these issues: may it be the only one issue with different ways to show up? If so, what steps shall I do to fix it?
Thank you very much,
Sergey |
Ismaelg
Tinkerer
Username: Ismaelg
Post Number: 73
Registered: 11-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, October 01, 2011 - 09:59 pm: |
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Hi,
I'm not familiar with that particular lens, but a possibility is that the lens focusing ring may have moved over the years and is no longer properly aligned. I have a similar issue with an older M42 lens. Try to calculate the actual focusing distance when the lens reads infinity. Let's say it is 10 meters. You would need to focus the lens to infinity, loose the focusing ring and move it until it reads 10 meters. Then it will be aligned and your lens reading vs. actual should be the same. That will also give it more travel to reach proper infinity.
Hope this helps.
Thanks!
Ismael |
Mareklew
Tinkerer
Username: Mareklew
Post Number: 238
Registered: 03-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, October 05, 2011 - 09:01 am: |
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Before you move any rings or else modify the lens, you have to figure out where the error lies.
I don't know your camera in detail unfortunately, but you need to find out which part of the system is out of alignment.
Easy test will cost you a roll of film. You have to proceed carefully and take notes what you did on which frame.
1st frame: focus at infinity BY THE SCALE (ignore the viewfinder) and take a photo of some distant landscape, that has also something in foreground.
Perfect would be freshly ploughed field with a forest line on horizon.
2nd frame: repeat by focusing BY THE FOCUSING SCREEN. If you can't achieve focus, note it down and take picture anyway.
3rd frame: take measuring band and put camera exactly 5m/15ft (or whatever exact distance marking does your lens have about this distance) away from a lamp post on a tiled sidewalk.
Measure the distance not from the front of the lens, but from where the film is. There's either a marking on the camera ( symbol: Φ) or you have to guess where the film plane is.
4th frame: repeat focusing by the focusing screen.
5th frame: same as 3rd, 2 meters (6ft) away, by scale
6th: as fifth, by screen,
7th: at minimum focusing distance as marked on the lens, by scale.
8th: like 7, by eye. If you can't achieve focus, note it down.
Use rest of film to take pictures of different static objects NOTING DOWN each time where did you focus (say, a car, focus on the left side mirror). If you can, take pairs of pictures: one using a measuring band and lens scale and one focusing by eye.
Evaluate the results:
If focus in pictures focused by eye is wrong, but ones focused by scale are right, then the mirror is out of adjustment or the focusing screen is installed improperly (likely if you have interchangeable screens).
If focus in pictures focused by eye is right, but ones focused by distance scale is wrong, AND you were unable to focus EITHER close or at infinity, then the distance scale is off.
If all pictures have focus in wrong place, but differently wrong in each pair, the camera was whacked hard at some point in the past. Check if the focus changes along the lamp post - if you leveled your camera and the post was vertical it should be all in or out of focus, skewed focal plane indicates mechanical damage of lens mount or lens itself.
If all picture pairs are IDENTICALLY off, then check your enlarger.
Marek |
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