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Rattl3dbyth3rush
Tinkerer
Username: Rattl3dbyth3rush
Post Number: 2
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 03:31 am: | 
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OK so, having recently acquired a back-up Pentax MX (with SMC Pentax 50mm lens), I thought I'd test it out with a roll of slide film.
As per, I fired off the 36 shots and sent the film off to be developed - anxiously awaiting the results.
Two weeks later, my slides arrived....
To my horror, ALL of the photos exposed on a wide(ish) aperture have a back-focusing issue!! In each shot, the subject intended to be in focus is blurred and instead I have a gloriously clear shot of the scenery behind (about 1-2 metres behind the intended focus point)!
So...
I took the lens off the camera and inspected the focus screen. Annoyingly, I then discovered that the focus screen was installed at a slight angle and was jammed on a small rivet of metal...
(Obviously) I then flipped the focus screen out of the body and re-installed it at its correct position.
So, my question is:
Do you think this will be the culprit? Do you think that if I put a new roll of film through the camera the shots will be in focus? Orrrr, does the focusing screen not impact the camera in this respect? If so, do you think I may have a bigger problem on my hands?
Additional points:
1. Whilst shooting the film, the camera consistently refused to focus to infinity!
2. There is no mirror cushion WHATSOEVER in the camera.
All help/advice is much appreciated!
Tom. |
Ismaelg
Tinkerer
Username: Ismaelg
Post Number: 95
Registered: 11-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 04:11 am: |
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That will definitely cause problems. As to whether it is the only reason, only a test roll can tell. Check the mirror and make sure it seats properly and nothing looks funny or out of way.
If fixing the focusing screen doesn't solve the infinity focusing issue, the problem could be related to lens adjustment. Once the back focusing issue is sorted, take some test shots using the scale on the lens. If the results show the scale is off that will most likely affect infinity focusing and the lens needs to be adjusted.
If you have access to a Pentax DSLR (gasp!) the lens can be tested separately.
The mirror bumper foam prevents the mirror from hitting hard solid on its way up, but the lack of it should not affect focusing as far as I know.
Good luck!
Thanks,
Ismael |
Rattl3dbyth3rush
Tinkerer
Username: Rattl3dbyth3rush
Post Number: 3
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 04:33 am: |
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Thanks for your help Ismael! Plenty of food for thought there.
Hahaa, I don't have access to a Pentax DSLR, however, I do have a Canon DSLR so I could test the lens out on that with the help of a mount converter! |
John_s
Tinkerer
Username: John_s
Post Number: 107
Registered: 07-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 06:58 am: |
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The position of the focusing screen is critical so I think you have to assume this was the problem, and put another roll through.
On my MX there is what seems to be an adjusting screw next to the mirror stop on the right hand side. I've never had occasion to try to adjust it but it could be another source of error.
Its not difficult to replace the mirror bumoer if you can get hold of a bit of sticky backed foam or felt. |
Ismaelg
Tinkerer
Username: Ismaelg
Post Number: 96
Registered: 11-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 08:11 am: |
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Beware that some lens mount adapters can cause infinity focusing issues because of registration distance differences. In other words, the lens sits farther away from the film plane/sensor thus preventing infinity focusing and altering the scale. It depends on the adapter.
However, thinking about it, you mention that in the shots the background was in focus, so it may well be related to the focusing screen issue...
Good luck!
Thanks,
Ismael |
Waynemel
Tinkerer
Username: Waynemel
Post Number: 407
Registered: 08-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 08:28 am: |
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Also, with an adapter, you probably cannot trust the distance scale markings on the lens. |
Rattl3dbyth3rush
Tinkerer
Username: Rattl3dbyth3rush
Post Number: 4
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 10:02 am: |
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Thanks for the feedback everyone.
I really appreciate it.
I'll run a new roll of film through over the coming days and see how I get on. Fingers crossed!! |
Rick_oleson
Tinkerer
Username: Rick_oleson
Post Number: 1262
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, December 14, 2012 - 05:16 pm: |
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"Back focusing" is a disparity between the position of the focusing screen and the position of the film plane; it is not a function of the lens, only of the body, and specifically of the viewfinder (either the screen or the mirror position).
It stands to reason that the camera would "refuse to focus to infinity" when the focus on film is much farther away than what you're seeing on the screen: the camera WAS focused to infinity, it's the viewfinder that was not. The screen being too high would cause this.
If I remember it correctly, I don't think there was a mirror cushion in the Pentax MX... but it's been a very long time since I had one. |
Rattl3dbyth3rush
Tinkerer
Username: Rattl3dbyth3rush
Post Number: 5
Registered: 04-2010
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, December 17, 2012 - 03:24 am: |
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jackpot. thanks for the help everyone 
hopefully my back-focusing problems are now a thing of the past!
Re: mirror cushion. the MX should have one, but over time they can crumble away and fall off. without one the mirror occasionally jams - mine is doing this at present. by contrast, my other MX has a nice little mirror cushion and never locks up. |
Steve_roberts
Tinkerer
Username: Steve_roberts
Post Number: 72
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, January 17, 2013 - 09:11 am: |
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I had a focus issue with my MX whereby the infinity focus was off and anything shot with the lens wide open just wasn't crisp. I noticed that the black underside of the mirror had been worn away over the years by the mirror stop, which is sharp edged component rather than a large area pad as on Spotmatics. I investigated the screw beside the stop but chickened out of adjusting it as it seemed very stiff and I had a sense of foreboding! Instead, I tweaked each of the three screws that set the position of the viewing screen to bring infinity focus back. Doing this only works around the problem, as the mirror is clearly slightly off 45 degrees and the focussing is only correct when you use the centre of the viewfinder - but then that's where the split image and microprism focussing aids are, so I don't see this as a problem. (Think I may have already mentioned this somewhere on the site.) I'm sure that after 30+ years just about every MX that's seen its share of use will be suffering from the back of the mirror being worn away (you have to look closely). I'd be interested to hear if anyone has ever successfully moved the screw near the mirror stop.
Steve |
