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06-06-2011, 08:04 PM   #1
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M42 to PK adapter question

I've tried 2 different generic Chinese M42 to PK adapters with my mirror lens, (One with the flange, and one without that supposedly allows infinity focus.) and after trying a few moon shots, neither will quite allow focus to infinity. Close, but no cigar.

After mounting the "infinity focus" adapter, the distance from the rear of the lens to the camera looks about the same as when using the flanged "non-infinity" version, so I suspect there's little difference in the thickness of the plate that butts up against the lens when it's screwed on.

What are the odds that if I seek out a genuine Pentax adapter I will be able to achieve infinity focus?

Cheers,
Bobbo :-)

06-06-2011, 09:04 PM   #2
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The thin-flange clones are just as thick as the Official Pentax adapter, so if you can't reach infinity with a clone, the mirror mount may need adjustment. Note than many mirrors and long-tube teles focus PAST infinity with a thin-flange adapter. I use thick-flange NIF adapters on both a Spiratone 400/6.3 tube and a Rubinar Makpo 1000/10 mirror, and reach infinity with both.

Have you tried focusing on fixed objects? With your lens on a secure tripod? Luna does move rather quickly; your 'focus' problem may be blurring due to an over-long exposure, or to some tripod instability. Even 1/100 sec may be too slow a shutter with a long lens. Try shooting a phone pole or the like, with your camera and lens stabilized, and see if focus is the problem. Are you shooting with Luna at the horizon, or overhead? Shooting through atmosphere can cause blurring too, especially warm air. The best Moonshot nights are cold and clear and dry.
06-06-2011, 10:50 PM   #3
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I'm a little confused here. Does the "infinity" adapter have a flange or not? If it does, then you may well not get infinity focus; if it doesn't, then why isn't the lens seating properly on the camera?

If the lens does seat properly, so that its base mates intimately with the camera mount, then if you're not able to focus, there could be something wrong with the lens's infinity end stop. But are you absolutely sure you're running out of focus adjustment?
06-07-2011, 04:35 AM   #4
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QuoteOriginally posted by RioRico Quote
Have you tried focusing on fixed objects?
Not at infinity. The reason I picked the moon to test the infinity focus is because I couldn't find a good sharp object at true infinity. How far out do you have to go to reach the infinity focus point? I used a stable tripod for the moon shots. My K100D is also equipped with a split screen, which showed the moon as in focus with the focusing ring at the end of it's range. The screen should be properly calibrated, as I've used a focusing chart with a 50/1.4 lens at f1.4 and shims to calibrate it.

QuoteOriginally posted by RioRico Quote
Luna does move rather quickly; your 'focus' problem may be blurring due to an over-long exposure, or to some tripod instability. Even 1/100 sec may be too slow a shutter with a long lens. Try shooting a phone pole or the like, with your camera and lens stabilized, and see if focus is the problem.
The tripod's very stable, so I don't think that's the issue. I've shot other subjects with it with no stability issues. Never thought of the shutter speed as a possible problem. I used exposure times of 1/10 to 1/30 sec. at ISO200. The moon was about a 45 degree angle overhead. it was a hot, muggy night, tho.

Is the mirror mount adjustment something I can do myself? If so, How?

Cheers,
Bobbo :-)

06-07-2011, 06:38 AM   #5
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QuoteOriginally posted by GibbyTheMole Quote
Not at infinity. The reason I picked the moon to test the infinity focus is because I couldn't find a good sharp object at true infinity. How far out do you have to go to reach the infinity focus point?
Infinity as far as the camera is concerned is when further increasing the distance makes no difference in the focus quality. Ten times the so-called Hyperfocal Distance is infinity for all practical purposes. For a typical 500mm 1:8 mirror lens the Hyperfocal distance is about 1mile so anything further than 10 miles away is at infinity. Online Depth of Field Calculator

QuoteQuote:
I used a stable tripod for the moon shots. My K100D is also equipped with a split screen, which showed the moon as in focus with the focusing ring at the end of it's range. The screen should be properly calibrated, as I've used a focusing chart with a 50/1.4 lens at f1.4 and shims to calibrate it.
Split prism focusing aids are optically correct; therefore the lens was focused at infinity so far as the prism was concerned. If the prism gives sharply focused photos for one lens then it should do just as well for any other lens.

QuoteQuote:
The tripod's very stable, so I don't think that's the issue. I've shot other subjects with it with no stability issues. Never thought of the shutter speed as a possible problem. I used exposure times of 1/10 to 1/30 sec. at ISO200. The moon was about a 45 degree angle overhead. it was a hot, muggy night, tho.
Typical exposures for a moon shot at ISO 200 are around 1/400 sec at f/8. Your photos are likely greatly overexposed and you are seeing the white fuzzy halo associated with overexposed edges.

Full Moon Exposure - Digital Photography Tips and Techniques

QuoteQuote:
Is the mirror mount adjustment something I can do myself? If so, How?

Cheers,
Bobbo :-)
I doubt it needs to be adjusted. You may be expecting more sharpness than possible for your lens in addition to the overexposure.
06-07-2011, 02:25 PM   #6
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QuoteOriginally posted by GibbyTheMole Quote


The tripod's very stable, so I don't think that's the issue. I've shot other subjects with it with no stability issues. Never thought of the shutter speed as a possible problem. I used exposure times of 1/10 to 1/30 sec. at ISO200. The moon was about a 45 degree angle overhead. it was a hot, muggy night, tho.

Is the mirror mount adjustment something I can do myself? If so, How?

Cheers,
Bobbo :-)
I have been photographing the moon with camera on tripod and the Tamron SP 350 mm f/5.6 mirror lens since 1982 (and, much to my wife's amazement, I continue to do so). Exposure times of your magnitude will NOT be a problem in respect of the Moon's apparent movement, but amospheric turbulence (bad seeing conditions) will show up very clearly. Thus, the Moon is not the very best infinity test object. May I suggest that you upload a sample picture. That would make an assessment of your problem (or lack of same) much easier.
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