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07-31-2015, 09:51 PM   #1
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Pentax 67 200mm f/4 default focus

I bought the lens with the intent of adapting it to my Nikon D5100 for wide field astro photography. . Seems to be nearly new and clean glass. Adapter is fine, using manual setting. I'll probably shoot at f/4. I have imaging software that will give the FWHM of a star to aid in focus. I realize that the closer than infinity focus requires a matching camera body. Not concerned with that, just infinity. Isn't it true that without a body the lens will default to infinity focus?
It's been cloudy for 3 months and no end in sight, so love to get this worry off my mind so I can keep or return the lens. Is there any way of adjusting the infinity stop a little if needs be? Imaging clouds, focus seems ok, but for those not familiar, FWHM on a star is extremely critical/accurate. -Thanks-Jack

07-31-2015, 10:34 PM   #2
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If your adapter provides the correct flange to sensor distance, and you turn the focusing ring so that the infinity mark is at the indicator (the focusing ring stops there) astro photos should be in focus on your Nikon. I'm not sure what you mean by "default focus". You should be able to check focus with your camera by focusing on things that are far away (a kilometer or more). Focus can be adjusted, but if the adapter is properly made you shouldn't need to do anything.
07-31-2015, 11:11 PM   #3
672
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Having shot this lens on a 645D and Z and Canon full frame, my experience is that it pretty much will not be very focused at f4. Maybe on film , but not on digital.
08-01-2015, 10:59 AM   #4
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QuoteOriginally posted by kalasinman Quote
I realize that the closer than infinity focus requires a matching camera body.
Not true.
QuoteOriginally posted by kalasinman Quote
Isn't it true that without a body the lens will default to infinity focus?
Again, not true.

08-01-2015, 11:16 PM   #5
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What isn't?

QuoteOriginally posted by desertscape Quote
Not true.


Again, not true.
I don't know. I was told by a vendor that there is an interaction between the lens and a body. In fact I did read in the manual that close focus issues are fixed by adjust 3 mirror screws in the body. My own tests indicate a lack of focus for objects 2m away, for example. Not a big deal. I don't need close focus, only infinity.
Bought the lens on Ebay and seems in near perfect shape. Flat frames look great, after some cleaning.
However if no sharp focus at infinity, it's useless to me. I can get a refund from Ebay, if I do it pretty soon. Of course, this is all academic without stars to test on. Clouds and more clouds.

---------- Post added 08-01-15 at 11:20 PM ----------

QuoteOriginally posted by 672 Quote
Having shot this lens on a 645D and Z and Canon full frame, my experience is that it pretty much will not be very focused at f4. Maybe on film , but not on digital.
I'll be using a Nikon APC sensor. That may help. What f/stop did you find a sharp focus at? I'm reluctant to use the iris, but could cut a mask for it.

---------- Post added 08-01-15 at 11:23 PM ----------

QuoteOriginally posted by pentaxus Quote
If your adapter provides the correct flange to sensor distance, and you turn the focusing ring so that the infinity mark is at the indicator (the focusing ring stops there) astro photos should be in focus on your Nikon. I'm not sure what you mean by "default focus". You should be able to check focus with your camera by focusing on things that are far away (a kilometer or more). Focus can be adjusted, but if the adapter is properly made you shouldn't need to do anything.
There is a difference between the focus of something "far away" and a point source hundreds of light years away. My capture software will measure FWHM in real time. It only takes a very tiny error to be obvious in an astro photo. Hence my concern.
08-02-2015, 12:27 PM   #6
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Lens adapters are commonly inaccurate in thickness and therefore many will not give an accurate infinity. Even an error of .002 inch will cause problems.

QuoteOriginally posted by kalasinman Quote
There is a difference between the focus of something "far away" and a point source hundreds of light years away.
This lens will render objects of 2 miles away as infinity and there will not be a focus difference between that and stars. Lens focusing is determined by the direction of the light rays entering your lens. At macro distances, the light is diverging , causing the focal plane to be pushed further back, behind the sensor/film. You could easily be using the lens at 220 to 250mm focal length (with extension tubes). At infinity distances, the light rays are parallel and focus is at the intended focal length intended by the designers; 200mm in this case.
08-03-2015, 02:01 AM   #7
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There were thin spots in the clouds last night. I set up on the tripod and took some 2 second snaps at f/4 ISO 2500. No stars visible, but there they were in the picture. They look fine except for the slight trailing, so I'm ok I think.
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