Re: [APML] Focus Problem with Pentax 67

[Ray Butler <ray.butler@nuigalway.ie>]

Hi Roland,

I checked out your page and the image excerpts make it much easier to 
see what went wrong. This looks to me like a misalignment or tilt 
between the optics and the focal plane. I can't see how film 
non-flatness (in the conventional sense) could cause such gross 
problems, in a gradient all over the entire frame, at f5.6.

You should first eliminate the camera body from your enquiries, as the 
police would say. Can you get your hands on any other Pentax 6x7 lens, 
say a 90mm or 105mm standard lens, and take some test shots with it? 
Even some tests in light polluted Brooklyn would do - all you need is to 
see a scattering of stars around the frame and examine them for 
sharpness. If they are good, then the camera's focal plane is correctly 
perpendicular to the optical axis - and the back is set correctly. The 
focus (pardon the pun) of suspicion would then move to the lens.

But the curious thing, is you say that on a different film the problems 
turned through 90 degrees. The 300mm lens only attaches at one 
orientation, so it is hardly the lens optics themselves that are at 
fault, unless an element is literally flopping around inside the lens. 
Does it rattle if you shake it?

Then take some terrestrial short-exposure infinity shots with the 300mm 
lens - say of the New York skyline, which has plenty of detail. If the 
lens is the cause of those severe aberrations, you will see it.

You could also get one of those cheap (~$20-30) Synta "collimation 
eyepieces" used to check the alignment of refractor optics. If you can 
centre it accurately at the back of the lens, with the front lens cap 
on, you can check for misaligned optics very easily: all the reflections 
from each glass surface should be concentric.

If there's nothing inherently misaligned in the lens optics, this would 
leave one last possibility. Your statement: "I attempted to have the 
three knurled-screws that held the lens front to do so snugly but 
without putting any stress on the lens" might be a clue. Attach the 
300mm lens to the camera, and the camera to something solid (such as 
your equatorial mounting), but don't support the front of the lens. Then 
push and pull the front of the lens and see how much you can flex it. 
This indicates the "give" in the bayonet mount, which may need to be 
tightened up.

I hope you can determine what's going on, one way or the other.

Ray "that's about all the diagnostics I can think of" Butler

>Thanks to everyone whose had some suggestions.  I've put up a web page
>with some of the info on these and my comments.  I'm trying to make my
>web site a documentary of my mistakes :-)
>
>    http://www.astrofoto.org/imaging/ssp2005.html
>
>There are no links from the menus or other pages to the above (yet), so
>the direct jump is the only way to get there.
>
>roland
>  
>


-- 
Dr. Ray Butler
Lecturer, Physics Department & Computational Astrophysics Laboratory,
National University of Ireland - Galway, 
University Road, Galway, Ireland. 
Web: www.nuigalway.ie/physics/ 	Email: ray.butler-AT-nuigalway.ie 
Tel: +353-91-493788 		FAX: +353-91-494584

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