Pentax SLR focus problem

mcgrattan · Jul 18, 2006

HI,

I have a Pentax P50 (newly acquired) which seems to have a focus problem.

Every shot is slightly out of focus and the focus alignment seems off -- i.e. if I focus through the mirror at something at infinity, when I look at the lens scale it reads 10 or 20m.

Now I presume this means the mirror isn't sitting in quite the right place.

However, I'm not sure how to adjust this. There seems to be a 'stop' that the mirror comes to rest against. Would I be right in assuming that I need to move this somehow (it looks like it may have a socket for a hex key in the middle)?

or do I need to add something to the stop e.g. a fine piece of rubber to slightly 'thicken' it?

Any suggestions?

thanks in advance.

John Koehrer · Jul 18, 2006

1) The likelyhood that the mirror is out of alignment isn't the first thing to check. The lens is. Will the lens focus to infinity?
2) Try using a ground glass to check the focus at the film plane.

resummerfield · Jul 18, 2006

Like John said, check the image at the film plane with a small ground glass or even a piece of frosted mylar, and check if it agrees with the image in the viewfinder. If not, the viewfinder focus screen needs adjusted. It is possible to do that yourself, but much better to send it off for repair.

mcgrattan · Jul 19, 2006

The lens is known to be good. I use it all the time on an EOS 650 with an adapter and on a couple of old screwmount SLRs.

I take it the likelihood then is that the focus screen needs shimmed?

I have also arranged to borrow a friend's known good K-mount 50mm lens to do a comparison.

Thanks for the advice.

Steve Smith · Jul 19, 2006

If you can see the whole image in the viewfinder, I would suggest that it is the focussing screen that is out of alignment rather than the mirror but it would be worth checking that the mirror drops to a proper seated position after each shot.

Eric's suggestion of using frosted mylar in place of the film to try to focus onto is a good one. We use this stuff by the ton (or is that tonne?). If you would like a piece I can post some to you. I use fine textured polyester sheet in place of ground glass on my 5x4" camera and for various lens tests.

Steve.

mcgrattan · Jul 19, 2006

What would people use as shim stock to shim the focus screen?

Some paper or thin card?

I presume it'd need to be something I can cut easily. Also, how would it be held in place? Just by the weight of the screen?

thanks for all the help, btw. Much appreciated.

I'm quite keen to make the effort as the camera is otherwise good -- meter, ergonomics, etc.

Jon Shiu · Jul 19, 2006

You can check the focus by putting a piece of frosted scotch tape across the film gate and then comparing this focus with the focus through the eyepiece.

Jon

mcgrattan · Jul 20, 2006

Thanks very much to Steve Smith who posted me some frosted mylar.

As far as I can tell, using a loupe on the mylar, the camera is fine. It looks like the (Vivitar) m42-K/mount adapter was off.

When I used a different -- Pentax brand -- adapter with one of my Pentax lenses it seemed to be perfectly in focus on the film plane and through the viewfinder.

Looking through a 135mm lens wide open there seemed to be good agreement between what I was seeing on the film plane and through the finder.

I don't quite understand why this should be the case - if the distance from the film plane to the lens is off because of the adapter shouldn't the distance to the focus screen be off by the same amount?

Looks like problem solved though.

Anyway, thanks again to Steve Smith and everyone for the advice.

Pentax SLR focus problem

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