Focusing screen details and questions

Big Dave · 11-29-2008, 02:43 PM

Hello,
Well I finally removed my focusing screen from my K100D super. I did this to get the screen dimensions for a future project. My body has a .3mm shim placed on top of the screen, giving a total thickness of 1.6mm. The screen is 1.3mm x 26.1mm x 17.8mm. The screen is placed with the Fresno surface facing up. This is unlike any of the film SLRs that I own. The body serial number is 2692771, purchased on 01/2008. Have any of you found this shim arrangement when you replaced your screen? If not then the screen thickness may not be critical. I always thought that the Fresno surface of the screen is were the image is manually focused. If the Fresno was facing down, you would have a common fixed reference for the screen and the thickness wouldn’t matter. With the Fresno facing up the focusing distance would change if the screen had a different thickness. The only reason that I can think of the Pentax doing it this way is to improve finder brightness. Screen thickness may account for the focusing problems that some people are having with third party screens.

Dave

*kristoffon* · 11-29-2008, 07:53 PM

Screen thickness is definitely critical. While installing a katz eye focusing screen on my K200D I accidentally let the shim drop out of its place. After putting everything together I was not happy because focusing on the screen wasn't proper. Eventually I realized I had put the shim flipped right to left, after fixing it focusing on the screen was spot on.

So if some micrometrical difference in thickness between sides of my shim caused focusing problems I'll assume focusing screen thickness will be very critical.

Big Dave · 11-29-2008, 08:13 PM

Kristoffon,
The focused image on the screen needs to be the same distance from the lens as the distance to the sensor for manual focusing. The 0.3mm shim in my camera must provide the exact required spacing when the screen is pressed against the pentaprisom sub-frame. Very interesting.

Dave

Originally Posted by kristoffon

Screen thickness is definitely critical. So if some micrometrical difference in thickness between sides of my shim caused focusing problems I'll assume focusing screen thickness will be very critical.