Mamiya 645 Cds meter error

[greg_prior]

I bought a used Cds meter prism for my 1000s at a photo swap. It read

3-4 f/stops off (had to set ASA to about 1000 to get f/16 at 1/125

under 'sunny 16' conditions), so I had the seller send it out to his

repairman to calibrate it. It came back a little better, maybe 2.5-3

stops off. Now I'm wondering if there is something in the body that

is losing the light. Are there wrong focusing screens? Missing

fresnel lens? Any other ideas?

 

Thanks, Greg

[custom film holders for fl]

Are you using a Mamiya focusing screen? If not, the light transmission probably isn't the same and thus would make the prism read an incorrect number.

 

Also, when you are doing the sunny 16 check, what is your method?

[greg_prior]

I believe it is a stock MS screen. It has the diagonal split in the middle with the micro-prism around that. It seems like an after-market fresnel screen brightner would make the meter read overexposed, not under?

 

I have tried making readings by just scanning around at mid-day to get an average reading, and taking readings off of a gray card. The readings vary 1/2 stop or so, but it is -1.5 to -2.5 stops. This still isn't real scientific, but I don't see how I could be that far off either.

 

Maybe this is just all the closer the repair shop could get it, but that just doesn't sound right. I checked that I'm not in M mode on the lens, or that the lens is for some other reason stopping down. I have used 4 different lenses. I always check that they are engaged with the meter pin. I wish I had another body to compare with...

 

Thanks for the help - any other ideas?

 

-Greg

[custom film holders for fl]

>>It seems like an after-market fresnel screen brightner would make the meter read overexposed, not under?<<

 

Maybe I am wrong, but here is my thinking... A brighter screen let more light through, thus making the camera meter think the scene is more brightly lit than it really is, therefore it recommends that you use a faster shutter speed/smaller aperture which would result in under exposure. I couldn't tell from your original post what "2.5-3 stops off" meant (under/over?).

 

I would take it back to the repairman, demonstrate the issue as you see it and ask for an explanation. They should be using a calibrated testing method. I have used your method as a rough guess but it sure leaves a lot of room for variation.

 

You might go to a local camera dealer and see if you can find a unit to use for comparison. Additionally, they might have another focusing screen which you could swap out for a minute to do a comparison test.

 

Just my thoughts...