# Light Meter Conversion



## saltface (Nov 20, 2008)

I've come across this old 35mm film camera that has a light meter in the body (but not through the lens). Since the meter is set for 50mm, I've had to do some guesswork with other focal lengths. I've found a formula of:

New_F-Stop = Old_F-Stop x ((50mm ± Other_Lens_Length)/50mm)

but this really limits what I can use. Does anybody know of software or a better formula? Thanks.


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## Helen B (Nov 20, 2008)

Where did you find that formula? There should be no need to change the f-number when you change lenses. That's the whole idea of using the f-number (a ratio of focal length/aperture diameter, in simple terms) instead of the actual aperture diameter.

Best,
Helen


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## saltface (Nov 20, 2008)

I guess that didn't occur to me.
It was on a paper that came with some extension tubes. (I see how it would definitely apply there).
I guess the meter is just off, then. Everything is underexposed.


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## Helen B (Nov 20, 2008)

How are you metering? What sort of a meter/camera is it? With external or handheld meters it is fairly important to ensure that you aren't metering too much from the sky, for example.

Best,
Helen


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## saltface (Nov 20, 2008)

It's a Mamiya/Sekor with no model number. 
I've just been doing averaging. I'm not sure of the angle it reads. The sensor is a little blue rectangle above the timer.


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## compur (Nov 20, 2008)

It's not unusual for the meters on old cameras to be off.

Test it against a camera/meter with known accuracy and compensate
accordingly.

You can compensate by adjusting the camera's ASA dial. 

Or, you can put a piece of wratten filter of appropriate density over the
non-TTL meter window to compensate for underexposure (or a filter on
the lens to compensate for overexposure).


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