How to test an in-camera (OM-1) light meter?

jdk01 said:

I am trying to test an OM-1 light meter. The light source is an ‘day-light emitting’ professional Aputure Amaran 200d lamp. I have a device to measure the actual LUX being emitted from the 200d as well as a modern sekonic analogue meter. Can I shine light directly into the camera lens? Or do I need to use my 45 degree 18% grey board? My experience is that the sekonic and LUX meter agree with the range of light levels being received directly from the 200d lamp. However the OM-1 shows different readings - 2 stops out. Is this a potential problem with the OM-1 or with the technique of not using the grey card? Your advice is appreciated.

Click to expand...

MattKing said:

Are you using the correct battery in the OM-1?
It was designed for a 1.3 volt 625 mercury battery.
Zinc air batteries - both hearing aid type and batteries adapted to fit in the 625 slot are close enough to that to work.
Some OM-1s have been adapted for use with a 1.5 volt silver oxide battery.
Some people buy diode equipped adapters that allow use of smaller silver oxide batteries in the adapter in the battery compartment.
I'm not sure whether the OM-1 can take some of the very last focusing screens that required exposure adjustment.

Click to expand...

wiltw said:

First of all, what battery are you using?! The OM-1 is known to be VARIABLY in error, depending upon the intensity of ambient light being measured when you put in a PX625 alkaline battery!

Click to expand...

jdk01 said:

Hi, I am using a new Wein battery. I will also try to obtain a 357 silver oxide. Thank you.

Click to expand...

wiltw said:

Do not bother with the 357 unless you KNOW your body was modified to regulate 1.5v down to 1.35v

An alternative is to buy yourself an MR-9 adapter, which takes a silver oxide battery and physically adapts the battery to fit in the space of a PX625 mercuric oxide cell AND regulates the voltage down from 1.5v to the needed 1.35v

BEWARE!: There are some pseudo MR-9 being sold, which simply force you to buy and insert expensive 1.35v air cells, which deteriorate even with zero usage, if the air seal has been removed.

Click to expand...

ic-racer said:

Personally I'd do the test outdoors in bright shade using two reflected meters.No need for a calibrated gray card white or black or any shade in between will do. One meter is the one in the camera and the other would be a hand-held meter. Ideally a hand-held view-meter.

View attachment 269521

Click to expand...