Focal length shouldn't really matter much, unless the difference in field of view is including something that will throw the meter off horribly (Such as trying to meter with a wide angle and including an ultra bright window.) Even a digital camera will work and let you collect more data.
A body offering a spot mode would be ideal.
In theory you shouldn't have much variation, unless the lenses involved have a massively different T-Stop rating from their F-Stop. (F-Stop being the mathematical value for how much light should be getting through, where as T-Stop is the measured value of what actually gets through at time of testing. Values can change due to fogging elements and the like, but usually are close enough if the lens is still optically good.)
The ISO/ASA rating is a standard, and should mean all the math works out the same, regardless of format type.
Your other option would be to get a dedicated hand held light meter. A good narrow spot meter is something I want to get, along with an incident flash meter. I've worked with other people's gear at times, and they really are nice tools to use if you can take the time and plan out your shots.
Originally Posted by Lrrpie-CT
I destroy drives for clients all the time. Sometimes, intentionally.