Tests are done. Both the Canon T2i and 5D Mk III have brighter viewfinders than Nikon’s FM 10. The FM 10 is, however, smaller and lighter.
I put the 28-135 zoom on the 5D, thinking that would be comparable to what was on the FM 10, but when I dug it out, I discovered it has a 35-70 f/3.5-4.8 lens on it!
The rebel has Sigma’s 18-250 f/3.5-6.3, which I find great for general use. Their new version is even smaller and lighter
So, anyway. not trusting my eyes, I set up a continuous fluorescent in a softbox at the edge of my desk, placed each camera in turn about the same distance from the softbox and used a Sekonic light meter to measure the light coming through the viewfinder. I set ISO to 8000 and shutter to 1/20th. Here are the apertures the light meter suggested:
At the shortest zoom settings
Rebel — 5.0
FM 10 — 5.6
5D — 6.3
At 70 mm, give or take
Rebel — 5.6
FM 10 — 5.0
5D — 5.6
Also, I put a Sigma 85 mm f/1.4 on the 5D, which resulted in a reading of 9.0, so as expected, if you use faster lenses, you get a brighter viewfinder. Canon’s 24 mm f/1.4 on the 5D scored 8.0
For giggles, I put the Sigma 85 on the Rebel. To my amazement, it also scored 9.0, so its viewfinder is not too shabby! Although, it only shows 95% of the scene so things tend to creep into the edges of the frame.