Dan, I consider the FM3A to be a superb viewfinder. Nikon made this VF very bright compared to cheaper models, so you will find the Leica & a high level SLR viewfinder like the FM3A both to be bright - as long as you are using a reasonably fast lens on the SLR. However, there are key differences between the two:
1. Although both viewfinders are large, bright, & easy to see through, the SLR viewfinder will change with each lens or zoom setting so that the desired picture will always fill up the viewfinder. The Leica VF will remain constant & you will compose your picture using framelines projected in the viewfinder window. A wide angle picture will fill up most of the viewfinder, but a telephoto shot will only use a relatively small portion of the middle of the viewfinder. In this way, the Leica VF allows you to factor in information outside the desired composition - useful when shooting people who are moving. On the other hand, the longer the telephoto lens on a RF camera, the smaller the framelines until it eventually becomes unworkable.
2. The magnification in the Leica VF will remain constant, depending on which model you have chosen regardless of what lens you are using. The magnification on the SLR VF will vary depending on the lens or zoom setting you are using, i.e. it will increase with a telephoto lens & decrease with a wide angle lens. As jja said the FM3A mag is .86 with a 50 mm lens set at infinity but varies up or down from there.
3. Viewing through the Leica VF is like looking through a window, independent of what the camera sees. Everything is always in focus, while the SLR view is out-of-focus until you adjust it. While the SLR gives you visual feedback & has DOF preview, it can be hard to focus when light is dim. The rangefinder mechanism is much easier to see & align in the viewfinder in low light. The SLR low light difficulties are compounded by some degree of light loss viewing through the lens in dim light even on the best of SLR viewfinders, while the Leica viewfinder will take advantage of any available light because it is jsut a clear window. In all lighting conditions, a rangefinder is a more precise focusing tool than through the lens focusing.
4. With a zoom lens on an SLR, you can play with composition until you get just the amount of the scene that you want. For the same function on a Leica, you can try different framelines to help decide how much of the scene you want to include before making a choice of lens. Of course, you can physically move closer or farther away with either camera type.
5. You can go to the leica website @
www.leica-camera.com or Stephen Gandy's website @
www.cameraquest.com to see examples of RF viewfinders with projected framelines.
I hope this helps.
Huck
