Has anyone figured out why... when you pop the battery in and out of your 35mm SLR camera, the viewfinder goes from dark and grainy to bright and grainless? I thought it was something with the lens, but it happens even when the lens is removed. I noticed on my Nikon D50, not sure if all SLR's do this.

Someone mentioned this before so it may have been answered, I couldnt find the thread.

when you turn off your camera, theviewfinder defaults to closed position, revealing a dark image. no magic, sorry. the thread about this earlier was answered the same way, although i can't find it either.

Do I understand correctly that you have lens on the camera body when you experience this? As it really depends on camera.
For example Canon EOS cameras deafault to open iris and Nikon cameras will close the iris when powered down. I guess that you can override closed iris without power via iris ring when the lens has iris ring.

Regs,
T

Toenis,
It happens with or without the lens.
Ben,
There is no mechanical "closing" going on in the viewfinder. The only explanation I can come up with is that there is a liquid crystal light shutter built into the viewfinder.

Here is a demo with a LED light, take a look...
http://media.putfile.com/D50-viewfinder