Why does my all my Nikon SLR viewfinders get so dark when macro extension tubes are used.?

Question

Both my D40 and D200 Nikon camera's do this. The view finder gets so dark when macro extension tubes are attached that it is difficult to focus, due to not being able to see the item you are photographing. My Canon XTi does not do this. The viewfinder is as clear as not having the extension tubes attached.

Answer 1

Older Nikon extension tubes do not actuate the aperture lever that opens up the aperture, so you're doing the composing with the lens fully stopped down and the finder will be very dark unless you manually open up the aperture while composing your image.

Answer 2

As you extend the focal length you let in less light, this applies to macro extension tubes as well as teleconverters and zooms.

With nikon you have the added complication of being able to use very old f-mount macro lenses with no aperture coupling (i.e. pre ttl metering and so always meter at set aperture as opposed to full aperture as on post ttl lenses, you want ai or later I believe not being a nikon expert)

I have an XTi and I can assure you that when I use a shifting barrel macro lens (such as my sigma 70mm) it gets visibly darker as the lense focuses closer, when I use manual bellows with an old manual lens the same thing happens.

I cannot think at all why the issue seems worse on your nikons (why do you have two contemporary systems?) with the d200 you can change finder screens for set applications.