In article <31879F1E.446B@csgrad.cs.vt.edu>, roland@csgrad.cs.vt.edu says...

>Using a 50mm lens with extension tubes what object/image ratio would >you expect to get?

Up to about 1:1 reasonably easily.

>Camera N90s, lens 50mm AF f1.8.
>What sizes to the extension tubes come in?

Nikon makes them in four lengths: 8, 14, 27, and about 50mm, I think...

>Also it was mentioned that you manually have to calculate the exposure,
>is this really true, won't the N90s and other autoexposure cameras
>calculate this for you, even though not offering AF?

Yes, the TTL ambient and flash metering will both compensate for the
lost brightness when using tubes.

>How does the image quality compare using close up filters versus
>extension tubes?

It depends on the lens design, magnification, aperture used, quality
of the close-up lens (the achromatic Nikon "T" series is excellent) -
so there is no easy answer, except that performance with simple
close-up lenses is generally (but not always) inferior to tubes.
The 50mm f1.8 Nikkor works well on tubes when stopped down, and it is
also quite sharp with the Sigma achromat made for the Sigma 90mm macro.
I would guess that the Nikkor 3T and 4T achromats would work well,
also (though close-up lenses have reduced front space for a given magnification compared with tubes, since they work by shortening the
effective focal-length of the lens).

>Last question, when using extension tubes do you get any form of
>viginetting on the photograph?

No. In fact the circle of sharp coverage (and certainly illumination)
of the lens is likely to be much greater than normal with the lens
extended (if it is extended enough, a lens designed for 35mm can
cover 4x5 film!).
Hope This Helps