50mm Lense

I've been looking at some 50mm lenses to use for close up with a set of extension tubes that i have. But do i go for a 1:2 ,1:4, or a 1:7. I assume that the smaller you go then the better the lense. I've noticed on e-bay that there are a lot of 1:7 and are called a prime lense what does that mean.

Richard

those numbers refer to the fstop. the smaller the number the wider (read more light) the aperture.
with macro, you'd be stopping the lens down to at least f8 and more likely f16-f22. The wider apertures then equate to brighter viewfinder, which can be helpful with macro rings which lower the amount of light getting through.
bottom line, any of those lenses will serve you well. I get great results using my 1.7.
prime refers to a single focal length lens, and most often refers to a single focal length lens in the 40-50mm (35mm) range which approximates the human eyes' feild of view.

Just to clarify a terminology misunderstanding:

The official designation on a lens is
1:1.2
1:1.4
1:1.7
1:2
1:2.8

Which correspond to maximum apertures of f1.2, f1.4, f1.7, f2 and f2.8 respectively. Many a time I've seen 1:2 lenses listed as f1.2, when they should be f2! A new f1.2 SMC-A series manual focus lens costs about £430 , second hand ranges from £150 to £350. An f2 lens second hand is about £10

The f1.2 (1:1.2) lenses are absolute monsters with beautiful, huge front glass elements

As Don says, the f1.7 Pentax lenses are superb - they have minimal distortion and are absolutely ideal for extension tube work

Matt