Widefield lenses advice

[Andrew*]

I have a little dilemma. I want to replace the Canon 450D's kit lens as it is not very fast and the IS is unneccessary. I got a Sigma 28mm f/1.8 lens from eBay for £100, but now I also have the opportunity to get an 18-50mm f/2.8 lens (also Sigma) for £200 and I'm wondering which I should go for.

The lens will be used for astronomical wide-fields only and no ordinary photography.

The 28mm is half the price, and faster, but the zoom lens is still quite fast and offers a great range of focal lengths.

A question to those who do wide fields - what focal length do you find most useful for this work and what lenses do you use the most?

Cheers

Andrew

P.S. Oh, and quick replies please if possible:D

[Psychobilly]

The wider you go the more distrortion you introduce into the field. For an APS-C sensor the sweet spot distortion wise comes out around 24mm.. I have a 10-20mm zoom and an 18-70...

Primes will generally give much better images than Zooms and are usually faster so you can close down a few stops to imrove the star shapes inthe corners and still be as fast as the zoom wide open...

Peter....

[Andrew*]

That advice'll do nicely. I'll stick with the 28mm f/1.8 then. It seems quite a popular f/l anyway.

Thanks Peter.

Andrew

[Psychobilly]

No Probs...

I have the 50mm f1.8 EF II and need to get a better "prime" wide angle

Peter...