camera lens focal length

[timelord (Al)]

Greetings, I,m in the market for a camera lens for my Canon 400D and was wondering what is the maximum focal length suitable for astrophotography ie f/2 3 4 etc.
I realise f/1,2 etc is preferable but as the prices for larger aperture lenses mutiply at phenomenal rates I,m loking for a little guidance.
What is the most popular lens for astro photos? I,ve seen lens prices as low as $250 to $2500 for canon lenses in the 50mm range thats 3 times the price of the camera!

[iceman (Mike)]

Hi timelord. What you're referring to is the focal ratio, not the focal length.

The focal length is the 50mm.

You're right - the lower the focal ratio, the faster the lens, which means more light comes in, meaning you need less exposure times. But good, fast lenses can be very expensive as you've found out.

The best budget fast lens for the Canon is the 50mm f/1.8. You can get these off ebay for $100-$150. They're great for the price, but if you use them at f/1.8 there's horrible abberations around the corners, like your image is going through a timewarp

What focal length were you looking at?

You can easily spend $1000-$2000 on a good L series fast lens, whether it's short or long focal length. They are expensive but by all reports, they're the best.

I've got a Sigma 17-70, f/2.8 which is a good mid-range lens, for the price of only $450-$500. A few IIS'ers use this lens for the astro work.

[[1ponders] (Paul)]

Check this thread out.
Which widefield lense on a budget

[timelord (Al)]

Thanx for the advise guys. looks like the sigma 17-70 might be the way to go in the meantime I'll keep a lookout for a canon L series when my budget allows .
Alex.