Is f4 enough

I am wanting to shot some astro photography and my wide lens is the canon 17-40 f4. Is it enough or do I need a faster lens?

What are you attaching it to?


But I'd guess that you are going to be disappointed with IQ regardless.

Faster lenses will always help but current sensor technology with higher usable ISO has come a long ways. Lenses that couldn't perform in the past has been given new life with modern sensors. What body are using?

I will be using a Canon 6D

Much depends on what you are looking to do but generally f/4 is fine. Just compensate with higher ISO or longer time or both.

If your shooting in a very dark place the f/4 will have a hard time on very short exposures unless it’s mounted on a tracking mount and aligned properly . But if your on a stationary tripod you’ll want exposures less than 20-secs unless your doing star trail images . F/2.8 or wider is highly suggested with shorter exposures cause you can use lower ISO to help minimize noise .

F4 will get you started. 2.8 or faster will be better but with your 6d you are at a good start. Wide field Astro is gear dependent but there is a ton to learn and the setup you already have will get you started. If you feel you need faster you can always get a rokinon 14 2.8.

Late reply but if you're willing and able to stack images for noise reduction, you'll be fine at f/4. Just be ready to take more images.