Camera lens diameter

[garypearson]

Another beginners question, which I hope someone can answer.

When calculating an exposure for taking photos of stars the lens aperture is important.

I want to take some wide field photos tonight.

My lens is a 10-20 mm f4-5.6 zoom.

The filter thread is 77mm.

For calculation of exposure time at 15 mm would I use aperture = front element size = 77 mm or 15 mm / f4 = 3.8 mm?

Obviously it will make a big difference!!!

Gary.

[Kaptain Klevtsov]

No not really Gary. The 15mm f/4 bit is all that you need, the filter thread is a lot bigger as its a wide angle lens and you don't want the filter in the frame as it would crop the corners.

Set the ISO to 400 or 800, stop down to one stop off fully open (f/5.6 or f/8) and try 30 seconds and see what you get. If you go for too long an exposure you'll get star trails, which are pretty, but maybe not what you want.

With astrophotography, its a try it and see thing. Not a problem with digital as the processing bit is free and there's no cost for the film.

Just have a go and see what you get, then adjust and try something different. It'll all work out when you figure out the settings that you need.

Kaptain Klevtsov

[garypearson]

Thanks,

I have just taken an hours worth of subs based on the longest exposure time I felt I could get away with for trial-free images. I am planning to combine them.

Although each sub has trial-free stars, the 40 or so that I took mean that from sub to sub the stars move. Can software like Registax rotate and shift the subs so that they line up, before stacking?

Gary.

No not really Gary. The 15mm f/4 bit is all that you need, the filter thread is a lot bigger as its a wide angle lens and you don't want the filter in the frame as it would crop the corners.

Set the ISO to 400 or 800, stop down to one stop off fully open (f/5.6 or f/8) and try 30 seconds and see what you get. If you go for too long an exposure you'll get star trails, which are pretty, but maybe not what you want.

With astrophotography, its a try it and see thing. Not a problem with digital as the processing bit is free and there's no cost for the film.

Just have a go and see what you get, then adjust and try something different. It'll all work out when you figure out the settings that you need.

Kaptain Klevtsov

[Kaptain Klevtsov]

You need to get Deep Sky Stacker and have a play.

http://deepskystacker.free.fr/english/index.html

Don't ask me how to work it as it hates me, but it does really work well.

Kaptain Klevtsov