Exposure times

[poider (Peter)]

I am curious about exposure times...
If I use a 10 mm lens I can expose a star photo for 30 seconds or so, any longer and the sky would be over exposed and too bright.
If I use a 90 mm lens then 10 seconds or so gives dark sky but trails, so if I were to buy a tracking mount It would eliminate the trails but if I leave the shutter open for 1 minute whilst tracking wouldn't that over expose the skyor is it all about taking a series of 30 second exposures with the tracking and then stack them together, if so then would it be just as easy to take a three hundred exposures at 10 seconds instead of 10 exposures of 30 seconds, When opening the shutter for more then 10 seconds or so what role does aperture play??

[Atmos (Colin)]

If you use the same camera in both situations then the aperture doesn't actually determine your exposure time, your F/Ratio does. For instance, a 10mm lens at F/5 means that you have a 2mm aperture, 10mm focal length. If you have a 90mm lens at F/5 you 18mm aperture and 90mm focal length.

What the focal ratio does is determine the amount of aperture at a given focal length and in turn the is the number of photons hitting each pixel. If the F/Ratio remains at F/5, as you increase your aperture or focal length, your 30s exposure time would remain constant but what would change is your resolution.

The reason why we are photographers tend to use 60-3600s individual exposures is that we're trying to get the sky background about the read noise of the camera. To give an example:

If you were to shoot with a standard 50mm lens at F/4 under dark skies you're probably looking at 120s exposures being your optimum time. At F/1.4 (VERY wide open) the sky background will be as bright at 15s as 120s at F/4. At F/9 you would require 600s to get the same background brightness. If you were to shoot with a 300mm at F/4 you would still need 120s to get the same background brightness or 600s at F/9 or 15s at F/1.4 (never seen a 300mm F/1.4 though).

[sil (Steve)]

Peter are you shooting after astronomical dark? before that time over exposing the sky is easy to do but after astronomical dark not so easy. btw at 90mm you should be able to expose for around 5sec without star trails.

Closing the aperture (a larger f number) will increase possible sharpness and reduce lens distortion of stars. Shooting with a camera on tripod is my normal method and stars in the middle of the field of view can be round while lens distortion makes them fan shaped in the corners. reducing aperture this effect is reduced.

light pollution and even over exposed sky can be dealt with in processing, but if your initial test shots look over exposed look at reducing ISO or increasing f stop (google "exposure triangle"). For shooting galaxies or nebulae you want longer exposure times rather than more and shorter exposures so you can collect as many photons as possible and have as much signal to work with in the shots.