Exposure question

[nickcowen]

If a correct exposure is 4 minutes iso 400 at f2.8 what would the exposure time be if the lens was at f16? Do you double the time per f stop? E.g. f4 8 minutes ?

[markmullen]

128 minutes, assuming you're on digital, longer on film to allow for reciprocity failure.

If you dropped back to ISO100 you're looking at 512 minutes, or nearly 9 hours.

Are you carving a niche photographing black cats in coal cellars?

[nickcowen]

No I was thinking of doing star trails with film just to finish a roll of

[markmullen]

The film box should have a reciprocity table inside, I'm not sure it will go that high though. You'll need somewhere very very dark for that length of exposure without light pollution.

[nickcowen]

Well maybe not star trails but at least star shots, I'll have a play and see if I come out with anything

[RobDickinson]

Anything longer than 30 seconds or so will give you star trails.

ISO 400 isnt high enough for good wide field non trail shots.

[DIW35]

For star trails or shots of stars in general, there's no point in stopping down to f/16. A lot of the stars out there are very faint, and using a wide aperture helps to capture some of the fainter stars. As has already been mentioned, really long exposures will result in a very pale sky due to light pollution, and reciprocity problems when using film can make getting a long exposure right quite difficult.