I need to take a long exposure photo of stars at night for a project, and don't know what film speed to use.

Question

My professor said we should use at least 400 speed, but a friend of mine who is into photography said we shouldn't be using that speed, because the stars are very bright against a dark sky. I will be away from the city lights, looking to Polaris to catch the stars rotating around it. I just need to know which film speed is best. Anyone have an idea??

Answer 1

You should probably use 400 speed film, as that's what your professor told you. (If you decide to use something slower, don't tell him as he might decide you're a troublemaker.)

But the REAL answer is, you can't really go by the film speed listed on the box for this.

Film speeds are rated based on exposures lasting a small fraction of a second.

With long exposures, you film reciprocity kicks in. At very long exposures, the paper you print negatives on can be more sensitive to light than high speed film.

This is a major reason why astronomers went to using ccd cameras - they don't have to worry about reciprocity.

100asa film with a lens set at F8 would be fine. Slower film would work too.

Your technique will affect your results more than the film or camera.

Try to get to a dark area away from "Light pollution". In a city, after a few minutes, the background of your sky will start getting very bright. You're also better off shooting under a "New Moon".

Here's a web site with help:

< http://photo.net/learn/astro/star-streak >

Here's one of my astronomy images - 2 hour exposure of M27 using an sbig st2000xm dedicated astro camera:

< http://pic50.picturetrail.com/VOL440/420540/13640785/216797120.jpg >

Good luck with your project.

Answer 2

Your friend probably didn´t realize how you were going to take the photo, by simply leaving the shutter open. The stars come out quicky if the exposure time is long enough. I would say 400 speed should do. Not too slow and not too grainy. I have seen startrail photos taken with 100 speed film.

Answer 3

I have used Ektachrome slide film (200 ASA) in 20 to 60 second exposure, using a 28 mm lens on a 35 mm camera (almost, but not quite a fish-eye) to take wide views of entire contellations. Working at f/2.

I found that 30 second exposures took me to 6th magnitude (showing what we see naked-eye from a dark spot) without showing star trails. The 60-second shots did show the beginning of star trails.

I was on a south-facing beach, on the south side of Barbados (nothing for a thousand miles to the south).

For longer exposures, I would use slower film to reduce graininess. There will be sufficient stray light from the sky to affect the larger grains in the film (larger grains are more sensitive than smaller grains) and slow speed films have less large grains (and the smaller grains are... even smaller).

If you are going to take exposures measured in hours... go slow.

Answer 4

Your professor is right. 400 MINIMUM. Astrophotography is nothing like regular photography so your friend is not well informed on the subject.

Your best results too will be if you take your film to a place that will develop it personally instead of through a fast machine. Tell them that it is astrophotography and they can develop with better contrast for you. It's more expensive but worth the extra effort.

Answer 5

going to record the stars rotating around polaris? that would be a lot of time. i'd call 100 speed or slower if available.

Answer 6

I would say 400 or less, longer exposure, the stars are very bright against the black sky