# Thread: Working out how long an exposure can be without getting star trails?

1. ## Working out how long an exposure can be without getting star trails?

I would like to photograph a building at night (4x5 bw film) but I don't want to have star trails or the moon move. Is there a formula I could use to work out how long I can keep the shutter open without getting apparent movement on the film? I presume the focal lenght and the desired restriction on translational movement at the film plane would be important factors?

2. ## Re: Working out how long an exposure can be without getting star trails?

The earth rotates one revolution (360°) every 24 hours. That's 15° per hour, or 1/4° per minute, or 1/240° per second.

Take a look at the angle of view of your lens and see how that compares with 1/240°.

- Leigh

3. ## Re: Working out how long an exposure can be without getting star trails?

Not really possible if it's dark enough to have the stars show up w/o additional lighting of the scene (buildings, etc).

4. Take a shot at twilight slightly underexposed and then double expose another shot when it's dark for the sky might work?
Erik

5. ## Re: Working out how long an exposure can be without getting star trails?

It depends a little bit on how far from the celestial pole you're pointing, how much enlargement the negative will have, and on how tolerant you are of dots looking like tiny sausages. My own personal "rule of thumb" for 35mm film is eight seconds. Longer than that and it annoys me (unless trailed stars were what I was after in the first place.)

Mike

6. ## Re: Working out how long an exposure can be without getting star trails?

Thank you all for your replies, doesn't a formula exist? I would be surprised if there is not one..

7. ## Re: Working out how long an exposure can be without getting star trails?

Originally Posted by l2oBiN
Thank you all for your replies, doesn't a formula exist? I would be surprised if there is not one..
Leigh gave you the formula in the second post.

8. ## Re: Working out how long an exposure can be without getting star trails?

Wouldn't it be possible to bring along a digital camera that has a zoom lens with an effective focal length equal to the focal length of the lens on your large format camera, make a few photographs at different shutter speeds, look at the results, find the one that works best, and use the same shutter speed on your large format camera? Assuming you worked fairly quickly I would think this would work but then astronomy, math, and similar subjects aren't my strong point so maybe I'm missing something.

9. ## Re: Working out how long an exposure can be without getting star trails?

A formula would help, but why not go out somewhere and do some practice shots and bracketing? Go again to account for variables of weather, haze, sky, etc. You might also try out different films. When it’s time to take your real shot, your notes from the field will be better than a formula in a book.

10. ## Re: Working out how long an exposure can be without getting star trails?

How about making the photograph on a cloudy night when the stars and moon are obscured?

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