Help: star shots

[Jonbar18]

I have been trying to get some nice shots of a starry sky. I don't have the opportunity to try now, as you can't see many starts in Toronto, but i'll soon be going on vacation and would love to try.
Anyways, my guess would have been a long exposure, but I have tried 30 sec and the sky still turned out blank.
TIA

[nemesis256]

this link is very useful

Photographing Star Trails

[ElCapitan]

Are you looking to to do star trails, detailed shots, or both? That makes a lot of difference in what to do. Also will you be shooting digital or film?

[dcclark]

I assume you'll shoot digital (since this is DPS, but you never know!). My first guess is that your exposure is way off. A starry sky is VERY dark, in photographic terms. To get stars to show up at 30 seconds, you'll need a very wide open aperture (f/3.5 at least, f/2.8 or more preferable!). You'll also probably need a higher ISO (at least 200, probably 400). If you're trying this in the city still, you may never be able to get more than a couple stars to show up -- you really need no light pollution.

If you want to take "still" shots (just stars, no motion -- no trails), then you'll need to open up your aperture as much as possible. Even f/1.8 isn't out of the question, but you'll have to work on getting focus all the way out to infinity (which can be tricky if your lens doesn't have a distance scale). You may also want to bump your ISO up fairly high. Then, 30 seconds is probably as long as you want to go -- even at that length, you'll see motion in the stars!

Now, what if you want star trails? In that case, you'll have much longer exposures. Try going into bulb mode, and shooting for at least 1 minute to get short trails. In this case, you can decrease the ISO (a good idea, to avoid too much noise), and perhaps close your aperture a bit -- but not too much, or else you'll black everything out again. My preferred method is to take a large number of 30 second (or 1 minute) exposures, and then "stack" them with photo stacking software. This creates a single image out of many similar images, letting the brightest parts (the stars) show through. (To see the results of that last method, check out my Quincy Boiler Stars).

[Jonbar18]

Originally Posted by dcclark

I assume you'll shoot digital (since this is DPS, but you never know!). My first guess is that your exposure is way off. A starry sky is VERY dark, in photographic terms. To get stars to show up at 30 seconds, you'll need a very wide open aperture (f/3.5 at least, f/2.8 or more preferable!). You'll also probably need a higher ISO (at least 200, probably 400). If you're trying this in the city still, you may never be able to get more than a couple stars to show up -- you really need no light pollution.

If you want to take "still" shots (just stars, no motion -- no trails), then you'll need to open up your aperture as much as possible. Even f/1.8 isn't out of the question, but you'll have to work on getting focus all the way out to infinity (which can be tricky if your lens doesn't have a distance scale). You may also want to bump your ISO up fairly high. Then, 30 seconds is probably as long as you want to go -- even at that length, you'll see motion in the stars!

Now, what if you want star trails? In that case, you'll have much longer exposures. Try going into bulb mode, and shooting for at least 1 minute to get short trails. In this case, you can decrease the ISO (a good idea, to avoid too much noise), and perhaps close your aperture a bit -- but not too much, or else you'll black everything out again. My preferred method is to take a large number of 30 second (or 1 minute) exposures, and then "stack" them with photo stacking software. This creates a single image out of many similar images, letting the brightest parts (the stars) show through. (To see the results of that last method, check out my Quincy Boiler Stars).

I'm using film, and I will be doing this outside the city.
I was considering the multi-shot technique, but after seeing it took 140 shots...I'd be broke!
I guess i'll just stick into into bulb mode and see how it goes. I'll share as soon/if it happens!
THX

[dcclark]

Yeah, on film, you'll have fewer problems. Calculate a proper exposure for LV 0 (light value). You can leave the shutter open just about as long as you'd like!

[Jonbar18]

Originally Posted by dcclark

Yeah, on film, you'll have fewer problems. Calculate a proper exposure for LV 0 (light value). You can leave the shutter open just about as long as you'd like!

Good old fashioned film!
So what would be a reasonable length for F/3.5?

[dcclark]

What's your ISO/ASA?

[Jonbar18]

Originally Posted by dcclark

What's your ISO/ASA?

200
200 letters :S

[dcclark]

Originally Posted by Jonbar18

200
200 letters :S

OK, this will be fun. Let's do some calculations with light values and exposure values! (Check me, I could be wrong.)

We're going to use LVs (Light Values) and EVs (Exposure Values). Each one is a single number which represents an amount of light. LVs are independent of ISO, EVs are not. EV 0 is defined as the same thing as LV 0 at ISO 100.

First, I made a mistake -- Light Value 0 (LV 0) is not starry sky -- I'm going to guess that you need, perhaps, LV -5. I could be a bit off on this, but it's a good starting point.

We'll convert LV to EV (exposure values):

LV 0 is defined as the same thing as EV 0 at ISO 100, so LV 0 = EV 1 at ISO 200.
That means that for you, LV -5 is the same as EV -4, or in other words, 4 stops darker than EV 0.

EV 0 is f/1 for 1 second, so EV -4 is an exposure 4 stops darker than f/1 for 1 second. First, f/1 is not so useful... let's bump it up to f/2.8, which is 3 stops darker that f/1. That means now that we need EV -7 (3 stops darker).

So we currently need an exposure 7 stops darker than f/2.8 for 1 second. Doubling the shutter speed is the same as increasing the EV by one stop, so we need 2^7 = 128 seconds.

So, it looks like you need about 128 seconds (say, about 2 minutes) at f/2.8, with your ISO 200 film! Note that that will definitely show star movement. If you were able to go to f/1.4 (2 stops faster), you could cut the shutter speed down to 32 seconds.

I'm looking for help from others to verify my calculations... these always mess me up.