Analyzing Star Trails, Part Three: Color And Intensity

In Part One, we explored the curvatures of star trails. Part Two dealt with the length of the trails. Here, we’ll talk about the color of star trails and how to record the most streaks.

Why are my star trails different colors? It’s a good question. I mean, shouldn’t the stars all be white, just as we see them with our eyes?

Ah, but we don’t see them as all white. Next time you’re out under a dark sky, wait at least 20 minutes for your eyes to adapt to the darkness and then study the sky. If you look with this in mind, you’ll see that the stars are many different colors, ranging from reddish to bluish. And since your camera can reveal those stars much better than you can see them with your naked eye, star photos reveal a lot more of the color. So the reason the star trails have different colors is because the stars have different colors.

Simple, eh? But you want to know WHY they have different colors, don’t you?

Night Waterfall-Star Trails-Light Painting

Nikon D800 and Nikon 14-24mm lens. For the star trails, I shot 52 frames at ISO 200, f/3.5, and a shutter speed of 4 minutes. For the waterfall, I shot separate frames and light painted the water with an LED flashlight and a blue gel filter. For the trees, I used a a green gel filter.

The color of stars comes from temperature. Cooler stars, such as Betelgeuse, appear reddish, while very hot stars, such as Rigel, are blue. The Sun is a medium-temperature star and appears yellow. At first, this might not make sense. A hot star should be RED, right? Red means hot, right? But think about what happens with heat sources on Earth. Low temperature flames, such as those from candles or matches, are yellow, while very hot flames, such as from a welder’s torch, are white or blue.

That’s all there is to it. Cooler stars appear in warmer tones, while hotter stars appear in cooler tones. Except for night photographers, that’s not all there is to it. We want to be able to record those colors as much as possible. Unfortunately, it’s not a simple matter of taking the picture and getting a sky full of nice colorful stars. There are so many things in both capture and post processing that can wipe out the color of stars and make them all appear white. In fact, that’s a big reason why people are often surprised when they see a photo with colorful stars. So many of the night-sky photos they see are full of only white stars.

For star trails, the most important consideration when making the photo is ISO. (I’ll have to save the post-processing talk for later on.) High ISOs tend to wash out the color because they are essentially causing over exposure in the stars. Aperture plays a role, too, but it isn’t as important. I shoot all star trails and pinpoint star scenes at mostly wide-open apertures because, as a rule, I’m trying to bring in as much light as possible in the shortest amount of time. But, of course, at a certain point, too much light comes in and washes out the stars. You could adjust this with the aperture, by stopping the lens down, but with all else being equal, it’s much better to lower the ISO first because lower ISOs give you better image quality, with less noise.

(Astrophotographers apply this basic rule when they shoot night-sky shots of pinpoint stars using longer focal-length lenses on tracking mounts. They shoot a lot of exposures at lower ISOs (400 to 1600, generally) and stack them, which gives them the effect of a higher ISO without the extra noise and loss of color.)

Night Waterfall-Star Trails-Light Painting

Nikon D800 and Nikon 14-24mm lens. For the star trails, I shot 337 frames at ISO 1600, f/3.5, and a shutter speed of 30 seconds. For the waterfall, I shot separate frames and light painted the water with an LED flashlight and a blue gel filter. For the trees, I used a a green gel filter.

Think about what happens when you shoot a star trail. Earth is in constant motion, so the instant you open the shutter, the star begins to streak. The shutter speed at which that streaking becomes obvious in the photo is the subject for the another post. For now, just remember that the star is constantly moving across the frame. For it to record at all, the aperture and ISO combination must capture enough of its light. If you keep the aperture as a constant, say f/2.8 or f/4—a typical setting for stars—the ISO will determine how much of the star’s light will record.

If the star is very bright to begin with, it won’t take much for it to record even at low ISOs of 100 or 200. (We old farts used to shoot star trails using Fujichrome Velvia film. That’s a film speed of 50!) Two things happen as you crank the ISO higher. One is that the star trail appears brighter. The other is that faint stars—those that might not record with lower ISOs—will show up in the image.

Pondering this, you might wonder why you wouldn’t want to crank that ISO on up to 3200 or even 6400 or higher so you can capture as many stars as possible and have them appear as bright as they can. Yes, you’ll get lots of stars—bright stars—if you do that, but they won’t have much color because they’ll be overexposed. Overexpose anything, even a black subject, and the color goes away. With stars, it doesn’t take much for that to happen.

Did you catch the upshot of the statement that, if the aperture remains constant, the ISO determines the star exposure? I didn’t say anything about the effect of shutter speed, because there isn’t any. It’s true, shutter speed has no effect whatsoever in the exposure of star trails. Aperture and ISO control the exposure exclusively. Shutter speed only affects how LONG the star trails will be, not how bright they will be. That’s because the stars are MOVING. They never occupy the same pixel space long enough for shutter speed (beyond a certain point) to have an appreciable effect on the exposure. Shutter speed does affect the exposure of the background sky, so you do have to take that into consideration.

Bodie Island Lighthouse-Cape Hatteras National Seashore-Star Trails

Nikon Nikkormat (I think) and, um, maybe a Nikon 28mm lens. Possibly Fujichrome Velvia film, and I'd say about f/4 and, oh, let's go with about 3.5 hours for the shutter speed.

(Oaky, I know I’m going to hear from someone refuting the above statement. Something about comparing a shutter speed of 10 seconds to 1/500th second and how the stars look in each. Yes, the star will be brighter in the 10-second shot. That’s because 10 seconds is long enough for the star to record all the light available from a given aperture/ISO combination before it moves on to the next pixel, while 1/500th is not. The actual shutter speed at which the sensor can’t record the entire star’s light available from the aperture/ISO combination varies according to focal length and the compass direction in which you shoot. Suffice it to say that if you’re shooting star trails, the shutter speed you use is FAR longer than this, and so it is safe to say that shutter speed has no effect of star trail exposure.)

You really didn’t need those last two paragraphs, did you? Sorry.

There’s another consideration with high ISOs and star trails. I said that as you crank up the ISO, faint stars begin to appear. The higher you go, the more stars you’ll record. Seems like that would be a good thing as long as you can retain the color in them, right? Well, that’s up to you.

Look at the two examples in this article. I shot the individual frames for the first one at ISO 200 and a shutter speed of 4 minutes. The second one was ISO 1600 and a shutter speed of 30 seconds. The aperture was f/3.5 for both, so the total accumulative exposure was the same for both. (ISO 1600/30 seconds = ISO 800/60 seconds = ISO 400/120 seconds = ISO 200/ 4 minutes) But look at the difference in the star trails. The high ISO shot recorded tons of star trails filling up the sky.

At ISO 1600, the stars still show some color, so it’s really just a matter of personal preference as to which way you want to go. Oh, there is one practical consideration, and for me it’s a biggie. In order to get the same length of trails using the high ISO shot, I had to shoot a LOT more frames. The first image is a stack of 52 4-minute exposures. The second one is a stack of 337 30-second exposures! And if you do the math, you’ll see that the 337 frames aren’t even enough to match the trail length in the first shot. I don’t know about you, but I have better things to do with my time than devote hours and hours to post-processing a 337-frame star trail. And besides, I like the look of the ISO 200 shot better, anyway.

The third photo is an old ASA 50 film shot. Notice how much more colorful the stars are in it? And if you look closely, you’ll see that the stars in the ISO 200 shot are more colorful than those in the ISO 1600 image.

And now I’m guessing you’d like some sort of formula or chart to help you determine which settings give you the best-looking stars. Sorry, no can do. I’m afraid there are just too many variables, among them the biggie of subjectivity. You’re going to have to do some testing with your own gear and use your own mind to figure out what the best settings are. But I can give you a general synopsis.

Low ISO/long shutter speed

With a low ISO (100 to 400), you get the more color in the stars and fewer stars show as trails. You also end up with fewer frames to work with in post processing.

High ISO/short shutter speed

With a high ISO (800 to 1600), you get less color in the stars (but you don’t lose all color) and more stars show as trails. You also end up a lot more frames to work with in post processing.

Really high ISO/really short shutter speed

If you go much above ISO 1600, you’ll lose a lot (or all) of the color in the stars and you’ll have a sky sardine-packed with white star trails. You’ll also have to deal with a huge number of frames in post processing.

My preference? I’ve never been a big fan of black and white and I prefer anchovies to sardines.

Show me the color!

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