Astrophotography

Ok, so I understand the basic concepts of photographing star trails. I imagine

there's more to it than just a long exposure at a larg-ish aperture/faster

film. Can anyone provide any insight into the subtleties and/or technicalities

behind this and other aspects of astro?

I've found when using a DSLR, instead of film, that in order to keep noise low, I have to shoot at low ISO (200), which results in longer exposures. Also my post processing is a little more intensive. There's lots of technical threads on the internet on astrophotography, including several forums devoted to the subject. If you're serious, you may want to do some research. Even the basic stuff like keeping condensation off the front of your lens, having a good tripod or altitude/azimuth tracking devices, clear focus screen (if they are made for your specific camera), cable releases, etc. should be checked out...again if you're seriously getting into this field.

Actually, I misspoke when I said faster film. I shoot a D200. Personally, I don't like using digital for really long exposures. I already have cable releases for my landscape stuff.... I don't know much about viewing screens. My tripod sucks, which is unfortunate as a landscaper.

Hi Christopher, I recently purchased a Gitzo tripod, it was the best investment I've made in a long time. I hope you can get a sturdy tripod soon, you won't know how you managed without it.

There are websites that are dedicated to astrophotography with forums.

I can't remember any of them but just google for them and you'll find

them.

 

You can of course do astrophotography with a camera, lens, and tripod,

but serious people use telescopes with mounts that track the movement

of the stars and laser crosshairs to fix on a guide star to make manual

fine tracking adjustments. Then the exposures can end up being 20

hours (4 different exposures 5 hours each night) and merge them

together using special software just for astrophotography. It can get

expensive quickly and is frustrating if you live in a city with lights.

If you want to get serious about it you need some sort of motor mount that will track the objects through the sky...

Jay, the city with lights won't be a problem, but I don't think I'll ever strive to shoot like what you described. My primary interests in astro are ways of incorporating the night sky into landscapes. One thing that keeps popping up is the idea that star exposure is not dependant on aperture designation, but rather physical size. Can anyone explain that?

<p>Stars are so distant that they are effectively point sources of light, i.e. they have no area. So regardless of focal length, they will always be projected onto the film plane as points. Therefore it doesn't matter how fast (in f-stops) your lens is, but rather, how much surface area (effective entrance pupil) you have to capture the photons from that point source. A larger entrance pupil captures more photons to project onto the point on the film, making it brighter.</p>

 

<p>Of course, a 100 mm lens at f/2.8 also captures more photons from an object with non-zero area than a 50 mm lens at f/2.8 (4x more due to the 4x larger entrance pupil). But this is exactly offset because the longer lens spreads out the image on the film (increasing the linear size by 2x and the area by 4x), so with normal subjects of non-zero area, the f-stop of the lens is what counts.</p>

 

<p>Focal length does matter when shooting stars though, because there is always some background light from the atmosphere, and that light has area, and the brightness of it is therefore dependent on the speed of your lens (i.e. the f-stop used). For example, a 50 mm f/1.4 lens has an effective entrance pupil equal to a 100 mm f/2.8 lens. Therefore the stars from those two lenses will be equally bright points. But the sky will be 2 stops (i.e. 4x) brighter with the 50 mm lens, and this may swamp your stars with background light.</p>

 

<p>You may find <a href="http://www.kodak.com/global/en/consumer/products/techInfo/p150/P_150.pdf">Kodak's guide (PDF file)</a> useful.</p>

One thing you will find, in practical terms, is that those pinpoints of light against an even dark background is that optical aberrations like coma and astigmatism can stand out quite starkly. Even good lenses shot at or close to wide open can give you star images that look like seagulls or UFO's that grow larger as you get away from the centre of the image. As you might expect the effect tends to be more pronounced with wider angle lenses. Closing down a couple stops can have a dramatic effect. Of course this will extend exposures which may mean either going to higher iso's if you were hoping to eliminate obvious star trails, or living with funky shaped stars.

The other thing to keep in mind is that, if you are checking out technical articles about exposure it can make a great deal of difference whether the author is talking about film or digital. Film suffers from reciprocity failure when exposures get longer than about 1 second. Thus doubling the length of the exposure will no longer give you a 1ev increase in the image. How much you'll have to increase the exposure to achieve this depends, among other things, on the particular film you're using. Digital sensors don't suffer from reciprocity failure.

Will coma abate as the lens is stopped down? I can't remember what Ansel Adams said in his books. I really only need the fast aperture for viewing. I need to be able to see EXACTLY what I'm looking at and where in the frame it sits.