Photography question

I'm sure there are plenty of photography nerds here. I has a question for you, or great ones.
I want to take some of those cool night sky pics of tonights meteor shower. I have a Nikon D60. I can get the shutter speed to stay open for about 30 seconds. I don't have a bulb thingy so I can't set it to bulb mode. What other settings am I looking for? I read that I should focus on the moon to get as close to infinity as possible, but what about other things like f stop and ISO?
I'm going to set it up to take 30 second pics back-to-back if I can.

I assume you're going to mount it on a tripod? If so, then note that 30 seconds is long enough that you'll get star trails in the resulting photos. If you want to avoid that, you need a motorized sky-tracking mount.

For astronomy, aperture is everything. You want the lens as wide-open as possible to gather as much light in as short a time as possible. The usual undesirable side effects of wide aperture are less relevant, since you're focused on infinity you're not concerned with depth of field, and the typical image quality issues that show up running the lens wide open are less of an issue with stars.

You probably also want a high ISO and a shorter exposure, because sensors are subject to thermal noise and the longer you keep it open, the more noise you get. Turning up the gain via higher ISO settings will usually give better results (up to a point). This is somewhat unintuitive, because at the shorter exposure that are more common for other types of photography this effect doesn't show up. Experiment and see what produces the best photos, that's the great thing about digital. :)

Thanks codrus. Yes, I want the star trails and the meteor streaks. I figured to open the aperture up as much as possible, and I just figured out how to do that. I'm glad you mentioned that because I wasn't sure.
So, you are saying incrust the ISO and reduce the exposure? Can you give me an example?

Thanks.

I would think oncreasing the ISO too much would cause increased noise beyond that of long exposures. I think ISO 800 on that camera would be a good compromise.

You should also be able to take multiple shots and "stack" them for more star trails and meteors. Just leave the camera on the tripod and don't move it.

bastomatic wrote: I would think oncreasing the ISO too much would cause increased noise beyond that of long exposures. I think ISO 800 on that camera would be a good compromise. You should also be able to take multiple shots and "stack" them for more star trails and meteors. Just leave the camera on the tripod and don't move it.

30 second exposures introduce significant noise into the image, generally more so than pumping up the ISO gain. The precise crossover point is going to vary with the camera and conditions, which is why I suggested experimenting with it.

Dr Boost: As for an example, the simplest way is to think of the sensor on your camera as a bucket that's collecting water. If you want a certain height of water in the bucket, you have three ways of getting it. You can open up the faucet further so that more water comes out -- this is analogous to opening up the aperture on the camera; you can hold the bucket under the faucet for a longer period of time -- analogous to increasing the shutter speed; and finally you can put a narrower bucket inside the first one so that you get to the desired water level with less water -- this is the ISO.

Everything is expressed in terms of "stops", which is a doubling of the light input. Increasing the aperture (F.stop) by 1.4x doubles the light, such as going from f/4 to f/5.6 (because aperture is a measure of radius, and light comes in based off area, area is related to radius squared, and 1.4 is approximately the square root of 2). Doubling the length of the shuttler will double the light (going from 1/100 to 1/50, for example), and doubling the ISO value (200 to 400) doubles the sensitivity of the sensor.

Each of these has other effects on the image too. A wider aperture results in less depth of field, a longer shutter time results in motion blur in the image, and increasing the ISO increases noise. Generally you want as low an ISO value as you can get (because there's no real value in noisy images, they just look bad), but you often trade off aperture vs shutter speed to get a desired effect. Imagine a given image is correctly exposed at ISO 400, shutter 1/100, aperture f/5.6 -- if you want to get a shallower depth of field, you could open up the aperture to f/2. That's 3 stops more light, so to compensate you would increase the shutter speed to 1/800. Alternately, you could reset it to ISO 100 (2 stops) and go with a shutter speed of 1/200 (1 stop) to get those same three stops. If you wanted to lengthen the shutter speed (say you're shooting a race car and you want the wheels to be blurred in the image) then you would go the other direction in all of those.

There's a great book on this topic called Understanding Exposure, which has lots of images in it to demonstrate the concepts.

So where are the pictures??

I didn't read those last two posts (before SVreX) until this morning. We went out earlier than was optimal and the pics have noise because I had the ISO and shutter settings all out of whack. I think I picked up two, but we were out 2+ hours before optimal viewing so it was kind of a dud. Eager to hear of Aussie was able to see much down in OH.