Today, we're diving into the nitty-gritty of camera settings. Remember - your camera is a time machine, so understanding ISO, aperture, and shutter speed is key to capturing stunning astrophotography shots.
First, throw out most of what you remember from daytime, terrestrial photography. Most of it doesn't apply when you're taking 10, 20, or 30 second exposures of the night sky. We don't care about matching skin tones, we'll never use flash, and we don't need to stop the action. Stars move v e r y s l o w l y so even if we took a 100 year exposure, we would maybe only see a pixel's worth of movement. In short, we're going to do things a little differently when we take pictures of the night sky.
Let's start with ISO. This controls your camera's sensitivity to light. For astrophotography, you'll usually want a somewhat high ISO, around 400-800, to capture the dim light from stars. All those photons can get pretty faint after traveling for millions of years. Like all things astrophotographic, though, there are tradeoffs. My Canon 80D has an ISO setting that goes up to 16,000 natively or 256,000 in "expansion mode." But higher ISO doesn't change the number of photons that hit the sensor, it just changes the sensor's sensitivity to those photons. Higher ISO values cause more noise in the image, and noise is the enemy of an astro image. So there must be a balance.
Some people will talk about the "native ISO" of a camera, but we can generally just set it to 400 or 800 or maybe 1600 and call it a day. Or a night, I guess. At this point, you're going to use Software to combine multiple images anyway, so ISO isn't really all that important. Just keep it as low as you can for best results.
Aperture is the opening in your lens that lets light in. For night shots, you want a wide aperture, like f/5.6 or lower, to allow as much light as possible. Don't worry if your lens isn't that fast; just use the fastest - or widest - aperture you have. Remember - smaller aperture numbers like 1.4, 2, and 2.8 are larger aperture openings than larger nubmers like 4.5, 5.6, and 8. Don't stress about aperture too much. Just remember than if you're going to take a 10 second exposure at f/5.6, you can take a 5 second exposure at f/2.8. So again, there is a balancing act. Sure, you can use f/16 on that 400mm/f5.6 with a 2X teleconverter and a 1.4X teleconverter stacked, but that 5 second exposure just became 40 seconds and if you don't have a tracking tripod, you just made star trails instead of stars.
Finally, shutter speed. This is how long your camera's sensor is exposed to light. For stars, a shutter speed of 5 to 10 seconds is ideal. Longer than that and you'll get star trails instead of stars. Unless that's your goal, in which case you want 30 second exposures or longer. If you're taking Milky Way shots, this is about right. If you're trying to take pictures of the Moon, you can use your daytime photography skills and take a 1/400 second exposure and adjust from there. If you're trying to take pictures of galaxies, then you're sort of using the wrong gear, but you'll be taking dozens or hundreds of 10 second exposures and constantly changing where your camera points; this is a topic for more advanced astrophotography.
Combine these settings to get the right exposure. It might take some experimenting, but that's part of the fun! Most time machines - I mean, cameras - have a setting called bracketing where the camera will automatically take a series of images with different exposure settings. Go ahead and try this if your camera has it, and see which exposure setting gives the best results.
In the next chapter, we'll talk about focusing in the dark and some tips for getting sharp images. Until then, clear skies, happy shooting, and I'll see you...in the dark!