Photographing comets is very much like doing other kinds of night sky photography. The basics are the same. Proper focus, high ISO setting, and long shutter speeds. Plus you need to find your subject in the night sky.
You will need a tripod, a camera with shutter speeds up to 30 seconds and/or bulb (“B”), and a lens you can manually focus with a focal length around 40-100mm.
You need a location as far from city lights as possible with an unobstructed view of the northwest horizon. You will also need clear skies, at least near the northwest horizon.
Finding Comet NEOWISE
One hour after sunset, local time, Comet NEOWISE will be in the northwest sky about “1/2 fist” above the horizon. Describing things in the night sky by “fists” is a simple way to tell you how far above the horizon your subject is. One fist is the width of your fist at arm’s length. One fist (depending on the size of your fist) is roughly 10 degrees. Straight up is 9 fists or 90 degrees.
There was so much sky glow last night that I could not see the comet with my unaided eyes. I didn’t see the comet until I took my first photo.
This sky map will help you. On July 14, Comet NEOWISE will be about 1/2 fist above the horizon and below and just to the right of the Big Dipper. Each night after tonight it will get a little higher, a little more towards the west, and more under the Big Dipper. It will also get a little fainter each night.
Camera and Lens Settings
Put your camera on a tripod and point it at the northwest horizon. Start with an ISO of 800 and an lens aperture of f/4. A shutter speed around 15 seconds should get you close to the right exposure. If the first photo is too light, use a faster shutter speed (8, 4 or 2 seconds). If your first image is too dark, go to a 30 second shutter speed. Don’t be afraid to try several shutter speeds so you have choices when you look at your images later on the computer.
As it gets darker you will need to change the ISO to higher settings to keep your shutter speeds from getting too long. So switch to ISO 1600 and then to ISO 3200.
Lens Focal Length
The longer the lens focal length you use, the bigger the comet will be, but you will have less sky area and less stars. A shorter focal length will give you more sky and stars but a shorter comet. The photo at the top of the page was created with a 15mm lens so you get more sky. The photo immediately above was photographed at 70mm lens so you get less sky but a bigger comet.
If this is your first time doing this I suggest a focal length around 50mm.
Focusing on Infinity
This is your biggest technical challenge. Your camera can’t autofocus on the night sky, and at night it is too dark through your camera’s viewfinder to manually focus.
If your lens isn’t focused on infinity, your stars will be little round blobs instead of points of light. So focusing your lens on infinity is important. This article tells you how to do that.
One simple way to do this is to show up early, focus on the distant horizon, turn off autofocus and don’t touch the focus ring for the rest of the night. Don’t touch the zoom ring either.
Don’t forget, if you are using a zoom lens, if you change the focal length you change the focus point. If you are focused on infinity at 50mm and you zoom to 70mm, the lens is no longer focused on infinity so you will need to refocus.
Depending on your lens focal length and shutter speeds the stars will look more like little white streaks (called star trails) instead of little white dots. If the streaks are too long for your tastes, use a shorter shutter speed and a higher ISO setting. So, for example, if you are shooting at ISO 1600 and 15 seconds and the start trails are too long, ISO 3200 and 8 seconds will give you the same exposure buy the star trails will only be half as long. There is a penalty for doing this. Higher ISO settings means you will have more digital noise in your photos.
If you want very minimal star trailing, use the “Rule of 500”. Divide the lens focal length into 500 to get the longest shutter speed you can use to minimize star trailing. With a 50mm focal length the longest shutter speed is 10 seconds (500/50). With a 100mm focal length, the longest shutter speed is 5 seconds (500/100).
For the photo above my focal length was 70mm so the longest safe shutter speeds is 7 seconds. I used a shutter speed of 13 seconds so there is some star trailing.
Comet Photo Data
Top of the page: Canon 5D Mark III camera, Canon EF 15mm lens at 15mm. f/4, 30 seconds, ISO 4000.
Middle of the page: Canon 5D Mark III camera, Canon EF 24-105mm lens at 70mm. f/4, 13 seconds, ISO 3200.