Milky Way Exposure Calculator

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Use this simple calculator to determine the best exposure to start with for photographing the Milky Way.

 

There are a few things that guide most astrophotographers when deciding which exposure settings to use for photographing the Milky Way. Astrophotography has a lot of variables that will affect what determine the best exposure. Some of them are from the environment: light pollution, moonlight, clouds, etc. and some are in the hands of the photographer: lens focal length, sensor size, minimum f/number, etc.

The calculator here outputs an exposure target of -8 EV which is what I would recommend starting with for most astrophotography in dark sky locations with a regular camera/tripod setup and no fancy tracking equipment. Assuming you don’t have a lot of light pollution or moonlight to deal with, these are very good exposure settings to start with. This calculator was originally featured in my recently updated How to Photograph the Milky Way article for a more complete explanation of the methods I use to make astrophotos.

Making Exposure Adjustments

The problem with this calculator is that it’s a one-size fits all solution so you may need to adjust based on your results. The calculator tries to determine the best settings to produce a neutral exposure. Usually, this means the resulting image may look unnaturally bright (because you expect an image of a dark sky to look dark) but don’t fret, you can reduce the exposure brightness in post processing which also often reduces noise in the image.

If your exposure is unusably noisy, you may need to reduce your Max ISO slightly or enable your in-camera Long Exposure Noise Reduction. If the stars are still streaking as star trails, you may need to reduce your shutter time. Light pollution and moonlight can also make the exposure too bright and a slow lens can end up forcing you to use either too high an ISO or too long a shutter speed for a proper exposure. While the settings suggested by the calculator are what I would use in 90% of my astrophotos, you’ll probably need to make some small adjustments. In order to develop a deeper understanding of how successful the exposure is, we usually cannot rely on what the photo looks like on the back of the LCD.

I recommend enabling the histogram view on your camera. Every digital camera is different, but all of them feature a way to view a graph of the exposure. The histogram is usually available by pressing “INFO” or “Display” or the Up/Down arrows when reviewing photos. It really depends on your camera so check your instruction manual.  Typically we will desire a histogram that shows peaks toward the center of the graph from left to right. Sometimes this is not possible if you are using a relatively slow lens and you may be forced to expose to the left or even underexpose.

See below for examples of histograms for various exposures of the Milky Way, and how to adjust for them.

how to read your camera's histogram

Understanding how to read your camera’s histogram will help you get the best results from your astrophotography.

Try to push your camera to the limits of its light gathering capability without compromising quality. Check and re-check your image review, zoom in on the LCD to check focus, review the histogram for exposure information and re-compose your frame often. Once you find an exposure you like, you can usually maintain the same exposure throughout the night. If you’re consistently exposing to the left or underexposing, you may need to look for a better lens for astrophotography. Check out my guide on How to Pick a Lens for Milky Way Photography or see my best lens lists for Canon, Nikon and Fuji cameras. You can also print out this histogram guide along with the rest of my Nightscapes Quick Guide to Astrophotography.

About the Calculator:

The calculator uses the exact guidelines that I use to figure out my exposures. Here’s what’s happening in the background for all you math-heads.

The shutter speed is calculated based on the focal length of your lens and the size of your camera’s sensor. Longer focal lengths and smaller sensors require shorter shutter speeds to prevent star trailing. This particular calculator uses the equation:

recommendedShutterspeed = 500/(focal length)/(crop factor)

Where (focal length) is your lens focal length (I recommend using your shortest focal length lens) and (crop factor) is the crop factor of your camera’s sensor relative to a full-frame 36mmx24mm sensor. For full-frame sensors it’s 1, APS-C sensors it’s 1.5 and 4/3 sensors it’s 2. I limit the maximum focal length to 300mm because it’s unlikely that you’ll be photographing through a longer lens without an equatorial mount anyway, at which point this calculator becomes useless because you would be able to take much longer exposures.

The calculation is based on the so-called “500 Rule” which many astrophotographers use to determine the shutter speed they should use to maximize light gathering without being long enough to make the stars trail across the sky. Your results will also vary depending on where you’re pointing your camera where photos near the celestial equator will show more star trailing for any given shutter speed. Finally, I don’t account for your camera’s sensor resolution but that’s intentional. At standard sizes and normal viewing distances, a 12 megapixel image looks the same as a 36 megapixel image. After all, most of us can’t look at photos on any more than an 8 megapixel screen anyways so those extra 28 megapixels aren’t making a difference. (4K UHD 16:9 television is 3840 × 2160 = 8.2 Megapixels.)

The f/number should generally be set to the lowest possible number, preferably f/2.8 or lower if your lens supports it. The calculator just uses the minimum f/number rating that you specified for your lens. Lenses with f/numbers of f/4.0 or higher are not recommended because they will force you to use higher ISO, resulting in noisier images. If you anticipate stopping down to reduce comatic aberration, enter the f/number that you will stop down to. Lenses with lower f/numbers are generally better for photographing the Milky Way. Check out my article on How to Pick a Lens for Milky Way Photography for a more complete explanation.

f/number = minimum lens f/number

The ISO is calculated based on your aperture and shutter speed both. It’s extrapolated in stops from a “standard” (-8 EV) Milky Way exposure of: 30 seconds, f/2.0, ISO 3200. For each stop of variation in the f/number, the ISO is adjusted reciprocally one stop to compensate for the change in brightness. Additionally, variations in shutter speed away from the “standard” 30 seconds will adjust the ISO based on an “ISO factor” where the factor is 0 near 30 seconds, drops to -1 above 60 seconds, and increases to 1 below 20 seconds, 2 below 10 seconds and 3 below 5 seconds. These aren’t discreet stops, but ranges that encompass the shutter speed stops of roughly 60 seconds, 30 seconds, 15 seconds, 7 seconds and 3 seconds. The resulting factor is applied as an exponent to 2 and multiplied by the ISO. The calculation stops at a maximum ISO of 409600 which is higher than most cameras can go anyway. The ISO calculation works on a series of nested if-statements which look kind of like this:

recommendedISO = IF (fnumber<=1.4 THEN 1600, ELSE IF (fnumber<=2 THEN 3200, ELSE IF(fnumber<=2.8 THEN 6400, ELSE IF(fnumber<=4 THEN 12800, ELSE IF(fnumber<=5.6 THEN 25600 ELSE 51200)))))*2^isofactor

Where isofactor = IF(recommendedShutterspeed>=60 THEN -1, ELSE IF(recommendedShutterspeed>=20 THEN 0, ELSE IF(recommendedShutterspeed>=10 THEN 1, ELSE IF(recommendedShutterspeed>=5 THEN 2, ELSE 3))))

Adding a max ISO limit that impedes on the standard suggested exposure will automatically compensate with increased shutter time at the reciprocal ratio of the calculated suggested ISO versus your inputted maximum ISO limitation if that limitation is lower than the recommended ISO. A warning notifying you that the longer shutter speed will results in star trails will show if your max ISO limit is lower than recommended.

ISOlimitedShutterSpeed = IF(maxISOLimit<=recommendedISO, THEN recommendedShutterspeed*(recommendedISO/MaxISOLimit) ELSE recommendedShutterspeed)

Finally, the exposure value is calculated with the equation for exposure value. The target exposure for shots of the night sky  is -8 EV. This calculation will tell you the actual exposure value of the calculated exposure settings. It might differ from -8 EV because of the “stopped” nature of the calculator but it should be pretty close to -8 EV if I did my math right.

EV = log2(fnumber^2/recommendedShutterspeed)

If you have any questions about the calculator or astrophotography in general, please feel free to comment below or contact me!

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