Milky Way Exposure Calculator

milky-way-exposure-time-calculator

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

Calculator

It’s not perfect for all situations or gear but the calculator here should give you a good place to start with your settings for shooting the night sky. If you’re new to night photography, you can start with the recommendations here and adjust to your preferences.

 

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.

Shutter Speed

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.)

A More Accurate Shutter Speed Calculator

One of our readers and fellow astrophotographers, Thomas Leber, has made an excellent calculator that can more accurately account for your sensor’s pixel density and output a recommended shutter speed bases on your personal threshold preference, in pixels, of how much star trailing you can accept. See his excellent writeup of the methodology and use his shutter speed calculator here.

F/Number

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

ISO

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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22 Responses

  1. Thomas Leber January 9, 2015 / 7:09 am

    Hi!
    Thx for this great tutorial and the nice calculator! I used it a lot and it helped me to make nice pictures for myself and my blog.
    After reading of the post I was all the time thinking that the rule of thumb for the shutter speed is a bit in accurate. I tried an other approach on my blog.
    Maybe you can find the time to review this approach.

    http://www.tl-photography.at/stars/shutter-speed-calculator-for-night-sky-photographing/
    http://www.tl-photography.at/stars/night-sky-photography-shutter-speed-calculator/

    Best,
    Thomas

    • Ian Norman January 9, 2015 / 2:41 pm

      Thomas, I read your article and liked it a lot. I think it makes a lot of sense. Taking into account the pixel density of the sensor is definitely a great way to back calculate the shutter time. I also like that you can use a threshold variable to give some leeway for movement. It’s the threshold that interests me more: what threshold values relative to the whole image size (in percentage perhaps) is acceptable for normal viewing? I think that being able to suggest a commonly accepted threshold level for standard viewing distances would be helpful. Especially with some of these new very high resolution sensors (like the Nikon D810 or Sony a7R), there almost always will be some star trailing, even at very short shutter speeds.

      I think that’s why the one-size-fits-all method of the 500 rule is OK as a best fit guide. A test shot, check and re-adjustment is then all that’s necessary.

      Also, I like your assumption of the sky as a cylinder but I’m wondering if we can do it one step greater and actually assume it’s a sphere. This would require inputting the latitude of the framing of the center of the image, and basing the calculation on the arc sweep of that particular latitude. In this regard, shots made closer to the poles will be able to use longer shutter speeds (just like in real life). This, however, would only be most useful for longer lenses where more of the whole of the image is on a similar latitude so maybe it’s better to just assume a cylinder. Just a thought…

      Overall, I really like it. I’ll link to your calculator in the article above.

    • Thomas Leber January 18, 2015 / 4:11 am

      Hi Ian!
      Thx a lot for the feedback and the link!

      A general percentage would be nice, but I think there is a lot of personal meanings about that, and it is heavily depending on the targeted end size of the picture.
      The idea I had was that you choose the threshold, and then the minimum mega pixel value you can life with. Let’s say you have a Nikon 810 and you targeted MP are 20. Then the calculator will give you a worst case value. As you said, the factor where you pointing at (north, south) influences the maximum time too. So the calculator ensures that, no matter in which direction you point, the shutter speed is not to low.

      But i’ll keep an eye on that and will play a bit with the thresholds. :)

      Your idea of a sphere is nice. In the beginning I thought also about a sphere, but since the exact coordinates and the pointing direction is not so easy to put in an web calculator, I used the cylinder.. :
      But, a friend of mine is programming an Open-Source Android App right now, there may will be the sphere implemented.

      Thanks a lot again! And continue with this really nice work here! After your review, i ordered today the Samyang 24mm 1.4. Hopefully mine will be ok a the first shipping. I also will go with an Focus Chip. Let’s see!

      Best Thomas

  2. Dale January 6, 2015 / 8:50 am

    Calculator’s not working anymore. Also, there’s a javascript error “TypeError: image is undefined” at scripts.js: 73.

    • Ian Norman January 6, 2015 / 11:13 pm

      Dale, I think I found the javascript error and fixed it. Seems to be working fine on my end. Might require a cache clearing on your browser but I hope it’s fixed for you too.

  3. Krzysiek December 18, 2014 / 2:32 pm

    How come, this calculator shows that i need shorter exposure for APSC sensor than Full Frame with the same other settings?

    • Ian Norman December 21, 2014 / 11:05 pm

      The calculator figures shutter speed based on effective focal length. The longer the effective focal length, the smaller the field of view and the shorter the exposure needed to reduce the effect of star trails. This is, of course, just a starting guideline.

  4. James April 24, 2014 / 2:08 am

    Off to Boa Vista later on this year which has very dark skies. Taking my 6d to hopefully capture some great shots of the Milky Way. Thank you so much for this great guide!

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