Tips

# Long Exposure Calculations

209 Seconds, by Bryan Davidson

A photograph shows a finite slice of time and it’s up to the photographer to choose how large that slice is.  A small slice can freeze time to show the details of a hummingbird’s wings or the sculptures created by a drop of water.  A large slice can show movement over a long time period compressed into a single frame.

Long exposures are challenging, but they can create images with tons of dramatic impact because they show the viewer something he or she can’t see in real life.  The problem is, most cameras, even DSLR’s don’t meter accurately in the dark and they will only give shutter speeds down to 30 seconds.  Luckily, you’re using a digital camera, so you can experiment for free!  So, simple, just set your camera to bulb mode (where the shutter is open for as long as the button is pressed), grab a stop watch and start experimenting, right?

Well, that could be very time consuming and frustrating.  Remember that an increase of 1 stop of light requires a doubling of exposure time.  That means if your 5 minute exposure is wrong, you may have to try 10 or 20 minutes to get it right.  On top of the exposure time itself, most cameras perform something called “dark frame subtraction” which helps lower noise on long exposures by taking an image with the shutter closed for a time equal to the previous exposure.  So you take a 5 minute exposure and then your camera takes another 5 minutes after that to become ready again.  You see how the time could add up fast?  There must be a better way.

Luckily, there is.  Again thanks to the wonders of digital photography, you can set the ISO speed to whatever you want, whenever you want.  Try this: crank up the ISO as high as it will go and then take some test shots at shorter exposure times.  You probably don’t use these higher ISO’s too much because the resulting pictures often look like crap.  My camera goes to ISO3200, but new cameras go to ISO12800 or higher.  Once you’ve honed in on the correct exposure, it’s time to do some math.  You want to aim for the lowest ISO possible because you’ll get more dynamic range and less noise.

1. Memorize or write down the correct exposure you found while experimenting (i.e. ISO3200 f/8 10 seconds)
2. Divide the ISO value by 2 and multiply the exposure time by 2 until you get to the desired ISO (preferably 100 or 200).  For the example above, this equals ISO100 f/8 320 seconds (5 minutes 20 seconds).
3. Set your camera to “bulb” mode (should be marked with a ‘B’, but if you can’t find it, check the manual) and set the ISO and aperture to the results from step 2.
4. Using a remote shutter release and a stopwatch, keep the shutter open for the amount of time you calculated.
5. Wait for what seems like forever until the dark frame subtraction is done.
6. Print!  (Haha, who prints pictures anymore, what is this 1931?  Post to facebook.)

If the time you compute in step 2 is just too long, you can also open up the aperture by 1 stop instead of doubling the exposure time for one jump down of ISO.  Remember that a smaller f-stop means a larger aperture (more light).  The list of whole f-stops is: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32.  Keep in mind that you’ll get the sharpest results out of your lens by setting the f-stop about 2 stops above the minimum (i.e. for an f/3.5 lens, f/8 – f/11 will be sharpest).  You can experiment during the day to get to know where your lens performs the best.

I hope this tip helps save you some time next time you’re out doing some night photography.  Don’t forget to pack an extra battery because the cold temperature and the longer exposures will both eat away at your battery life.  If anyone has any other long exposure tips to add, feel free to leave a comment.