A Practical Guide to Long Exposure Photography
Step 1: Check the weather
A day with a cloudless sky is more suitable for having fun with your family and friends than for a long exposure. Likewise, it can’t rain forever so do not resign yourself to an afternoon with your PlayStation if the weather is not optimal. You should take advantage of satellite images rather than the meteorological sites, trying to figure out if there is an incoming storm, or if the downpour is about to end. To capture a long exposure you need moving objects, so a cloudy sky or a choppy sea is the ideal situation.
Step 2: Visit the location well in advance
Scout the location ahead of time, as you’ll need a lot of time to find the perfect composition (or at least more time than the one needed for a “short exposure”). In fact, in a long exposure, the world is completely different from how you see it with your own eyes. The secret is to try to visualize with your mind how the subject will move in the scene, in order to take advantage of the leading lines and contrasts that they will create. Try not to put the sun into the composition because its movement will ruin the shot and it will create an area of overexposure that is not recoverable. If you cannot avoid the sun, wait for it to hide behind a cloud.
Step 3: Use a tripod
As soon as you roughly define your composition, mount your camera on a tripod. We need a tripod because even just a few seconds of exposure will result in a blurry image if we hold the camera with only our hands. Install all the accessories such as the remote shutter release and, if you use slot-in filters, the filter holder. However, do not install your ND filter yet: this is crucial to achieve a great long exposure.
Step 4: Compose the image and lock focus
Refine your composition, focus on the subject and lock the focus. If you are using manual focus, just focus, but if you are using the autofocus mode, you should focus by half-pressing the shutter button, and once the focus has been made, while still holding down the shutter button halfway, push the lever from Auto Focus to Manual. In this way, your camera will maintain the focus (or alternatively you could use back-button focus).
Step 5: Set the exposure
Now set your camera to Manual (M) mode or Aperture Priority (A/Av) mode. Then set the aperture to an appropriate value for the scene (for landscapes I suggest between f/8 and f/11) and take a “Test Shot”: take a picture as if you wanted to capture an image without any long exposure effect.
The test is complete when you get a “correct exposure”. To determine if the exposure is correct, check the histogram, your best friend while shooting on the field (do not trust your display, it is too bright). It is true, there is no universally correct histogram, but there are histograms that are universally incorrect, namely moved completely to the right (the image is overexposed) or completely to the left (the image is underexposed). Once the test shot is successful, write down the shutter speed you used for that test shot.
Step 6: Add your filter
It’s finally time to use our ND filter, so let’s install it in on our camera. If the filter is very strong, for example a 10 stop ND, you will immediately notice that you’re no longer able to see the scene through the viewfinder or the Live View. Do not worry, because if you have followed the guide up to this point you will notice that we have already made the composition, and the focus too! You are blind, but your camera will see everything perfectly.
Step 7: Switch to Bulb mode
On your camera set the shooting mode to Bulb (B) in order to take over the thirty-second limit of the camera. Do not change any of the other settings (ISO and aperture) used in the test shot.
It is finally time to take our long exposure shot. But how long should you leave the shutter open? It is less difficult than you might expect. First of all, recollect the shutter speed that you noted from the “Test Shot” you did in Step Five above. Now you must compensate by the number of stops introduced by the filter, doubling the time for every stop introduced by the filter. For example, if your test shot was ½ second, adding 10 stops will get a shutter speed of approximately 120 seconds. There you have your shutter speed.
No need to be stuck in math: on the internet, you can easily find conversion tables and applications for your smartphone that will do the conversion for you!
Step 8: Check the histogram again
Once you’ve taken the shot with the calculated shutter speed, check the histogram again. If the new histogram is approximately equal to the one of the test shot, mission accomplished and congratulation: you achieved your first long exposure image!
If it is shifted too far to the right or to the left, don’t worry, probably it’s just because the light condition changed or because the filter density is not so accurate. We have just to apply an adjustment: if the histogram is too much shifted to the left (and so the image appears a bit darker than expected) repeat the shot increasing the shutter speed value. On the other side, if the histogram appears too shifted to the right (and so the image is too bright than expected), repeat the shot decreasing the shutter speed.
Easy, isn’t it? Now fill your backpack with your camera and filters and go to practice in the field. For any doubt or if you need any help, don’t hesitate to ask questions!