This type of photography is not without its challenges, however. Even with cameras that offer very high ISO settings, cleaner images will always come from using a lower ISO and a long shutter speed. The problem arises when the required shutter speed drops below the longest programmed speed of the camera (usually 30 seconds). This is where the “bulb” feature of a camera comes in handy because it holds the shutter open for as long as the shutter button is pressed. (A wired cable release is helpful here, as it can lock the shutter open without needing to keep your finger pressed on the button.)
Unfortunately, since the bulb setting could be anywhere between 30 seconds and three days, your camera’s built-in light meter ceases to become useful. It will show you that you need more than 30 seconds, but how do you know if you need 35 seconds or 35 minutes?
One way is simply to guess and check, but with such long exposures, this can become grueling. Photographer Karl Taylor offers up a simpler technique on PetaPixel. Yes, it does involve a bit of math, but it’s quite straightforward.
As Taylor explains in the above video, take test photos at shorter shutter speeds, raising the ISO and opening the aperture until you get a proper exposure. At that point, dial back your ISO to its base value and stop your aperture down to get the desired depth of field. Next, add the equivalent amount of time to the shutter speed to achieve the same level of exposure.
Taylor goes over the process step by step, one stop at a time, to reveal that his test shot of one second at f/2.8 and ISO 6400 is equivalent to 16 minutes at f/11 and ISO 100. As you become more familiar with exposure values, you should be able to quickly add and subtract stops in your head, but even moving up and down one stop at a time is faster than taking multiple test shots until you get to 16 minutes.