There’s something absolutely magical about long exposure photography.  Rough water and waves turn to smooth patterns, cars vanish, and choppy clouds turn into smooth streams in the sky.  I love using these types of shots for architecture – all the distracting details disappear and you can feel a sense of motion.  But setting the right exposure can be tricky for several reasons.  First, if you need to use a neutral density (ND) filter to get a long shutter, your camera’s meter probably isn’t up to the task and you get exposures that are far from perfect.  Second, trial and error is extremely painful when each exposure takes several minutes.  You don’t want to be doing that over and over!

I use a very simple trick to find the right exposure:  I find the best exposure using a test shot at high ISO, and shoot my final image at the right ISO.  It’s dead simple.  If you need 4 seconds at ISO 6400, then you need 4 minutes at ISO 100.  My basic process is:

• Compose the image and focus on a tripod before putting an ND filter on the camera (so you can see what you’re doing).
• Once you’ve added the ND filter, set the ISO to 6400 and use the meter to get a starting shutter speed.
• Take a test shot, evaluate the histogram, and try a new shutter speed as needed.
• Once the histogram looks right, switch from ISO 6400 to 100, and use convert whatever shutter speed you had in seconds to minutes

You can make small variations on this as needed.  For example, if I want to go even longer on my D810, I’ll click my ISO down to 64 and bump up my exposure by 2/3rds of a stop from there.  In the video below, I show the whole process, but first I’ll add a few more general tips:

• Get high quality neutral density filters.  Even if you can get the exposure at f/22, you’re losing image sharpness and bringing out a lot of dust spots with such small apertures.  I like to shoot at f/8-f/11 as much as possible.  See my gear page for a list of the filters I recommend.
• I strongly recommend getting a digital bulb timer, such as the Nikon MC-36 Remote Shutter Release.  Manual shutter releases are cheaper, but I’ve had to redo many shots when I didn’t stop the exposure in time – plus it’s more relaxing to not have to watch the clock so carefully.