A Basic Guide to Long Exposures

Creating interesting effects with an ND filter

By jgmurphy88 in Articles and Tips on Jan 22, 2015

Long exposure photography is a trend on the rise, with a lot of people trying out this technique now that the tools to do so are becoming more accessible. Long exposures have gotten a lot of attention in landscape magazines, and on photo sharing sites. Long exposures are a really unique effect achieved by letting your camera shutter remain open to soak up light, which has the added effect of capturing movement if any parts of the image aren't stationary.

When to Shoot

Long exposures really shine in a couple areas: capturing things in motion, and enhancing lights already present, bringing out even dim sources. There are a few subjects that always make for good shots. Long exposures can make water look like mist, streak clouds across the sky, and leave trails with any kind of light in motion. Car headlights leave bright trails across the image, and with a long enough exposure you can actually capture the stair trails as the Earth rotates.

A long exposure can also have the side effect of making people disappear from your photos, if they quickly pass through the image. Across a 30 second (or longer) exposure, they won't really show up. Long exposures really shine at night, as they can compensate for the general lack of light around you. Look for things in motion. If you're shooting architecture and you have a cloudless sky, a long exposure won't do much for you.

How to Shoot

Once you've found a good subject, set up your tripod.  Make sure the camera is well-secured, and weight your tripod if necessary. To take any long exposure, the camera needs to be absolutely still. Any kind of movement, and you'll wind up with motion blur.

In order to keep the camera still, you might want to invest in a remote shutter: a small piece of equipment that plugs into the camera body, letting you hit the shutter without touching or bumping your camera, risking nudging blur into your shot. It's also a lot easier to use one when in bulb mode.

Set your camera to aperture priority mode, which will keep the aperture fixed and determine the appropriate settings to make that aperture work. An aperture in the neighborhood of f/8 - f/11 is a good starting place for landscapes. Set your lens to autofocus (or dial in focus manually), and turn off image stabilization if you have it: it'll give you motion blur. 

Take a test shot in aperture priority mode to make sure your exposure is right: check your histogram, your monitor isn't a great indicator. If the exposure looks good, it's time to add your Neutral Density (ND) filter, a filter that will drop the exposure of your image without changing the color. Be sure to note the shutter speed when you're taking the test shot.

Because so much light is entering the lens, you'll need an ND filter to be able to make any kind of effort in daylight, or you'll just end up with a completely overexposed image: it doesn't take much to go to total white.

Now it's time to choose your exposure time. Most cameras have a built-in shutter speed setting of up to 30 seconds, but you can switch over to bulb mode to exceed that limit. Don't change your aperture or ISO settings at this point: you've dialed in what you need, and though you won't be able to see through the viewfinder with the ND filter on it, your camera knows what it's doing.

So how long should your exposure be? There are some formulae you can use to work it out if you want, but there are easier ways. There are smartphone apps and conversion charts for easy reference. You can also do it the old fashioned way and guess and check: if 60 seconds is underexposed, try 90. Ideally, your histogram should match the histogram from your test shot. Use the following table to convert:

The best way to learn is to practice. Scout out some locations ahead of time: rivers, waterfalls, seascapes, or just a busy street at night, and you'll soon be on your way to mastering the technique.

Want to read more about using an ND filter?  Check out mattb123's LightCraft Rapid ND Review.  Also, stay tuned for an advanced guide on long exposure composite photography next week.