Into the World of Long Exposures

A couple months ago, I wrote a column on this site about freezing motion using very short burst of flash light, in order to capture phenomena that are too fast for our eye to perceive. This time, we are going to explore the opposite end of the time scale, using very long exposures to underline the passage of time and create unusual images and alien worlds.

Technically, a long exposure can be defined as any exposure that lasts from a few seconds to minutes, hours or even days. Some photographs have been made by letting the shutter open for months! We are going to examine the various tools and techniques that are needed for long exposure photography and understand how even the most mundane subjects can be transformed into art, with the right combination of vision and craftsmanship.

Tools of the trade

Whereas nowadays digital cameras offer an incredible level of convenience with respect to film, the latter still has an advantage when it comes to really long exposures: a film camera, even one with lots of electronics, will use very little power when taking a picture and can keep the shutter open for days without using up the charge of its battery. On the contrary, a digital camera’s battery will only last a few hours, at best, if the camera is in constant use.

On the other hand, film suffers from the phenomenon called reciprocity failure. In the simplest possible terms, what happens is that, as the amount of light hitting the film decreases below certain limits, the time needed for a correct exposure increases more than linearly. This makes the camera meter useless and manually computing exposure times must take into account the appropriate correction factors. Add to this that you won’t be able to judge the exposure by looking at the histogram on the LCD monitor and you’ll understand why digital is so much more convenient.

Limiting ourselves to the realm of digital, the good news is that it is not necessary to acquire costly professional equipment for most of the techniques that will be presented in this article. Even a compact camera will suffice, provided that it has a manual exposure mode and, possibly, manual focus. A “bulb” mode, whereby the shutter can be kept open for as long as needed, is necessary for exposures longer than 30 seconds. It is also useful to have a mount thread so that it can be mounted on a tripod.

A DSLR will probably be best, however. It gives more versatility, better image quality at high ISOs, and generally much longer battery life. A DSLR, and some high-end compact cameras too, will also be capable of saving RAW files; while this is not absolutely necessary, it can help when doing post-processing.

The second, almost mandatory, tool that you should have is a tripod. In the absence of a tripod, you will have to find somewhere to rest your camera upon, so that it doesn’t move during the exposure.

Finally, it is recommended to have a remote release, either one that attached to the camera via a cable, or an infra-red remote command. This will ensure that the camera does not get any shake from your pressing of the shutter button.

In what follows we are going to present one of the most popular sub-genres of long exposure photography: fine art landscapes. In one of the next columns, we will provide some useful suggestions for shooting star trails instead.

Fine art landscapes

This is a genre that can be quite difficult to define precisely, especially because it’s hard to pin down what “fine art” means to most people, but if you have trouble figuring it out, you should look at the works of Micheal Kenna and Joel Tijntjelaar.

The specific effect that these artists are usually looking for is the smoothing of the movement of waves in the sea, so much that the surface of the sea becomes an almost uniform, cotton-like substance, or the movement of clouds turning into long, smooth streaks. These effects tend to give landscapes an ethereal, almost otherworldly appearance.

In order to achieve this effect, exposure times on the order of minutes are normally necessary and obtaining those times means reducing the amount of light entering the lens. Unfortunately, there is only so much that you can do using only the camera controls. For example, on a bright sunny day, at noon, the correct shutter speed at ISO 100 and f/32 would be on the order of 1/25s, which is definitely not long. Short of shooting at night, there is only one way of letting less light in, and that is using a neutral density filter. A neutral density filter, or ND for short, is a filter that blocks most of the light coming through the lens, without affecting the scene’s colors (hence the “neutral”).

ND filters come in different densities, each one corresponding to a number of stops of exposure reduction. A 2-stop ND filter will reduce the exposure by 2 stops, therefore requiring 4 times as much time for a proper exposure. The thickest ND filters that are commercially available will reduce exposure by as much as 10 stops, corresponding to a factor of 1,000. That means that our hypothetical 1/25s exposure time will have to be brought up to 40 seconds, which can be enough to smooth out a raging sea. For more prolonged exposures, more than one filter can be stacked together.

Aside from their strength, which can be expressed as a number of stops, or as a fraction, ND filters are characterized by their shape: you can either have circular filters that screw onto the end of the lens, or rectangular filters that slide into suitable adapters. Both shapes have advantages and disadvantages, and none is obviously better than the other one, but I’m accustomed to using rectangular ones.

Some companies also produce variable ND filters, whose strength can be varied by rotating them, as you would rotate a polarizing filter.

Operating procedures

Do not forget that beautiful photographs always require interesting subjects and pleasing compositions. Do not fall into the trap of thinking that a technique like long exposure can be a surrogate of vision and craftsmanship.

So, once you have you subject, you composition, and your framing put down, the first thing to do is to meter the scene without the filter. It might be impossible to see anything with the filter on. Now it would also be a good time to focus the scene and switch to manual focus.

After you have determined the correct exposure, multiply the shutter time by the the factor corresponding to the number of stops of your filter. For example, an exposure of 1/100s will become 1/25s with a 2-stop filter, or 10s with a 10-stop filter. You can certainly find tables online which list the correct exposures with respect to a specific filter strength.

If the light conditions are changing, like around sunrise or sunset, you should take that into consideration and maybe shorten or lengthen the exposure by a factor of between 30% and 50%. If the resulting time is more than what can be set in the camera, you will have to set it to “bulb” mode.

Now it’s time to mount the filter, being careful not to move the focus; lock the mirror up, if your camera has a mirror that can be locked up, and shoot using your remote release. If in bulb mode, use a timer to know when the exposure time that you have determined previously has elapsed, then close the shutter. Time to review your picture in the back LCD, compute a more correct time if it came out over- or under-exposed, and possibly shoot again.

Keep in mind that the pictures you have taken are just a starting point. They will probably look flat and lifeless. The colors might also be off because the supposedly neutral filter is not so neutral after all, and introduced a distinct color cast. Time to bring them into your favorite image processing software and tweak them until they really shine.

Five tips for better long exposures

• Composition is king. A badly composed photograph will not become great just by using a long exposure.
• If shooting vast landscapes, try to include some element of foreground interest, like rocks or piers jutting into the sea.
• If shooting at night, consider painting objects in the foreground with a flashlight to avoid them being mere silhouettes.
• When the sky is included in the image, fast moving clouds will create interesting streaks and patterns, whereas a cloudless sky will be boring. In the latter case, compose to include less of the sky.
• Try to use a long exposure to make moving people completely disappear from a crowded place.