Background Exposure

No! Not those kind of night moves! I’m talking about things you can do with your camera at night.

There are two important elements of photography present at night that most people don’t expect: light and color. It is, after all, dark and “dark” is defined as the absence of light, right? Well unless you’re in a mine tunnel underneath the ground, there is always light. It may come from street lamps, or the moon, or even the stars, and it may not be bright enough for a human eye to see, but it is there and a camera can see it.

The same goes for color. The human eye “sensor” is made up of two types of cell: rods and cones. The sensors that are most sensitive to light, the rods, are not sensitive to color and so at night everything looks somewhat black & white. A camera sensor is equally sensitive to color in low light as it is in high light. You can get some colorful photographs at night that are especially startling because they did not appear colorful while taking the (low light) picture.

Night often has one other element important to photography: contrast. This is both good and bad. It’s good because contrast is interesting. It’s bad because it frequently exceeds the dynamic range of your camera and thus either has washed-out highlights or large areas of total blackness. Film is better than digital sensors in this regard because it over-exposes gracefully, leaving highlights that have more detail and fewer color artifacts around them.

Ah, but digital has the power of computer post-processing… if you can capture the information in the first place. To do this, you’re going to need these things:

1. DSLR: A point-and-shoot just isn’t going to cut it here. You need a camera with very low noise (i.e. the ability to go to ISO1600 or higher) and that means the physically big sensor that only DSLRs have. You also want a self-timer and “mirror lock-up” (sometimes called “exposure delay”).

2. Tripod: There is no way to hand-hold decent night exposures. You must have something that can lock all axi of movement and hold your camera completely steady for several minutes, even with you fiddling controls.

3. Adobe Photoshop: I’m sure there are other software packages that can do HDR, but I don’t know many of them off-hand. Gimp will not support this until v3 and that’s still a long time away. I’ve used EasyHDR, an inexpensive product that produces excellent results, but isn’t as flexible as Photoshop (surprise, surprise). Please leave a comment if you want to describe some other HDR-capable program.

Set up your tripod and frame the image you want. Set everything to manual, including white balance. I frequently use F8 to get a good depth-of-field. Focus and then lock the focus by switching from “auto-focus” to “manual”. Set your ISO sensitivity down as low as it can go (i.e. ISO50 or ISO100). You want to make sure there is as little noise in the images as possible and this is the best way to accomplish that. Noise is typically random and so over an extended exposure it will be reasonably uniform, like a light haze, easily reduced or removed simply by shifting the black point slightly. A high ISO setting will instead amplify individual errors and make the image look “grainy”.

Calculate the manual exposure of the darkest area you want clearly visible. You can do this by using a hand-held meter, or the spot-meter function of your camera, or you can just guess until you get it right (my preference). The shutter time will be quite long, probably on the order of 15 to 30 seconds if you’re using F8 like I do.

Program your camera with a selfdg-timer of 2 seconds or more and to have some delay between lifting the mirror and starting the exposure. The self-timer gives the camera time to stabilize on the tripod after you depress the shutter release and the exposure delay lets it stabilize again from vibrations caused by lifting the mirror. Also, if you have a “vibration reduction” or “image stabilization” lens, turn that feature off. These technologies can get confused by the steadiness of a tripod.

Take your first picture and verify the result. The dark areas you’re interested in should be well illuminated and the light areas will be flashing because they are over-exposed.

Now reduce the shutter time by a factor somewhere around 1/4 to 1/16 (3-4 stops) and take another picture. Don’t change any other setting! Repeat this until there are no over-exposed areas, usually 2-3 times. You have now captured the complete dynamic range of the scene, albeit over several exposures.

Bring these images to your computer and start Photoshop. Go to the “file” menu and select “automate”, “merge to HDR”. Then select all of the exposures for a particular image. I’ve used both RAW and JPEG sources for this and never been able to tell the difference. If you choose a raw file, the converter will use whatever settings you used last time, so open one directly, first, and configure everything as you like it.

Check the “try to automatically align images” box. It’ll take longer, but it’s worth it. There’s a good chance that miniscule movements occurred while changing the exposure settings and this is the easiest way to correct for that. Click “convert”. Wait. EasyHDR does not yet have this option, so you’ll have to open each file individually and manually align them.

When it’s done, you’ll get a full image preview and a slider you can move around to see the full dynamic range of the image. Changing this value only alters the display, not the image in any way. When you’re happy, click “OK”.

There isn’t much you can do with an HDR image until you convert it to a normal 8-bit or 16-bit image. You do this by going under the “image” menu and changing the “mode” to “16 bit”. At this point, you’ll get a dialog asking how to map the HDR image to a normal 16-bit image. The possibilities for his are more than I want to get in to in this article, but I recommend Sean McHugh’s article on tonal mapping with HDR images.

Because of the extended dynamic range in the converted image, most of these 16 bits, perhaps even all, will be data instead of noise. You can thus apply a “curves” adjustment layer to get the exact display you want without amplifying noise from the camera’s sensor.

Good luck! If you post some of your great night shots, drop me a comment with a link to them.