June - sun You are here: Home Techniques Light metering


Introduction

Proper exposure times are essential with any kind of photography, especially when you use slide film that has a narrow exposure range. Since the sky can have huge differences in brightness, it requires a bit of experience using your exposure meter properly. In some cases you won't get away with just aiming your camera at some part of the sky and taking a photo, putting all your faith in the camera's light meter.

The way to properly expose a sunset photo is to meter on the medium bright areas in the frame, resulting in the photo at top. The circle indicates the location metered on. If you were to meter on the brightest areas, near the horizon, the photo would be underexposed (middle photo). If you would meter on the darker areas overhead, the photo would be overexposed (lower photo).

Example of bad metering: underexposed sunsets

The exposure errors that I most hear are complaints about underexposed sunset pictures. This is probably because many people photograph sunsets (compared to other things in the sky) because they are pretty. I am taking this example because it is easy to explain how to meter sunsets correctly, and it applies to several other subjects in the sky.

Imagine taking a picture of a sunset, while the sun is low in the sky, or maybe just set. If you meter the frame while you have your camera aimed, the camera meter will see the sun, or clouds very near by the sun, that are much brighter than the rest of the sky (which is what you want to expose properly). The camera light meter thus thinks the frame has a high brightness and meters according to a short shutter speed and/or high f/ratio. The result is that the clouds very near the sun become properly exposed, while the rest of the sky becomes several stops underexposed and thus very dark to black.

An underexposed sky with some very small clouds near the sun that are properly exposed, is much worse than having a properly exposed sky with the small clouds near the sun overexposed; this looks much more acceptable.

Manual metering

The way to correctly meter a sunset is to meter on a small part of the sky away from the sun, any part of the sky that you want properly exposed, and then use those exposure settings while taking a picture of the sky with the sun (or other very bright parts of the sky) in it.

This technique requires that the camera either has some kind of exposure-lock (meaning you can lock the exposure settings while you move the camera to a different part of the sky), or that it has manual exposure.

The picture below illustrates this technique. It is important to remember this if you consider yourself a beginner, because you need to use this technique for many weather subjects! I will call this technique manual metering for the sake of reference. Not only does it apply to taking pictures that need to be properly exposed when a bright object is in the picture, but also to pictures with a large dark area in the picture that you want to keep dark on the photo (e.g. a photo of surface fog taken during the night).

Subjects that require manual metering

Some subjects that need to be metered manually are most atmospheric optical phenomena such as cloud coronas, iridescence (usually), all halos (if near the sun); some cloud types such as bright cumuliform clouds, noctilucent clouds, and in general any clouds that you photograph with a polarizing filter; most astronomy such as wide-sky astrophotography, zodiacal light etc. (although most camera light meters are useless for such low light levels anyway), sunrise/sunset colors, and last but not least snow.

Snow and ice are notoriously difficult to expose properly. Because snow is so bright, when you meter a snowscape the photo will turn out one or two stops too dark - the snow will be medium grey and all other parts of the photo will be dark or black.

Using a grey-card

A good technique here is to lay down an 18% grey card in the snow and meter on that card, without any snow in the metering frame. You will then be able to determine the correct exposure time for grey areas in your frame, and with that exposure setting the snow will still appear white on the final frame.

Grey cards are available from most professional photo stores. Kodak makes good-quality grey cards that reflect 18% of the incoming light, while the reverse of the card is white, useful for white-balancing digital cameras. Another company that makes high-quality grey cards is Sekonic. These cards come with full instructions on how to use them, not only for reflective light metering but also other techniques for which they are useful.

Tips for proper metering

    With overcast stormy skies, try metering on the brightest parts of the clouds. The photo will be fairly dark, making the clouds look more menacing.

  • if you are unsure about which parts of the frame or sky to meter on, take several photos, bracketing your exposures (i.e. taking several exposures each with one or half a stop difference around the exposure setting that you think is right).

  • use as few different types of film as possible for your general photography. Not all film responds the same way to light for different subjects.

  • periodically calibrate the light meter of the camera, especially if you obtain an unusually large number of improperly exposed photos. You can calibrate (or at least check) the light meter by comparing its response to that of light meters of other cameras.

  • always try to meter on areas in your photo that are as neutral grey as possible. If there are no such areas, try to meter with many different colors in the frame. This gives more accurate readings.

  • never meter on the foreground if you are taking pictures of the sky. The foreground is most often a few stops darker than the sky. Conversely, never meter on the sky if you are taking pictures of something on the ground.