Exposure

Exposure & metering go hand in hand. I’ll start with exposure this week & discuss camera metering next week.

Although some Digital Single Lens Reflex (DSLR) cameras can record 12 bit or even 14 bit levels of exposure, the majority of point & shoot cameras record 8 bits of electrical levels from the sensor. 8 bits provides 256 steps between pure black & pure white. It is the job of the camera’s metering system (or, when operating in the manual mode, the photographer) to insure shutter speed, aperture & ISO are properly set so that the important elements of the image falls within this range. The term used to describe the darkest to lightest is dynamic range. It is either given as a ratio, or, more often, as the number of “f” stops.

A high contrast daylight image may have a dynamic range well over 14 f stops, or a ratio of 1:10,000. Somehow, the photographer needs to fit this wide range into the rather narrow capability of the camera’s sensor. Even the human eye is limited to a range of about 10 – 14 f stops (although some argue that by using the iris, squinting, etc we can actually see as wide as 24 f stops).

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Film photographers have always had to deal with this problem – print film has a dynamic range of about 8 stops while slide film is limited to around 5-6. Depending on the model of your digital camera, it may have a dynamic range equal to or even better than slide film, but it will not be capable of recording the worst case real world conditions (well, at least in one exposure.) There are techniques that allow combining separate exposures in camera or post processing that can extend dynamic range (usually called HDR – High Dynamic Range.) Although the theoretical range of an 8 bit converter is 256 bits or 8 f stops, a typical point & shoot camera has a dynamic range of about 5 f stops; a 14 bit DSLR will have more.

There are metering techniques that help the film photographer avoid over exposing or underexposing the critical part of their image, however the digital photographer has an added advantage – the ability to examine the image immediately after recording it & making additional exposures to correct for problems. If your camera shows histograms, you have one of the most useful tools provided for the digital photographer.

A histogram is a graph of the level of each bit recorded by the sensor. A typical 8 bit histogram looks like this:

This is a histogram for the image below. The left side is the darkest part of the image (0 bits), the center the midtones (128 bits), and the right side the brightest (256 bits). The height is determined by the number of pixels at each level from 0 to 256.

This one shows that the exposure is acceptable – it doesn’t appear to push the bulk of the data to either end.

Underexposure will shift the data towards the left, overexposure towards the right:

One additional point – the histogram is a record of data. If your image doesn’t contain any information that is dark (or bright), or has holes in the tonal range, nothing is going to appear in that portion of the histogram. Some good examples of unusual histograms & the images that go with them are available at The Luminous Landscape website, which, by the way, has some excellent tutorials.

Although there really isn’t such a thing as a “bad” histogram because you might want most of the image black (or white), you do want to avoid your data spilling off either end. Anything past the right edge is data above 256 bits – it cannot be recorded so the results is clear or pure white – the data above 256 bits is thrown away and results in a burned out image.  Check the clouds in the overexposed image above. Although you also throw away data below the left side, that is in the shadows, so is usually not as obvious that it is missing. It is a good idea to keep the histogram as far right as possible, but not go over. Underexposed images (those shifted to the left) will look “muddy”; check the underexposed image above.  Unlike the overexposed image, an underexposed photograph can sometimes be corrected with editing software to print or view on a monitor, however it will end up noisier than one properly exposed.

Many cameras provide a warning that you have gone over the upper brightness limit by blinking overexposed portions of the image. If this happens, you need to force the camera to a smaller aperture or faster shutter speed by either switching to manual or, if your camera has the adjustment, use exposure compensation, or, unfortunately, live with burned out portions of your image.

If you don’t have exposure compensation or manual settings, try repositioning the camera so the meter system provides correct exposure. If the composition is not what you want, half press the shutter. On many cameras this locks the focus and exposure. While holding the shutter half pressed, recompose the scene to what you want & finish pressing the shutter. This technique is particularly useful if your camera can be set to spot metering.

It is important to accept the fact that unless you can take multiple images at different exposures without moving the camera and use HDR techniques to combine them, your camera cannot record the range provided by a high contrast scene. You must choose the portion of the image that matches the exposure range your camera is capable of recording. Use the histogram to confirm you have made the correct exposure.

Next week I’ll suggest some additional techniques you can use to produce correctly exposed images.