Whether taking photographs using a digital camera or on film, correct exposure is crucial to getting great results. This means applying the available light with the camera to make an image with the digital image sensor or film within the dynamic range of the imaging medium (digital sensor or film). Digital sensors and film have limitations in how wide a range of brightness levels they can handle while capturing detail in each brightness range, from Very Dark to Very Bright.

The most common fault shown in digital pictures is burnt out highlights, often seen as skies devoid of detail, just clear white, or sometimes even with areas of false color. You also don't need to look far to find pictures lacking in shadow detail, with empty black areas where there should be subject.

Digital cameras and color transparence films have a dynamic range of about 5 f-Stops. This is a fairly narrow range of tones compared to what the eye sees. Rendering the ‘Correct’ exposure goes beyond fitting all available light into an arbitrary brightness range. The final image actually must satisfy the ‘Eye’ to be effective. To this end, ‘Correct Exposure’ means balancing the range of brightness levels to what 'looks good'. The relatively narrow range of brightness levels a digital camera can render means that to please the eye, maintaining highlight image detail will make for the most pleasing result. Shadows are usually not as ‘Eye Catching’ as bright parts of a picture. So, expose for the brighter parts of a scene and the results will be ‘Right’.

One f-stop increments (5 zones) on a 256 step gray scale
recordable by a digital camera.  When considering a color RGB image,
each color, Red, Green, and Blue have 256 color tonal values.

Contrast is a related problem, and many digital pictures are far too bright and contrasty, while others show a bright sky with everything else looking dull and dark.

Film Response vs Digital Response
Digital does react differently to light compared to film. Film sensitivity lags until light level reaches a threshold. At that point light begins its effect. The developed film will have a low density whether or not it is developed. This results from the density of the film base and a small amount of non-light induced development where the effect of the developer at a given temperature reacts with the photographic emulsion. Film base and the development effect combine to be called – fog.

Above the threshold level, the density (after development) starts to rise at first very gradually, then more steeply to reach a particular value. The slow rise part of the curve (see figure at top of next page) is called the toe of the curve. The middle of the curve where it is rising steadily is called the straight-line section. Eventually as the exposure is increased it starts to have less effect, and the curve gets less steep, eventually leveling off, with further exposure having no effect (in fact it can even cause a reduction in density.)

The area of the curve where it starts to get less steep is known as the shoulder of the curve. The part of the curve we normally use for photography starts just above film base + fog, on the toe of the curve and continues into the shoulder area (some films actually have little or no straight line area.)

Slope and Contrast
The contrast of the image depends on the slope of the curve - the steeper the slope the higher the contrast. This means that normal photographs tend to have low contrast in the shadows (and possibly in the highlights, if these are well up on the shoulder.) Photographic paper can help to put a little contrast back into the shadows, but it also often has the effect of increasing the highlight separation more than is required.

Digital Differences - Saturation
Digital sensors behave differently than film. Like film they have a threshold, below which there is not enough light to have any effect. Above that point, the exposure starts to increase more or less directly proportional to the increase in light - they have virtually no toe. When any sensor cell gets saturated with photons during the exposure period, adding more light has absolutely no effect, so it has little or no shoulder either, simply going direct from the straight line to a constant maximum value.

Practical Consequences
Using film, overexposure on negative film gives dense highlights that are hard to print. With digital there is simply nothing there to work with. A small difference in exposure that would simply mean a little extra exposure in printing with film will make a digital image totally unusable.

You do have to expose transparency film carefully to avoid overexposure, but the difference is even more marked with digital imaging.

Underexposure works similarly with both - you get no information either from clear film or a sensor cell with no current. Above the threshold however, we get better shadow separation with digital, but the low exposure response of the sensor is much more subject to noise. We need to make sure it is only used (if necessary) to record the darkest parts of our subject. When we have scenes without dark areas, it is important to expose to keep these away from this area.

A Simple Rule
The simple practical exposure rule for digital is to peg the brightest highlights (excluding such things as light sources and their direct reflections) at just below the maximum response.

Unfortunately, the automatic exposure systems of most DSLRs seem to be calibrated to produce correct results on film, i.e., 18% gray. Film exposures are generally based on the assumption that subjects generally have an average reflectance corresponding to a medium grey tone - the well-known 'gray card' carried by some photographers and photographed in scenes or used for exposure meter readings. With film, if you get this mid-point correct the rest of the scene will generally fall into place sufficiently well. This approach requires adjustments when using for digital.

Camera Replaces Meter
Digital exposures need to be based on highlights. These could be measured with a spot-meter or, better, found from an incident light reading, as even the brightest highlight cannot reflect more than is falling on it. Of course incident readings are often impractical when the subject is very large or distant as with a landscape. However digital cameras can actually do the job better, using the image review screen and the histogram display.

Practical Exposure
Good digital cameras can be set to give warning of overexposure by flashing blown-out areas on the LCD review screen. Checking the review histogram will provide a reliable and definitive way to confirm optimum exposure.

Judging highlights
At this point a certain amount of judgment needs to come into play. A well-exposed image will give no large areas of flashing highlights. There can be small areas, perhaps very brightly lit clouds in the sky, or actual light sources that can be allowed to burn out. Specular reflections of the sun – from windows or shiny metal can also be expected to be overexposed.

Where direct reflections are concerned, rather than trying to alter the exposure you may have to change your camera position slightly to lessen or eliminate them. A polarizing filter can also help control reflections. If reflections are not large and are not causing flare, then they may be appropriate in the image in any case.

If there are large flashing highlights, dial in less exposure and make a revised exposure. Experience is the best teacher. Bracketing can also be useful in obtaining the best exposure. Many DSLR allow automatic bracketing. I usually try -1/3 stop bracket increments if only small areas are concerned, -2/3 as a general guide, and -1 stop if the effect is severe. It seldom takes more than a couple of exposures to find the maximum usable. Conveniently, digital allows you to delete the earlier exposures.