Knowing Your Dynamic Range

Dynamic Range is critical in digital photography, more so than with film. First, film is more forgiving because its response to light tails off gently at either end of the scale, while a digital sensor has a more abrupt limit. So if you over-expose, don’t expect to be able to recover much information from the blown highlights, and much the same applies to under exposure.

Currently the biggest limitation to dynamic range of camera sensors is the noise floor-the darkest part at which real content detail can be distinguished from noise. The greater the dynamic range, the less it is likely to produce noise at low level brightness shots.

The second importance is that dynamic range varies between brands and models of camera. With film it hardly mattered which camera you used, but with digital, the dynamic range depends on several factors, and manufacturers often withhold full information to keep a competitive advantage. A great deal of research goes into this area, with constant development, and considerable secrecy.

If you have a high-end Digital SLR, you can reasonably expect the dynamic range to be better than the cheaper model, but another factor is how new the model is. Often, a newer second-tier camera will out-perform the older top-of-the-line model. It makes sense to know your camera’s abilities. Once you do, you can put this information to practical use by having a pretty good idea and without calling on good help from extra lighting or reflectors.

The main factors that control the dynamic range of the sensor are the bit-depth, the size of the individual photosites, and the noise characteristics,

Bit-Depth

Greater bit-depth means that the sensor has the capacity to record a wider range of brightness, so a 14-bit sensor potentially records more than a 12-bit sensor. To take advantage of this, however, the sensor needs other qualities. One of these is the size of the photosites. The larger they are, the better, and the less prone they are to noise at low levels. Some sensors have extra technologies to improve the range, such as Fujifilm’s Super CCD SR II, which has two photo-diodes in each photosite – a larger one for general use, and a smaller one that reacts to high brightness levels.

Noise Characteristics

Noise has its biggest effect when there is least light, as in shadows and low lighting. More than bit depth, it limits the dynamic range of a sensor at the lower end. This is known as the noise floor, and you can see its effect by trying to open up shadows in any image beyond a modest amount. If you do this in a Raw converter, when you increase the exposure or brightness control beyond a certain point, you will simply be revealing more noise than real detail.

Check Your Dynamic Range

Photograph a gray card under even lighting that doesn’t change, defocus the lens strongly for an even tone. First, shoot at an average exposure setting then take a series of frames at one-half or one-third f-stop intervals, both darker and lighter, at least 6 f-stops darker and at least 4 f-stops lighter, so that you have a range from completely under-exposed to completely over-exposed. In a browser, database, or image editing program that allows you to measure the brightness, assemble the images in order, and label each with their exposure. Measure the values on the usual 8-bit scale from 0 to 255. Find and mark the step on the left-hand side that measures 5 (anything less is effectively black). Find and mark the step on the right-hand side that measures 250 (anything higher is effectively white). These two extremes set the dynamic range. If your camera shoots Raw, measure the values as you open each image in a Raw converter.