Dynamic Range

Dynamic range, tonal response, and contrast Dynamic range (or exposure range) is the range of light levels a camera can capture, usually measured in f-stops, EV (exposure value), or zones (all factors of two in exposure). It is closely related to noise: high noise implies low dynamic range. It is also related to the tonal response— the relationship between light and pixel level (shown below). Contrast, also known as gamma, is the slope of the tonal response curve. High contrast (shown on the right) usually involves loss of dynamic range— loss of detail, or clipping, in highlights or shadows— when the image is displayed. (The image file often has a greater dynamic range than the displayed image.) Dynamic range, tonal response, and gamma are measured (A) by Stepchart using a transmission step wedge, preferably one with a maximum density at lest 4.0 (equivalent to 13.3 f-stops), or (B) by Dynamic Range, a postprocessor for Stepchart that uses results from up to four differently-exposed reflective stephart images. The latter technique is often the most convenient because it doesn't require a special light source in a darkened room.

original | clipped

Dynamic range is a strong function of pixel area, which is proportional to the number of electrons a pixel can store. It is invariably better in DSLRs (which have relatively large pixels; at least 5 microns square) than in compact digital cameras. It can be maximized by setting the camera at the lowest ISO speed.

Displaying images with large dynamic ranges (which can be well over 1000:1; 10 f-stops) can be problematic in printed media, which has a maximum dynamic range of about 100:1 (a little over 6 f-stops; 200:1 at the absolute maximum). Reducing contrast can make the image look flat and dull. Some processing is usually required, like applying an "S" curve or dodging and burning (lightening or darkening) portions of the image. Contrast masking is a particularly effective approach. Tonal quality and dynamic range in digital cameras offers additional tips.

Lenses have an intrinsic contrast— the higher the better. But lens contrast isn't exactly intrinsic. It results from flare light— light originating inside and outside the lens's field of view that bounces between lens elements and off the inside of the lens barrel. Flare light tends to fog the image and obscure shadow detail: it reduces dynamic range. To our knowledge there is no solid standard for measuring lens contrast. But you can measure it, or at least compare it for different lenses and apertures, by running Stepchart on a Q-13 (or equivalent) chart mounted on a white board that extends beyond the camera's field of view. (Recall, contrast = gamma.) There would be less flare light, hence higher contrast, with a black board. Actual lens contrast depends strongly on the scene.