What is dynamic range, in photography?
... and why is it important?
"Stops" are measures of light, just as inches are measures of length.
The human eye, at any one time, can see the difference between around 12 stops of light. A digital camera however can only see around 8-10 stops of light. This range of stops is known as "dynamic range":
This means that your eye can discern details in shadows and highlights that your camera cannot. In the diagram above, the camera would record as pure black everything to the left of the end of the arrow at the black end, and pure white everything to the right of the end of the arrow at the white end. But your eye would still be able to make out shades of grey in these two areas.
As a photographer, you have to decide where you want the dynamic range of your camera to fall, always remembering its limitations. You have to pick: do you want detail in the shadows, in the highlights, or neither? Usually you'll want one or the other, and changing the exposure accordingly is known as "exposing for the shadows" or "exposing to the left", and "exposing for the highlights" or "exposing to the right". But remember, if the range of tones in your image contains both very dark and very bright tones, you won't be able to expose both correctly in a single shot because the camera simply can't resolve all the detail. You have to pick - shadows or highlights?
This is how it looks in practice:
Using HDR techniques to fix the problem
If you have software that will combine images in HDR (High Dynamic Range), you can take 2 or more shots, each from exactly the same place, but each with a different tone correctly exposed. You stack the images in the software, which then works out to take detail from the highlights and shadows as appropriate. This is the result from an HDR edit of the images above:
If you are close enough to your subject you can use fill flash or reflectors to bounce a bit of light into the shadows and reduce the total dynamic range of the image, but for shots like the cathedral ceiling, you just aren't close enough.
Want to know more?
My free, online beginner's photography workshop takes you right back to basics so you can understand why the camera picks the midtones to expose correctly, and how exactly to override it to expose to the left or right. Join A Year With My Camera now, and get started today: