Understanding Your Digital Camera's Dynamic Range
There
is a lot written (some of it accurate, some not) about the dynamic
range of digital cameras and how this range differs from that of film
cameras. I find it interesting and useful to read the varying
(sometimes wildly varying) opinions about it, but I sometimes end up more
confused about people's understanding of dynamic range by the end of an hour of reading than I was at the
start--largely because the experts tend to disagree pretty emphatically
(and if you read some of the online bulletin board exchanges, pretty
rudely, too) about just how digital cameras stack up against film
cameras in dynamic range.
No one argues about how a digital camera's
dynamic range is actually defined: in very simple terms it's the range
between the darkest shadows and the brightest highlights in which the
camera can record detail. And it's pretty much accepted that a digital
camera's contrast range is narrower than film's (particularly
color-negative film); in other words, most experts agree that a good
quality color negative film, properly exposed, can capture and record a wider range
of tones than a digital camera (also, only when properly exposed). The
actual dynamic range of a particular digital camera depends on a lot of
factors, including the size and design of your camera's digital sensor.
Your camera manual will provide some insight into those particulars.
Generally, the larger the sensor your camera has, the wider its dynamic
range; which is why a full-frame digital camera--one that has a sensor
that's the same size as a frame of 35mm film--usually has a wider
dynamic range than a camera with a smaller sensor (typically most CCD sensors are smaller than CMOS sensors--but not always). In fact, overall,
all aspects of image quality get better with a larger sensor.
The key thing to keep in mind is that your camera does indeed have a
limit to its ability to record contrast and if you try to exceed that
range, something is going to give. Not maybe, definitely--either the
highlights or the shadows (or both) are going to get lost. How your
camera reacts to a very contrasty scene (and which end of the scale it
dumps) depends almost entirely on how you expose things. If a scene has
very bright highlights and very dark shadows, you can usually (not
always) expose to capture one end of the range knowing that you are
willingly sacrificing the other. In other words, if you really want to
record detail in bright highlights (and normally you should--there are
exceptions), then you will have to cut the exposure to bring highlight
detail into range. Doing this, however, will cause the shadows to lose
all detail. On the other hand, if you set exposure for the shadows, you
will gain detail there, but lose most highlight detail. The contrast
range never changes for a particular scene at a particular moment, but
by altering the exposure, you shift which part of that range you're
going to record with detail.
There will be times, of
course, when the scene will wildly exceed your camera's contrast range
and even your ability to only record a portion of that range. In the
scene here (shot in the Chincoteague National Wildlife Refuge in
Virginia), for example, the presence of specular highlights made it
impossible for me to hold the very bright highlights on the water. If I
had exposed for them (and theoretically that might be possible), the
rest of the scene simply would have gone black or near black. Blehh!
Instead, I simply chose to expose the best I could for the grasses and
the silhouette of the goose in the foreground and let those
super-bright highlights just blow out. Yes, I could have shifted the
exposure down a bit, perhaps underexposing by two or more stops from
what I shot it at, but then the light airy feeling of the brilliant
morning sun would be lost. Detail or not, I liked the way the
highlights turned to a wash of specular highlights. Also, since I shot
this in RAW format, I could easily have corrected the exposure to a
fair degree in editing but chose not to do that.
Two other things to keep in mind about dynamic range: One is that camera makers
are improving it and stretching it all the time--in a few years, the
range is probably going to be extraordinary. Two, in many situations
you can use a technique called high-dynamic-range imaging to vastly
extend the dynamic range of digital images. Essentially with that technique you take a series of different exposures of the same subject and then use HDR software to merge them together. I'll talk about the latter
technique in a future tip. In the meantime, if you want to read more
about it, Ferrell McCollough has written a great book on the topic
called Complete Guide to High Dynamic Range Digital Photography (A Lark Photography Book)
.
