Increasing Image Contrast Without Sacrificing Shadow &
Highlight Detail
Contrast is a very important concept in photography; it's one of the main
visual cues that enable us to perceive detail and distinguish between various
elements in a photograph. But contrast is also normally the sworn enemy of
dynamic range, which is the difference between the brightest and dimmest light
levels a camera can record in a single frame. Digital cameras use a tone curve
to control the conversion of the sensor data into an image, and to control the
transition from black to white. Tone curves typically are S-shaped; steep in the
middle, and flatten out at the ends. The top flattened area is called the
shoulder, and controls the transition to pure white, and the bottom flattened
area of the curve is called the toe, and controls the transition to black.
The toe of the default 1Ds tone curve is fairly long, and the slope of the
center of the curve is fairly steep as a result. This keeps noise in the shadow
areas very low, and produces fairly "punchy" looking images from the
camera, but also reduces the effective dynamic range of the camera considerably,
because most of the output color values are coming from the upper midtones of
the sensor. Here is an example:
Exposure was set manually, to keep the sky and the sunlit areas of the grass and
building from washing out to white. The long toe of the standard contrast tone
curve pushes the brightness of the shadow areas down very low, so they are
almost black. The details of the reflection in the window are lost for the most
part. The image has lots of contrast, but in this case it's a bad thing because
the detail level of the shadow areas is severely compromised as a result.
Compare to the image below, shot with my 1DS
Low Contrast tone curve:
As you can see this rendition of the image has much better shadow detail, but
it's kind of flat-looking. There is much more perceptible detail in the shadow
areas now, because a much greater portion of the dynamic range of the sensor is
incorporated into making the final image. But that means that the overall
contrast level is significantly lower. Obviously, we don't want to increase
contrast in the normal way, because that takes us right back where we started.
The solution is to increase contrast locally instead of globally. Instead of
applying contrast equally to all parts of the image, (globally) apply it
selectively to increase contrast locally without increasing the overall contrast
to unacceptable levels. The easiest way to do this is with Photoshop's USM
filter, using a small Amount value and a large Radius value. This will
compare pixels in the image to surrounding pixels, and adjust their brightness
level compared to the average brightness level of their neighbors. In the
example below, I used an amount of 90 and a radius of 120 while running my Midtone
Sharpen LAB 2X Action. As you can see, we now have the best of both worlds:
the dynamic range of the scene is fit into the image much better now, but the
image is no longer flat-looking, and is sharp and detailed throughout.
Both of the sample images are downsized 1Ds JPEGs exposed at 1/400, f/5.6, ISO
100.
Related Articles:
1Ds Metering Strategies
1Ds Tone Curves
Sharpening Actions
