November 5 2008
An essential part of digital photography is using editing software to fine-tune the exposure. The levels adjustments are the first stop for setting contrast and brightness.
The Levels tool is available on many photo editing programs, including Adobe Photoshop and the Gimp. This is a first look at some of its capabilities.
A key component of the levels dialog box is an exposure histogram. Here it is again, that histogram from the DSLR’s LCD screen. When checking exposure in the digital camera the main function is to make sure the image fits in the histogram.
Precise exposure is not critical in the camera because the Levels function is able to fine-tune the final exposure. What is critical in the camera is not to crowd light levels at either end of the histogram resulting in the loss of fine detail in the shadows or the bright areas.
In the levels dialog box the three triangular sliders displayed below the histogram control the exposure adjustments.
There are 256 values available for tone ranging from darkest black to whitest white in the image produced by a digital camera. Sometimes not all are used and this can result in flat lifeless looking images lacking in contrast. The levels tool allows for the expansion of the images tonal range, increasing the contrast so the blacks are blacker and the whites are whiter.
Shadows and Black
If there are no pixels showing on the extreme left of the histogram move the left slider to the right to the first pixel. This sets the point for maximum black in the image.
This slider is usually black, reinforcing that this is for adjusting black levels.
Highlights and White
Similarly, the slider on the right sets for the white point of the image, and the slider is colored white. Moving the slider with the mouse to the left to the first pixels registered on the histogram sets the maximum white level.
The vary amount of adjustment applied depending on the scene. A naturally low contrast scene requires less aggressive application of extra contrast. The final decision is creative and depends on the photographer’s interpretation of the image.
This is the middle slider and moves automatically with the outside sliders to stay at a gray midpoint. This is the last of three slider adjustments, making the overall image darker or brighter.
The appearance of the image changes with the adjustments but they are not permanent until confirmation with the OK button.
In Photoshop, there are numerical values, labeled Input Levels, for each of the sliders above the histogram. The black starts at 0 and the white at 255. The middle slider starts at 1.0 and reduces if the slider is adjusted to the right producing a darker image. Changing these numbers directly instead of moving the slider is another way of using the levels dialog box.
Photoshop Clipping Preview
Holding down the Alt key and the mouse button at the same time on the black or white slider produces a display of clipped pixels in the image.
This is allows the photographer to see when the levels adjustment starts as the slider moves, and where it happens in the image. Sometimes clipping in unimportant areas is necessary to get the desired contrast in the main areas of interest.
There are two examples of images before and after levels adjustments. There are screen shots of the Photoshop levels dialog box of the yacht picture.