| ||||||||||||||
|
Whether you are shooting with a digital camera or are editing an image in the computer, an understanding of histograms is so useful as to be essentially mandatory. Histograms give you essential feedback on the overall exposure and the contrast range of an image. When shooting digitally, checking the histogram of what you just shot tells you if you nailed the exposure; if the exposure is poor, then you have a chance to reshoot after correcting the exposure. This is something you can't do with film. With film, you have to wait to get your prints or slides back, and by then it's too late to reshoot the scene. Score one for digital cameras! (Not all digital cameras have a histogram feature, a feature I strongly recommend you require when shopping for a new camera.) When editing an image in the computer or when setting up a film scan, examining the image histogram is vital for getting optimal results.
|
image histogram of a scene with a full tonal range | |
image histogram of a dark scene |
image histogram of a light scene |
Now let's see how underexposure and overexposure look in a histogram. The key is to note whether the histogram seems to be "piling up" at one side or the other. If you can check the overflow channel (rightmost) and underflow channel (leftmost), so much the better: if these channels contain any significant amount of pixels, you have over- or underexposure. In the first histogram below, the image is underexposed, with lots of dark tones collapsed into pure black. The second shows an overexposed image, with lots of light tones pushed into pure white. If the histogram is piling up at the dark end, if it looks like it wants to continue off the left edge of the plot, you have an underexposed image. Similarly, if the histogram is piling up at the light end and looks like it wants to continue off the right edge of the plot, you have an overexposed image.
histogram of an underexposed image |
histogram of an overexposed image |
The same principle applies when scanning film into a digital computer file. If the histogram of your scan shows that you underexposed or overexposed in the scanning, you can rescan to get it right. In fact, most scanning software lets you set the dark and bright limits of the output histogram before the scan is performed, so you are guaranteed to get a suitable scan the first time.
Histograms are also vital when manipulating a digital image in the computer. The histogram feature of your image editing program will reveal if the print you are creating will be too dark or too light, and you can adjust the image accordingly.
So far I've only spoken of image histograms in terms of how light or dark the overall image is, i.e. overall exposure. They are even more useful than that, for they also give you information on the overall contrast of an image. Overall contrast is the range of tones in the image. An image with a wide tonal range -- very dark darks and very bright lights -- has a high contrast. An image with a limited tonal range -- the darks aren't much darker than the lights -- is a low contrast picture. Depending on the subject, sometimes high contrast looks better and sometimes low contrast looks better.
The more spread out a histogram is -- that is, the farther apart the dark end of pixel values is from the light end -- the higher the contrast of the image.When shooting with a digital camera, you are pretty much stuck with the tonal range of the scene you are photographing. But if you are scanning film or editing images in the computer, you get to adjust the histogram in the software to achieve the overall contrast you want for that image. As an example, below are two versions of the same image along with their histograms. One has low contrast and one has high contrast. The histograms reveal a broader range of tones for the higher contrast image. In this case, the high contrast image looks better. It has more snap, crackle, and pop.
high contrast image | lower contrast image |
histogram of high contrast image |
histogram of lower contrast image |
VISIT THE GALLERY |
Timothy Edberg / 6511 Homestake Dr. South / Bowie, MD
/ 20720
(301) 809-5857 / 1-877-471-6414 (toll-free)
www.edbergphoto.com