The HistogramWhat is it? How do I use it?
|
More confusion surrounds this term than just about any other term in digital photography. Why the confusion is there, we won't go into because I'm not a psychologist (or a deranged serial killer either, but I digress).
What we will do is to explain what it is, what it can do for you, and why it's important.
If you don't know what a histogram is, this is what an optimal one looks like.
Histograms don't exist in a vacuum. They are always related to a specific image. It's a way to graphically display information about a particular image.
Basically, it is a graph of your digital image's brightness, or "luminosity." The method used to determine brightness is for your camera to count every single pixel in your image.
It then totals the number of pixels that are black (luminosity value = "0"), the number that are white (luminosity value = "255"), and the 254 different values of gray, in between. (I told you we'll explain it, I didn't say it will make prefect sense.)
This is the image which produced the above graph. Since there's obviously no "gray," you might be scratching your head about now.
Humans see colors. Cameras see shades of gray, which are then converted to color. The key point is that the horizontal axis shows the brightness level; darker to the left, brighter to the right.
The vertical axis shows the relative number of pixels existing for each brightness level or luminosity.
Your camera should have the ability to display this information on your LCD screen. For a well-balanced photo like above, you should expect to have a medium-sized "mountain," with no gaps.
If your image does not contain any white, the right side of the graph will be empty, or "flat." Similarly, if there are no black pixels, the left side will be flat.
If you have a 100% black-and-white image, expect to find pretty much that there is one tall line on the left side (representing all the black pixels) and one tall line on the right side (for all the white pixels).
How Can We Use It?
That's what the histogram is. Now let's figure out how we can use it to produce better photographs. After you snap an image, check your digicam's histogram. If there is a nice range of exposure, you should see a medium-sized mountain range with no gaps (no absence of shades of gray).If it's heavily weighted towards the left (with few or no pixels toward the right), your photograph will probably be underexposed (too dark). Either re-position yourself and/or recompose the image with more light, or try to brighten the image by using exposure compensation.
If you wait until you get home and then discover an underexposed image, you can't do anything. But if you get into the habit of checking results immediately after taking a shot, you should be able to salvage at least some images. At a minimum, check questionable pictures.
Conversely, when the graph is weighted towards the right, your image will most likely be overexposed (too light). Follow the above tips, or use filters (such as neutral density), to darken the image.
Final Thoughts
The histogram is only a tool. Use common sense when evaluating it. For example, if you're capturing photos of a bright sunrise or desert scene, the graph's information will be shifted way to the right, but that's OK. Do not attempt to correct for this! Conversely, don't correct for histograms shifted to the left if you're shooting dark scenes like skylines at night.
If possible, avoid shooting scenes that produce histograms weighted heavily towards both the far right and left sides. Photographing human subjects standing in front of a brightly lit sky will produce this type of result.
High contrast situations produce quite a challenge to taking outdoor portraits. If you can't avoid this type of situation, try using graduated neutral density filters. However, the easier solution is to recompose the shot in a more exposure-friendly environment!
 | To Support/Share This Site, Please Click Here: |  |
