Histogram

If you're not familiar with the term "histogram" it is essentially a bar chart with no spaces between the bars. Photographically speaking it is a graphic representation of the exposure of a photo.

After you take a photo and look at it on the LCD most cameras will allow you to view the histogram. You will have to check your manual as to how to display it, but I imagine many of you have seen it accidentally. The image at left is a histogram of the tones in the photo below.

The histogram comprises 256 vertical bars. A bar at the extreme left represents the number of pixels that are completely black and a bar at the extreme right represents the number of pixels that are completely white. The histogram here shows that there are very few bars at the ends and that is, normally, what we want. Completely black or white areas have absolutely no detail and usually we would like to keep those areas to a minimum and here we have.

If we under expose our shot we have a histogram that looks like the one below

notice that the bars are shoved way to the left. Many pixels are either black or very close to it and therefore there is a lot of detail lost in the image, usually not what we want. Looking at the histogram after you shoot can let you know if you have an exposure problem which

might be hard to see if you are outdoors in bright light. In this image the center, left, and right areas show little detail and the whole photo just looks too dark.

In a well exposed photo we usually expect the histogram to be spread out to cover the whole range of dark to light. One exception to this might be a photo on a foggy day. Then the histogram may be a hump in the middle that does not reach either end. The image would then look very flat and low in contrast which is what you want in a fog shot.

Finally we have an example of an image that is over exposed. It is way too bright there a no pixels that are black and very few in all the darker areas of the image. On the bright side (right side) of the histogram is a large part of the image information the pure white and almost pure white areas have a lot of pixels and consequently

will have very little detail. The image looks very washed out. Only the areas that were very dark in the above image show much detail at all.

Bottom line here is to turn on your histogram display and look at it after you take a shot. There is no such thing as a correct histogram, but we usually want to avoid those that are shoved to one side or the other and especially those that then do not reach the other end.

For the more advanced shooter here is a warning. If you shoot RAW the histogram is NOT of the RAW image it is of the JPEG version of the image created by the cameras computer and may show over or under exposure where there is none. It can still be used as a guide but notice in the processing in the computer whether what you see in the histogram is a problem in the final image as it may not be. Here is an area where experience will be the best teacher.

Have fun!