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Today's e-tip:

What they are and
why they're important

Your digital camera doesn't see the way you do. Understanding the differences in the way you and your camera sees is the start of taking great pictures.
Histograms: What they are and why they're important
Histograms are graphs that show you the distribution of tonal values in your image. This can provide you with a way to identify if the exposure is correct or needs adjustment.

When you take a picture, assuming you’re not shooting in manual mode, your camera makes some decisions on how to expose the image. Exposure is determined by 4 things:
  • The amount of light on your subject;
  • ISO speed;
  • The lens aperture (f-stop); and
  • The shutter speed.
You don’t always have control over the first item, but you or your camera can decide on how to set the latter 3 items. It’s these settings that determine if your picture comes out too light, too dark, or just right.

As you are probably aware, your camera doesn’t always get the exposure right. Sometimes pictures come out too light or dark. The worst-case scenario is when important areas of an image are so bright or dark, that you will lose all detail in those areas. For example, if you let your camera determine the exposure for a sunset, you’ll most likely lose all detail (and color) in the sky – the most important part of your image. This is called "clipping"– where the highlights or shadows are clipped, or lost.


Enter the Histogram
A "histogram" is a graphical representation of your picture’s exposure. You can look at it and instantly see graphically what your image looks like.

The graph shows the distribution of pure black, pure white and all tones in between. A well exposed image should have all of these tonal values.

Histograms make a lot of sense, once you understand them.

The image to the right is a well-exposed image. There are blacks, whites and many tones in between.

The histogram for this image looks like this:


At the far right of the histogram, you can see a thin white vertical bar – this represents everything in the image that is pure white – mainly the white part of the building. This bar reaches the highest point in the histogram, indicating that there is more pure white in the image than any other tonal value.

The thin short vertical bar on the far left represents all of the parts of the image that are pure black.

Everything in the middle, represents all of the other tonal values which are neither pure white or pure black.

The above histogram is called a "luninance" histogram - it combines all three primary colors in one graph: Red, Green & Blue. Often, histograms show each color separately. These are called RGB histograms:


How to Read a Histogram
Here are three images below (click to enlarge). The image on the left is under-exposed – too dark. The image on the right is over-exposed – too light. And the image in the center is just right. The histograms above the images tell the story. The middle histogram is well-balanced – the graph is spread nicely throughout the range of black to white.

The histogram on the left is skewed to the left – the tonal values are bunched towards "black". Likewise, the histogram on the right is skewed to the right – towards "white". Whenever you see this "bunching" of tonal values, up against the right or left side of the histogram, you might want to be suspect of the exposure.
(Click the above image for a larger version)

Just by looking at the histograms, you can get an idea of what the tonal values in the image looks like.

Your Camera's Display Lies!
When you view an image on your camera’s LCD panel, you really can’t tell if you have a good image or not. The panel is just too small. There are two main issues:

Sharpness: The LCD is too small to evaluate sharpness. An image that appears sharp on your LCD screen, may, in fact, be quite blurry.

Exposure: If you are shooting outdoors on a sunny day, it’s hard to evaluate if an image is well exposed because of the ambient sunlight. The conditions may be just too bright to tell if the image is well exposed. In addition, the display itself may be too bright or dark.

This is where the histogram comes in. By quickly viewing the histogram, you can tell if you’ve overexposed or underexposed an image. If you have, then you can use exposure compensation feature of your camera to compensate (PLUS exposure compensation to lighten; MINUS to darken - see this eTip for more info on exposure compensation).

Another thing to look for
If the graph doesn’t extend to the far right or left, as in the case below, this generally means that the image is low contrast, and doesn’t look very good. (see how the ends of the graph don’t extend all the way to pure white or black). In this case, the graph is "bunched-up" in the middle.

Through post-processing, you can use a "Levels" adjustment to spread the graph from the far right to the far left. This will give the image a full-range of tonal values, and improve how it looks:

Histograms Don't Always Tell the Tale
If your image should have a lot of bright or dark area, the histogram may not be "well balanced", but the exposure may still be good. Take this image, for example:

The histogram looks like this:

So, when viewing a histogram, you have to base your judgement on what type of image you’re taking before you can decide if the image exposure is OK. Since this image has so much dark sky, you would expect this bunching to be on the left-side of the histogram. But, since there’s no detail in the sky anyway, this is expected, and OK.

The mysterious "Blinking"
This has nothing to do with histograms, but it’s related. I’ll bet that many of you have seen a strange flashing or blinking when you look at the image you just took on your camera’s display. This is generally an option you can turn on or off through your camera’s settings. It’s something you should leave on, however. It’s a "highlight warning", which tells you which parts (the blinking parts) of your image have NO detail, because these areas are overexposed. Often it’s a clear signal that you need to reduce your exposure – this is best done with the exposure compensation feature on your camera (see this eTip for more info on exposure compensation).

Histograms can be very useful tools, but, like everything else in life, there are exceptions. Learn to read your camera’s histogram, and you can improve your exposures.

Related products to help you take better pictures:

ProDot Shutter Button Upgrade - Black
The ProDot Shutter Button Upgrade We find the ProDot to be one of the best, and least expensive, accessories we own..

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