Histograms
Summary
- Histograms are graphs which represent the brightness of the image.
- The left most part of the histogram is pure black.
- The right most part of the histogram is pure white.
- The middle is broken up into 3 sections: Shadows, midtones and highlights.
- You can view the histogram after an image is taken by turning on the appropriate setting.
- Blinkies is another tool that can be used to find the areas in your photos which are over/under exposed.
What is a histogram?
Histograms are a graphical representation of the number of pixels contained in an image and their respective luminance. An image can have a pixel luminance of pure black (0% brightness), pure white (100% brightness) or anywhere in between. The tones in-between black and white are divided into 3 sections: shadows, midtones and highlights. As you can see in the graph below the x-axis is dived into these areas/sections. Something that is not always clearly explained is that the y-axis represents the percentage of pixels in the image that are within a particular luminance range.
Why are histograms useful?
When you first start out, this graph might seem like useless information. I mean why do I care how many pixels are within a specific luminance range?
In fact, the histogram is one of the most useful tools that help photographers properly expose images. As we learned in the metering article, the built in light meter has problems correctly exposing the image in some instances and determining if an image is properly exposed is really hard to do when all you can look at is the small screen on the back of your camera. However, by looking at the histogram your camera provides, you can immediately tell if there are parts of the image that are over exposed (the histogram is skewed all the way to the right) or under exposed (the histogram is skewed all the way to the left). When an area of the image is under exposed, the pixels within this area are entirely black and contain no information (This is called shadow clipping.). The opposite is true for areas of an image that are over exposed. i.e. The pixels in the area that is over exposed are entirely white (This is called highlight clipping.). A little bit of clipping is expected when photographing something that is supposed to be bright (like the sun) or dark (like the shadows from the inside of a cave).
Over Exposed
Under Exposed
Since these pixels contain no information there is nothing that you can do to fix the image in post-processing software. However, it should be noted that both of these issues can be avoided by adjusting the exposure setting on your camera.
With all this being said, histograms will not tell you if you have a "good image". For example, you can have a stunning image with a white subject and a white background and the histogram will be completely skewed to the right. This doesn't mean you have a bad image. It just means that your image is made up of mostly white pixels, which is what you wanted.
However, if you captured an image and the histogram is unintentionally skewed to one side then you know that you need to recompose your photo.
DSLR Cameras will automatically Display the histogram
Normally, after taking a picture, your camera will show you the image you just took. (If your camera does not display the image, the image review setting needs to be turned on under the playback menu.) It is worth noting that this can be changed so that the histogram graph is shown. Expand the sections below to learn how to enable this feature for different camera brands:
- To turn on the histogram do the following:
- Menu >Playback Icon >Playback Display Options >RGB Histogram
- The playback display option setting is also where you can turn the "blinkies" on or off.
- Note: When reviewing an image you can press the up/down button on directional pad to cycle through the different displays you enabled in the playback display options.
- To turn on the histogram do the following:
- Menu >Playback Icon >Histogram Display >Brightness/RGB >RGB
- This is also where you can turn the "blinkies" on or off.
- Note: When reviewing an image you can press the info button to cycle through the different displays you enabled.
- To turn on the histogram do the following:
- Menu >Ger icon >DISP button >Histogram
- The DISP setting is also where you can turn the "blinkies" on or off.
- This is done automatically once histograms are enabled.
- Note: When reviewing an image you can press the DISP button to cycle through the different displays you enabled.
Color Histograms
So far we have just talked about the standard grey histogram. However, your camera has the ability to display more advanced histograms using the red, green, blue (rgb) color space. The RGB histogram graph is still displaying the same information but now the information is broken up by color. This means that the RGB histogram displays the luminance / brightness of each color so we can see if any one color is over/under exposed. Cameras will either display a single histogram graph per color or will display a single graph with the colors overlapping each other. (The standard grey histogram combines all this information into a single graph.)
What are blinkies?
One draw back of using the histogram is that it does not tell you what part of your image is over/under exposed. However, there is a tool generally called "blinkies", some cameras call it highlights (Nikon), highlight alert (Canon) or zebra (Sony), that will help you determine what area of your image is exposed incorrectly. Please note that the drop down menu above has a note about turning this feature on.
This feature gets its name as it will blink over the area that are over/under exposed once enabled. This features doesn’t give you as much information as the histogram but it is easier to see what parts of your image is causing the histogram to skew to the edge.
HDR images
Are your images still producing over/under exposed areas even though you have looked at the histogram and adjusted your exposure setting multiple times? If this is the case it may be because the dynamic range (fancy talk meaning there are dark shadows and bright highlights) of the image is too wide for a single exposure. If this is the case, taking multiple exposures of the same image and then combining them into a single image may be your solution. This is called an HDR (High Dynamic Range) image. The images rotating below is an example of the kind images that are taken to make an HDR photo. Below those images is the end result image. Further explanation can be found in the metering article.
