The image above was shot at ISO 3200 and over exposed by two stops. I showed the results of an exposure test and resulting noise here. This post is a follow-up going into more detail about using the Histogram. The Histogram is the ‘picture of light and shadow’ in chart form. Let’s take a look at using it.
This is the Photoshop Histogram showing all of the color channels in the black to white image. When a histogram piles information up against either, or both, walls it is showing that the areas are lacking detail. The higher the piles the more detail is lost. The right hand side is revealing the highlights and the left is the shadows. Note that even though the image is black and white all color channels are recording information.
Histogram before pressing your shutter button
Many cameras now give you the ability to view the Histogram live on your viewfinder. A histogram is a marvelous tool that helps make decisions on the correct exposure. I keep my histogram active at all times to help decide my final exposure. Let’s do a quick review on what a histogram is showing.
Histogram after the shutter press
If you do not have the ability to see the histogram before exposing, you have the opportunity after, for review. In other words, you can still check on the exposure values using the ‘picture of the light.’ Above you can see the view from the back of the camera from three separate exposures. If you don’t see a view such as this when reviewing your photos look for a button that reads ‘DISPLAY’ or ‘DSP’ when in Play Mode. When viewing images you can change the display mode on your camera for a view similar to above.
The left image is two stops under exposed according to the meter as evidenced by the -2 and the information piled to the left of the histogram. The middle image is showing proper exposure in which information is well within the histogram. The right image is showing a +2 and you can see the red and blue channels are showing over exposure.
In spite of the fact the middle histogram is showing a ‘proper’ exposure (see below) the one that was two stops over exposed was used to make a file with the least amount of noise. See the article and files here.
NOTE: It is important to be aware, especially when trying to push the limits of ETTR, that the histogram you see is of the jpeg camera processed image. If you are capturing JPEG files what you see is what you get. If you are making RAW files you have more room than is shown. My recommendation is to push the envelope with your camera in different situations. Test. And, test some more. Then you’ll know your gear and its limitations. This will keep you from leaving information in the noisy area of your file or throwing highlight info away.
As always, I suggest that you not believe anything anyone has to say as far as technique goes until you have tried and proved it for yourself.
Yours in Creative Photography, Bob