Histogram; secret or simple?
How to use it in the field and at home.
What does it tell us?
The Histogram is actually simple to understand.
The histogram is a graphic that represents the tonal range of your image.
The black in the image is on the left, the white in the picture is on the right, and between these two extremes are all the grayscales. The scale, left to right, is 0 – 255. The peaks mean that these particular tones have the most pixels in the image relative to the other 255 values.
RGB and luminance Histogram
There are 2 different histograms available, either on your camera or in the software you use on PC.
The RGB histogram is divided into Red, Green and Blue colors; hence the name RGB. Professionals may use each color individually.
Add to these 3 channels the luminance – or exposure – histogram and you have 4 possibilities of working with an image.
Let us begin with the RGB histogram using this image.
The color red.
Notice the 4 checkboxes underneath the histogram. On your PC you will usually have the possibility to turn each channel on or off.
Your eyes are drawn to the red in the image and the histogram tells you that from dark red to light red everything is evenly distributed.
Now let us add green to the mix.
The red has receded and green has taken over. Specifically dark green has a high percentage in the image, shown on the histogram by the green spike at the left. You should know that green has a slightly higher weight in the algorithm because the human eye is fine-tuned to the color green.
When we add blue and luminance, we get this.
The dominant pixels, the spike, still represents an abundance of dark green.
Although it sometimes makes sense to turn everything on and work with all of this, you will find it much easier to work with luminance alone. The luminance histogram brings the exposure into a graphic.
The luminance histogram of the flower above shows you – on camera – whether or not the exposure is what you wanted.
Right or wrong graph
There is no right histogram, just as there is no right exposure.
We create art by understanding the rules; and then bending or breaking them as we see fit.
However, you do not want peaks touching the right or left side of the graph. This means that you either have dark areas with no detail, or white without detail.
Let us take a look at some images along with their histograms.
Out there – in the field – this is a good histogram to see for a quick review of the exposure.
It tells you that the image is balanced nicely between dark and light. When you get back home, you may only want to make minor adjustments to the exposure.
Low-key image, or just plain too dark?
This histogram lets you know that there are no details to be seen in the shadows. Most pixels are right up against that dark wall to the left.
If I remember correctly, this was a color image and I wanted to capture it with the dark tunnel of the trees. It ended up on the cutting floor because I did not get what I wanted.
High key or plain old too much light?
Too many pixels are pushing up against the right hand wall. It would be worth taking another shot after adjusting the exposure because of the lack of details in the sky. This is an example of how a histogram on camera can help you adjust and retake an image in the field.
This one is different from the last image. It is called blown out, or you may hear the term washed out. Looking back at the last histogram, you see more pixels in the mid-range, so there are some details there. In this image, you can see using the histogram that it is mainly whites. The sky, water and wood are much too light to see structure in them.
Highlight warning, also called blinkies, should be turned on in your camera. This is a useful tool that shows blinking in the blown out area of the preview on your camera. For this particular image, you would see the sky alternating with a pure white or red sky, telling you to readjust the exposure and capture again.
Breaking the rules
An interesting challenge here.
Dark woods in the background with mother and child sitting in direct sunlight.
Using my old camera, I could not zoom in any closer. Moving in was not an option either without mom getting upset.
What to do? Expose for her face and let the dark be dominant in the histogram. Then try post processing and cropping on PC to get the best image possible.
I ended up keeping this one, even though it could have been much better.
Easy to understand
So the histogram is no big secret reserved for those who meditate on histograms for hours every day.
It is a very useful tool in the field because you can see blown out whites and shadows that will turn out too dark. After readjusting your exposure, or in some cases using a fill flash, you can recapture the image and get what you wanted.
You may also decide to break the rules and deal with it in post processing at home.
As always, we have not covered every single aspect on this subject, so if you have questions or comments, it only takes a minute or two to ask in the comment section.