Cameras can show a graph divided by 3 vertical lines. What does this graph describe about the picture, and how can I learn how to read it?
What does this graph describe about the picture, and how can I learn how to read it?
That graph is a histogram showing the relative frequency of different pixel brightnesses. Pixel brightness values are on the x-axis, increasing from 0 (completely dark) on the left to max (brightest) on the right. The y-axis shows the relative frequency of occurrences of a given value. So, if the graph is very low for a given point on the x-axis, then the image has few pixels of that brightness; if it's very high, there are many pixels with that brightness. If the left side of the graph is a lot higher than the right, then the image will have lots of dark pixels; if the right side is a lot higher than the left, lots of bright pixels. If the graph looks flat, then there's an even distribution of pixel brightnesses (that's pretty unusual).
Here's an example image:
And a brightness histogram for that image looks like this:
One thing you can tell from the histogram is that only a few pixels are completely dark or completely bright, which means that very little detail is being lost due to under- or overexposure.
A lot of cameras can also show a histogram that's actually three different histograms layered together. These show the values of the red, green, and blue components of each pixel:
This is the same idea as the combined brightness histogram, but you get the individual color components broken out. That can help you judge white balance and also see if any individual channel is being clipped at either end.
How to read the Histogram
A histogram is a graphical representation of the pixels exposed in your image. The left side of the graph represents the blacks or shadows, the right side represents the highlights or bright areas and the middle section is mid-tones (middle or 18% grey). How high the peaks reach represent the number of pixels in that particular tone. Each tone from 0-255 (0 being black and 255 being white) is one pixel wide on the graph, so imagine the histogram as a bar graph all squished together with no spaces between each bar.
When the histogram tells you to adjust your exposure
Gaps on either end indicate you are missing information and your exposure can be shifted safely without losing detail. When your graph is shifted too far in one direction or the other so that it does not even touch the other edge – that means you can safely shift your exposure to cover more of the range of tones.
What do the spikes up the sides mean?
Spikes up the left or right edge indicate “clipping” of that tone and loss of detail in that area. Clipped areas are often unrecoverable, especially in the highlight area but it is generally advised to expose so you your graph just touches the right edge and keep your highlight details. It is usually easier to recover some shadow detail and retain a decent image, than try and create highlight detail that isn’t there on the file.