Graphs and numbers sometimes can be a little intimidating. Histogram is one graph that is anything but that. If I can understand it, I’m sure most people can.
Let me try and explain what I understand of the histogram – To me it is : “A graph that plots the luminosity of each pixel on my sensor on a scale from 0 to 255”.
The Histogram comes in form of an overall Luminance and also for each of the channels (Red, Green and Blue). Lets consider the Luminance histogram only for this part.
If i have a 10 megapixel camera, it is going to be a graph containing 10 x 10^6 dots. One for each pixel. A value of 0 for any pixel means that the pixel is completely black and a value of 255 for any pixel means that that pixel is completely white.
Thats what it basically does. Once the Histogram has been plotted, there are a few things that can be interpreted from it
- The Tonal Range of the image
- Is the image overall darker or
- Is the image overall brighter
- Do I need to correct the exposure
The Histogram is generally divided vertically into five sections
BLACK | SHADOWS | MID-TONES | HIGHLIGHTS | WHITES
You would generally want your images having most pixels in the mid-tones section. At-least thats what the camera tries to achieve when it meters. If your graph is sticking too much to the left, assume that it is by and large a dark image. If your graph is sticking mostly to the right, then it is largely a bright image. Take a look at the image below for an explanation of different kinds of histograms
Please note that the wider your histogram is, the more tonal range your images have. So for e.g. if you click a scene in fog, it is highly unlikely that you would have a histogram spread across the entire range because fog makes the scenes less contrasty (less tonal range). Take a look at the image below on the contrary. There is a wide range of tonality and the histogram shows that really well with a wide graph.
Having seen the histogram, you would realize that while the histogram is good at telling you about the tonal values of each pixels and giving an overall picture of whether you have too many dark or bright pixels, it really cannot tell you if the exposure is correct or not because it cannot identify your subject. Having said that, it is a very useful tool to understand the overall exposure of your frame and I wouldn’t hesitate in always keeping it on while reviewing a frame. This clubbed with the Highlight-Alert tool helps immensely
As seen in the image above, the graph (just look to the luminosity graph – first one) tells you that there are some elements which are sticking to the extreme right, thus leading to a blow-out. In this particular case, the blow-out is on the subject (as shown by the Highlight-Alert).
Take a look at the image below which shows you how to enable the histogram in Canon and Nikon
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