How to read a histogram? [duplicate]

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?

• Do you mean histogram? – thebtm Mar 2 '17 at 15:25
• could be....dont realy know what its called – SE is Evil - Support Monica Mar 2 '17 at 15:51
• Do you have an example photo of what you are trying to look at because histograms and spectrograms are different things? If you are looking for a Histogram, this question has already been answered. – thebtm Mar 2 '17 at 16:00

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.

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.

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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.

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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.

• Speaking of laziness, you've copied the 3 main paragraphs in your answer verbatim from the linked page without a block quote or quotations marks. – Caleb Mar 2 '17 at 16:47
• @Caleb I did state the source in the beginning – Janardan S Mar 2 '17 at 16:48
• I see that, but without some marking the reader can't tell that all but the last line of your answer is copied directly. Even with the "From:" this seems poorly attributed. Also, the last line seems out of place on a SE site. There are LOTS of questions here that could be answered via Google, but people come here for the high quality, concise, original answers. – Caleb Mar 2 '17 at 16:52
• @Caleb Well, there are hundreds of really good webpages on histograms in the net. One just has to search 'camera histogram' in Google. – Janardan S Mar 2 '17 at 16:57
• @Janas There is nothing wrong with quoting supporting information. But the point is using the source to clarify, expand, or otherwise support your answer, rather than having an entire answer be wholesale copy/paste from somebody else. The distinction is this: A link-only answer that just points the reader to go somewhere else is a poor Stack Exchange answer. But an answer that consists only of copy/pasting blocks of text from that source is plagiarism, or at least skirting close to it. – scottbb Mar 2 '17 at 18:01