Histograms – what are they and how do I read one?

Histograms are readings on your camera LCD or Image editing software, which shows the detail of how your photograph has been exposed.

The exposure consists of the levels of brightness and shadow – commonly known as the tonal range

It graphs the tones in your image from black (on the left) to white (on the right).

It is a good idea to get familiar with the histogram so that when you are taking pictures, the exposure is as balanced as possible, right from the start.

Photographers will often say, take your pictures in RAW format which will allow you to rectify any errors afterwards using photo editing software. The problem is that this is only correct it to a certain extent. Some of the loss of detail due to either too much highlight or shadow, will have been permanently lost and adjustments to regain the over-highlighted areas will result in grainy or ‘noisy’ pictures.

The Histogram can, on all DSLR as well as a lot of compact digitals, indicate the colour levels. The basis of those colours are red, green and blue. Termed the RGB (Red, Green, Blue) value.

The benefit in using a histogram is when you look at the small display on your camera the picture may look great only to find when you download it onto a PC screen, you find the image is under or over-exposed. It can tell you this whilst you are taking the pictures and allow you to retake immediately.

Ok, so how do we read it?

Christmas tree worm

Exposing towards the left with the graphics peaking towards the left of the graph, means accentuated shadows – perhaps a dark background or foreground as in this image.

Turbinate Coral

overexposed histogram

Exposure peaking towards the right means either overexposed highlights, called ‘clipping highlights’ or a lot of white in the image. In this image the graph climbing up the etreme right indicates some highlights have been lost.

Tricolour Blenny

This is quite well balanced but as you will notice the graph doesn't reach the left side indicating it is slightly over exposed and some shadow needs to be restored with your image editing software.

The objective is the have the graphic peaks centered, tapering off at each side, the best is to have the graphic actually touch each end.

It should be clear that there is no correct or incorrect histogram, it all depends on what is desired out of the final image.

The black and white graph is more than adequate unless there is a need to specifically check colour – perhaps when you are getting deeper and colours like red are being absorbed by water depth. Strobes, if you are using them may be a factor and how much power you are setting them at. It will be able to tell you how much red is being replaced by your strobe.

Most of the time your photo editing software has the ability to replace individual colors, particularly if shot in RAW format.

The red, green and blue graphics would be adjusted towards the centre to ensure a balanced colouring of the image.

Click here to learn about photography underwater

Click here to read about more technical tools like Histograms

Click here to learn about using Aperture priority on your camera

Click here to learn about using Shutter priority on your camera

For further reading on histograms click here

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