Wednesday, October 14, 2009

Using histograms on your camera


The Abbey ceiling on the Mont St Michel, Normandy, France presents and extreme exposure challenge.

Do histograms on your camera's LCD really matter and what benefits could you derive from understanding your histogram?

First though, what is a histogram?

At it's most basic level it is a graphic representation that uses a bar graph to show the proportionate distribution of the pixels in your image, ranged from black on the left to pure white on the right. In other words the bars that peak the highest in your histogram show that there are a lot of pixels with that particular tonal value in your image.

If most pixels are on the left of your histogram your image is mostly dark. If on the other hand the longest bars are on the right then most of your picture is bright. Simple. So if the bars are all heaped up on the right hand side then your image is 'clipped' in the highlights, which means there are tones of pure bright white. Another phrase commonly used is that you have areas in your image that are 'burnt out'.

There are different types of histograms. Some show luminance and others provide colour information (RGB). Here's a link to one of the best detailed, technical explanations that I've found on the net, Sean T. McHugh's tutorial on his site Cambridge in Colour.

But are histograms really useful to photographers

The answer for me is mixed. Yes and no. When I'm shooting I have enough knowledge and feel for light not to need to bother looking at histograms. The last thing I can afford is to be peering at the back of my camera and studying the information while all the action happens and I miss the shot.

Histograms do provide a useful tool to photographers who need to learn how their digital camera and its meter react to a given light situation. So like trainer wheels on your first bicycle, they can really speed up the learning process until you're cycling along with perfect balance and everything becomes automatic.

In complicated lighting situations with flash mixed in I will take a peak at my histogram just to confirm everything is fine - but that's driven more by my fear of messing up than anything else. By the way a healthy fear of messing up and recognition of one's fallibility is a good thing in my view and it has saved me on many an occasion.

Ultimately every photographers' goal should be to understand exactly how your camera meter is going to interpret a given situation and having anticipated what it is going to do you can tweak your settings with exposure compensation. So you're ready for the moment when it happens in front of your lens. It's no good getting the shot and then looking at your histogram and finding out you've clipped all the detail out of your highlights. OK so you adjust your exposure for the situation but in the meantime that shot has gone, and probably another one while you were peering at your histogram on your LCD display. Not good.

I shoot 90% of the time in AV mode (aperture priority). The camera is always trying to pull exposure into the middle of your histogram. Your camera is trying to give you a nice bell shaped curve in the middle because it is designed to put most values in the mid-grey (safe) range.

However if you want to produce exciting images with dramatic light then you have to break away from the dull mid-grey world and live on the edge, close up to that bright white or in the deep shadows. Drive the camera, don't let it drive you. Remember you don't get exceptional images by playing it safe. To continue the driving metaphor, no Grand Prix races are going to be won by a racing driver going at 70mph down the middle of the track. Use all the available road. Just don't spin off and crash.

To sum up: learn to see the world like your camera meter sees it, anticipate, adjust and concentrate on getting the image, not the histogram on the back of your camera.

Good luck,
Paul
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