There’s no need to be embarrassed for asking “What is a histogram?”. While your camera’s histogram is one of the more important tools at your disposal, many photographers are unaware of its capabilities. In this post we’ll answer the question, What is a histogram? And we’ll also answer some of the more common questions around how to read a histogram, where to find it and what you should be looking for on that tiny graph.
What is a histogram: all your questions answered
I’m new to photography: what is a histogram and what does it do?
A histogram is simply a visual guide to the range of tones or brightness levels in an image. It can be viewed on your camera’s LCD screen alongside an image after you’ve taken it, or displayed during Live View shooting, but either way it shows the same thing: a small graph that displays what tones an image is made up of.
So why do I need it?
Because it shows the spread of brightness levels in a scene, it is the most effective way to judge the exposure of an image. It also gives an indication as to whether you need to make the exposure brighter or darker.
The standard histogram found on digital SLRs measures the brightness on a 256-step scale. The far left of the graph represents 0, or pure black, and the far right of the graph represents 255, or pure white.
The steps between these points are filled with all the possible shades in between, with the centre of the axis being the midtone brightness level.
The height of the histogram at each point along this 256-point scale is an indication of the number of pixels in the shot at that level of brightness. The histogram’s size and shape will change depending on the mix of tones in the scene.
Right, so is there a particular histogram shape – an ideal exposure – that I should be looking for?
Well, the histogram is merely a reflection of what is in front of the camera. If the scene or subject is dark, the histogram should be bunched towards the left, or dark, end of the scale; if the scene or subject is bright, it should be over to the right (if it helps you to remember which is which, right is bright!).
If a scene is mainly made up of midtones – a brown horse in a green field, for instance – then the histogram should be humped roughly in the centre of the scale, but there’s no such thing as a ‘perfect’ histogram.
That said, there are occasions when you might need to make adjustments to get a more accurate histogram, and consequently a more accurate exposure.
Is my camera’s built-in exposure meter not very accurate then?
As we discovered when talking about exposure last issue, your camera won’t always get it right automatically.
Because the built-in light meter is designed to achieve an exposure with an average range of midtones, scenes that are much brighter than midtone (such as bright white snowscapes) will appear grey, or under-exposed; scenes that are much darker than midtone (such as shadow-packed night scenes), will also appear grey, or over-exposed.
The way to rectify this is to use Exposure Compensation to make pictures brighter or darker. To do this, you need to press the ‘+/-’ button and turn your camera’s control wheel. Turn it clockwise to brighten up an image, or anti-clockwise to darken it.
This is where the histogram is most useful, as you can gauge how much brighter or darker to make the image.
Take a shot of a white sandy beach on a bright sunny day, for instance, and you know the histogram should be closer to the right of the scale.
If it’s not, dial-in some positive Exposure Compensation and keep checking the histogram until it is. Make sure you don’t go too far, though, otherwise the histogram will become ‘clipped’.
I’ve heard of the term ‘clipping’ before. Why is this such a bad thing?
If the histogram runs off the left or right of the scale, it means that part of that picture information is being lost. If the histogram is clipped on the left, the darkest shadows are said to be ‘crushed’, as they will be pure black, with no detail.
This is fine if you want, say, shadows or the night sky to be pure black, but even with shadows and night skies you usually want some texture or detail.
When the histogram is clipped on the right, the highlights are said to be ‘blown’, and will render as pure white, again without any texture or detail.
That said, there are times when capturing clipped highlights will be unavoidable. Specular highlights – those bright, reflected spots of light that appear on shiny objects – are supposed to be bright white, and don’t actually contain any important texture or detail.
So I should always make sure there’s space between the right side of the histogram and the edge of the scale?
Yes, if you can, but don’t leave too much of a gap. You should always try and expose digital images so that they’re as bright as possible, but not clipped, because for technical reasons the sensor can record more tonal information towards the bright end of the histogram.
Is there ever a chance that the histogram has got it wrong?
No, it’s simply a representation of tonal range. On some cameras, the graph is small, so it can be hard to judge whether the edges of the histogram are at the extremes, but the highlight alert feature will indicate any over-exposed areas.