Using the camera's histogram
What is a histogram
The histogram is a graph that you can read on your camera that represents the exposure of your image from the blacks and shadows on the left of the graph, through the mid-tones in the middle of the graph, to the highlights on the right. The height of any peak in the histogram is determined by the number of pixels in any specific tone.
The histogram shows the range of tones in an image from pure black on the left to pure white on the right
How is this useful?
Looking at an image's histogram can help you determine if an image is exposed in the way that you want it. There is no perfect shape to a histogram – it will depend on what any image is like as to how the histogram looks. For example an image with a balanced range of tones from shadows to highlights may have an fairly balanced histogram, but a dark image will more likely have peaks that are predominantly on the left side of the graph. Conversely a bright image will have a histogram predominantly peaking on the right side. None of these is wrong, but each is related to the content of the image. However a histogram can show you if there are problem areas in an image – as we'll see later.
An image with a fairly balanced range of tones produces a fairly balanced histogram.
From the histogram we can see that there or no pure blacks or pure whites and that there are more darker tones than lighter.
Darker and lighter images
But we don't always want our images to be tonally balanced – sometimes we deliberately shoot dark or light images – it will depend on your subject matter and location as well as your vision for the image.
This image is deliberately shot dark with the lighter areas appearing in the face.
The histogram for this image is pushed to the left showing that most of the pixels in the image are dark, and some that are completely black, but none are white.
This image is deliberately light with virtually no shadow areas.
The histogram shows that there are no blacks, and that most of the pixels are light – some even pure white (far right edge).
An image with a range of tonal values from blacks to whites...
…and the histogram shows quite a lot of dark tones, including some black (these are mostly in the subject's shirt), fewer mid-tones (in the wall, shadows on the face), and quite a lot of highlights (hands, face and wall), including pure white (in the light reflected in the eyes, shirt button and parts of the hair).
How does this help us?
When you check your exposure using your camera's lcd screen you are seeing an uncalibrated representation of the image. In other words you can't really know if what you are seeing there is what the image is really like – and what it will be like when viewed on a big screen under ideal conditions, or when the image appears somewhere else like in a print. Your camera's screen may be set too bright or too dark. But to complicate things further if you look at it in bright light the same image will appear different to when you are looking at it in dark conditions.
This is where the histogram comes in. With practice you'll be able to tell by looking at the image's histogram if your image is over- or under-exposed.
'Clipping' the highlights and shadows
Spikes on the far left or far right edges indicate that there is 'clipping' in the whites and black – in other words there will be no detail in those areas. This doesn't mean you should never have pure blacks or whites in an image, but you should be aware that if you do then there will be no detail to recover later in your post-shoot workflow.
As a general rule of thumb you should make sure your exposure creates a histogram that just touches the right and left edges, but keep in mind that there may be some times when you want completely black or completely white pixels in the shot. These are probably rare occasions however, and for most of your images you will want to avoid 'clipping'.
Why avoid clipping?
The camera cannot always record all the tonal values in a scene. Imagine a situation where there is a very bright sky and very deep shadows in a landscape. The camera will struggle to record the detail in the sky and in the shadows at the same time. In this situation you may have to choose what you expose for – sky or shadows. If you choose to expose for the sky (to keep the detail of the bright clouds for example), if you're not very careful you may find that you to turn the deepest shadow areas to complete black. AND if you do there will be no recoverable detail post-shoot in those areas. Or you may decide to expose for the shadows to maintain the details there, and run the risk of 'blowing out' (clipping) the highlights. It can be very difficult in situations like that – particularly in very bright conditions to tell from your camera's playback of the image how exposed the image really is and whether the blacks or whites are clipped. The histogram will tell you this.
Clipping warnings
You can also turn on the clipping warning on your camera so that when you are viewing an image where the blacks or whites are clipped, that part of the image will flash. This can be helpful in these extreme situations in conjunction with viewing the histogram, but it can also be quite annoying, so you may decide to keep it switched off most of the time.
Using the histogram when shooting
When you are shooting, frame your shot, check your exposure scale, and then take the shot. Check the image, and check the histogram. Based on what these tell you you will either leave the exposure as it is and carry on shooting, or adjust the exposure to make the image darker or lighter. Do this by using the Exposure Compensation button or dial (shooting in Aperture or Shutter Priority) or adjusting the Aperture and/or Shutter speed individually (shooting in Manual). Shoot and check the image and histogram again.
Get into the habit of quickly checking the histogram – it will eventually become easy for you to understand exactly what it means, and what, if anything to do about it.