Understanding the Histogram

Female nude portrait from photographer Mark Laurie of Calgary Alberta

The histogram on the back of your camera is an amazing device for interpreting your exposure. You need to understand what it is reading, what it is telling you, and how to interpret it, to harness its power.

First the broad strokes: essentially the histogram tells you the placement of all the grey tones in the image. It does not “see” colour. The left side is the amount of pixels in the image that are dark to black. Moving right, the very middle tells you the volume of pixels that are mid tones or 50% grey. On the right side is the volume of pixels that are light to white.

These are also called shadow, midtone, and highlights.

Understanding the Histogram on your digital camera settings

The more the histogram shifts to the left,
the darker the overall image is.

 

 

Understanding the Histogram on your digital camera settings

The more it shifts to the right,
the lighter the overall image is (often called highkey).

 

 

If the bulk of the density is in the middle, then the image is very flat with little contrast.

If you take a reference colour balance set – like whibal or any device that has a black and white and a grey tone – and photograph that with perfect exposure, you should see three lines only in your histogram, one at each end for the black and white cards and one dead centre for the grey card. If it is shifted in either direction, then you have overexposed or underexposed the image.

It’s important to know the histogram is pulling its information from the image style of the jpg version, even if you are shooting in camera raw. Camera raw has a larger range of exposure than a jpg, so the jpg does not represent the real full range your camera raw file will hold. Your histogram will also change a bit, with the same scene, if you change your jpg picture settings.

Different cameras have these settings named differently. Essentially the camera’s computer pre-processes the capture data to confirm to the picture style chosen. Camera Raw does not alter the file, it’s like a negative so you an twist any way as often as you like yet still reset to original exposure settings.

Another critical thing to understand is the brightness of the captured image you see on the back of the camera does not relate fully to the exposure of your image. You can control the brightness of your LCD screen independently of the image exposure brightness. That is why it is so dangerous to judge the exposure of your image off the LCD screen. 

The histogram does not judge the brightness levels in the image. For example if you have an image that has a lot of bright reflective chrome, it sees overexposure in the highlights. You will never get detail in those reflections so you don’t need to try to adjust exposure.

Understanding the Histogram on your digital camera settings

The spikes, gaps, etc in the histogram are not good or bad –  each line simply tells you if there are any pixels in the image that hold that shade of grey. The histogram covers 256 shades of grey from pure black to pure white. Each numbered grey shade of the 256 corresponds to a specific shade of grey that may or may not be in any given image. The height of the line tells you how many shades or tones are in that specific shade of grey. So if the line is high or low does not matter, it’s just telling you how many pixels in the image fall in that very precise shade of grey, throughout the whole image.

What does matter is where the ends are. Digital sensors capture highlight detail better than shadow detail. They don’t handle the dark end very well where it’s underexposed. That is where you get the grain and noise.

If your image is underexposed (a chronic problem), then there is detail you will never be able to recover. As your exposure becomes properly exposed you will see the bulk of the histogram lines move to the right.

Understanding the Histogram on your digital camera settingsIn a full tonal range image you will see the vertical lines spread out across the histogram. Ideally you would like the highlights that you need not to get to the far right, and the shadow detail you need not get to the far left.

When you are taking photographs, you have to know or remember this really important reality. The eye can see an amazing range of detail, more than the sensor can. The sensor can see more detail and exposure than a monitor can show you, and a monitor can see detail in blacks and highlights in a wider range than paper can. You need to have your histogram showing you an exposure that your output device can resolve – be it paper or monitor.

Spend some time observing how your camera’s sensor sees the images relative to what you can print or see on the monitor. From film to digital exposure has always been one of the most critical elements to get right for a brilliant image. Armed with this understanding of the histogram tool on your camera, you should be able to really refine the quality of your images.

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