Nailing the Exposure-The Histogram

I shot all my early images on large format film, with a hand held light meter, so I can tell you there are a few big things that make digital cameras such a huge leap from film. Such as…

• Instant feedback – you can see it the moment you shoot it.
• Much broader dynamic range – digital captures the extremes so much better.
• The Histogram… Why? Keep reading…

It’s been a while between posts. This article follows directly on from my previous posts about shooting into the light… you can read the previous post HERE>>

Why the Histogram?

So why is the Histogram so revolutionary, and why do I think it’s one of the best things about digital photography? Three things spring to mind immediately…

• It’s fast – I can see at a glance my exposure, and thus all the highlight and shadow detail available for adjustment in post production.
• Since I use a mirrorless camera, I have the histogram in my viewfinder and can make instantaneous exposure decisions, even during hi-intensity photography like festivals or sports.
• I can control noise in my images more effectively (See the next post for more on this).

All this becomes especially important in tricky lighting situations such as scenes with extreme contrast or shooting into the light.

What is the Histogram?

The Histogram is literally a graph of your picture… Your camera converts your image to greyscale (actually luminosity, but greyscale will do for now), then divides it up into 256 levels of brightness. Black at level 0 (0% brightness), to white, at level 255 (100% brightness), this becomes the horizontal axis across the bottom of the graph.

The vertical axis denotes the number of pixels that fall in each brightness level. So you have a visual representation of the brightness of all the pixels from within your image.

So why can’t I just look at the Jpeg preview on the back of my camera?

Well you can, and while the preview doesn’t actually lie, it does distort the truth. Why?

• The preview is small, so big areas of dense black shadows just look like small black blobs, and thus not very significant… until you see them on your computer, or worse, on your prints.
• Little patches of white that don’t seem offensive on the small screen of your camera can be disastrous when you discover your lovely cloud filled sky is featureless white… Or a textured white brick wall, or the sunlight reflecting off the side of someone’s face…
• Images look better on your camera screen when you under-expose, colours appear richer. But, you will often be losing important details in the shadow areas, which you won’t notice on such a small image. The histogram allows you to see what you really have.
• There is a lot of hidden information in raw files (see image below), but if it’s hidden, how do you know it’s there? Hint: The histogram….

The majority of my post production involves the balancing of light and shadow, which means having details in both shadows and highlights. Knowing what details I have captured is very important, and doubly so if the image is to be printed.

So how do you read the histogram?

The histogram shows all the brightness values of all the pixels in your image, from full black on the left, through shadows, to midtowns, to highlights and then pure white on the far right (see the image 2 above).

So all the dark areas, especially shadows will appear on the left of the graph, the mid tones in the centre and the brighter areas to the right. This shows you how you have exposed the image, and how much of each brightness level you sensor has captured or will capture – in the case of the preview histogram (what you see in a mirrorless camera viewfinder before you take the shot… but only if this function is turned on…).

Important things to remember are…

• Any pixels on the extreme left (climbing the left wall) of the histogram are featureless black… The shadows are “Blocking up”, meaning no detail.
• Any pixels on the extreme right (climbing the right wall) are “Clipping” and will be featureless white… also, no detail.
• NB: Don’t worry so much about the height of the graph, it can be distracting but it’s just the number of pixels in each zone.

Note: your camera doesn’t know what it’s looking at, there is no AI involved here (yet)… Your histogram won’t tell you if it’s a good image, whether it’s focused, if your subject is worthy or if you have nailed your composition. What the Histogram does tell you is everything you need to know about the exposure.

Pushing the Histogram around

If you increase your exposure, by either:

• Slowing down the shutter speed, opening your aperture, increasing your ISO – if using Manual Mode (M).
• or increasing your exposure compensation (+1, +2 etc…) – In Aperture Priority (A or Av), Shutter Priority (S or Tv) or Program (P) modes.

You will shift the whole histogram to the right. Conversely if you do the opposite you will shift it to the left.

What you will see is…

• As you shift the histogram to the right your preview image will get brighter and the colours will wash out.
• As you shift it to the left your image will get darker and the colours richer.

It’s tempting to push the histogram to the left as you preview image looks so much better, and this is the most common response. But if you are shooting in raw (instead of Jpeg), you may want to try a different strategy that I will explain in the next post.

What are the 3 histograms I sometimes see?

The 3 extra histograms you will see when reviewing your images on the back of your camera are the complete RGB channels. Red, Green & Blue light, which together make up the light spectrum that is visible to us and our cameras.

In the image above, which is taken of the back of my camera while reviewing an image, you can see all 4 histograms. There is usually a display function button that toggles this view on and off, or in some case you scroll down to see these.

The white histogram is luminosity, or supposed to be. Actually it’s a combination of two out of three of the RGB channels… Don’t worry too much about this.

The important thing to note is that the white “brightness” histogram is the main one you should be using most of the time, and it’s the only one you will see in preview mode (in your viewfinder or rear camera screen when you are composing the shot).

The RGB channels you should take note of when you have strong dominant colours, such as autumn leaves in your shot as with the example above, or important micro highlights, such as small highlights in those same leaves.

You can see in the example above that the red channel is very dominant, especially in the mid and high tones, so you can see why you might want to check that this is in range (not clipping in the red) before you walk on to the next shot.

The blue channel is very strong in the shadows as this image was taken in the early morning, and all the shadow areas were lit by a bright blue sky.

Conclusion

We have all taken those shots that are truly awesome, unrepeatable images, that were soooo close to brilliance… if only the highlights weren’t blown out… (Or the shadows totally black…)

Learning to read your histogram (and using it) is one of the best things you can do for your photography generally, it will take the guess work out of exposure and allow you to concentrate on the creative aspects, which are much more fun.

In the next post I’ll expand on the use of the Histogram with a technique you may or may not have heard of.