SLR photography — understanding the histogram

After you take a shot, your digital SLR will often display a little graph on its screen. It might look confusing, but it's a very handy tool to use. And it's dead easy to understand once someone explains it to you. Which is what I'm about to do now.

What is the histogram saying?

The histogram is a graph showing the amount of different levels of dark, light and in-between tones.

Huh? Don't worry. It will make sense in a little while. But for now, let's just say that the left side of the histogram tells you how much black stuff there is in the shot. The right side shows how much white stuff. And the bit in between shows how much is, well, in between.

Making sense of this stuff with some examples

Imagine you left your lens cap on when you took a photo. You'd end up with a totally underexposed black image like my first example, shown below.

Shown at left is the 'photo', and at right is its histogram. It's a simple image, so you get a simple histogram. See how the histogram says that the amount of blacks goes right to the top of the graph. Meaning that there is real lot of black.

Now have a look at the next example. This time the photo has been totally overexposed, resulting in a completely white image.

You'll see in each of those examples, the histogram is a visual representation of the amount of black or white.

Hopefully this will start making sense now, but I'll add another example (below), showing how a pic full of nothing but mid greys would look in a histogram.

That's enough of the blank images.

Next example shows a typical real photo. You'll see that all of the tonal values in the photo, as the histogram illustrates, lie comfortably in between the extremes of black and white. Now I'll just make the point here that in this shot, the graph makes a nice even hill shape. At first you might think a nice even hill shape is good but it's neither good nor bad. The important thing is that it all sits inside the borders defined by the right and left edges of the graph.

And just how is this making us better photographers?

Let's imagine that you're outside taking a photo on a sunny day. You look at the little screen display on the back of your camera to see if the picture turned out okay. Well you think it did, but it's kind of difficult to judge from looking at that screen in bright sunlight. And you can't run back home to check things out on your computer after every shot.

So you look at the histogram. In the shot of the Noisy Miner bird above, you'll see that the entire shot is made up of tonal values that sit comfortably in between the extremes of white and black. The histogram makes that point very clear, even on that tiny screen in bright daylight. This tells you your camera was able to capture the full range of lights, darks, and in-betweens in the shot. If everything else kind of looks okay, then most likely the shot is correctly exposed.

But what about the one below?

See how on the far right of the histogram, the graph makes a sudden jump to disappear off the edge? That's a warning sign. It means there's a good chance part of the shot has been overexposed.

Sure enough, when you look at the resulting photo on your computer back at home, you'll see that the top of the wing was completely blown out (overexposed). There is absolutely no detail there, and the result will be a giant ugly blob of zero-detail nothingness (that's a term you won't find in many camera books) in your photograph. Even if you try to tweak the image using something like Photoshop, you'll be disappointed. Because the camera's sensor didn't capture any detail at all in that part of the scene.

So, if your histogram jumps up off the right edge like it did with this picture, you need to take another shot, but this time you know you need to reduce the exposure.

Likewise, if the histogram shows the graph disappearing off the left edge then you need to take another shot, but this time increasing the exposure.

How do you adjust your exposure if the camera keeps choosing its settings automatically?

If you're in Time Value mode and you choose a faster shutter speed to reduce the exposure, the camera makes up for it by picking a bigger aperture for you. Which cancels out your change. Likewise, In Aperture Priority mode, the camera adjusts the shutter speed when you change the Aperture. So how do you over-ride this and adjust the exposure?

In this situation, I like to use the camera's Exposure Compensation function. You tell the camera to make things lighter or darker than it normally would. (I explain more about Exposure Compensation here.) Your camera's manual will tell you how to make your camera do this. You see, camera manuals are hopeless for some things, but really good for looking up this kind of thing. In the case of the bird shot above, I'd tell the camera to adjust its exposure by about 1 stop down.

Thanks to your histogram, you ended up taking that next shot, but this time it was correctly exposed.

See? It really did make you into a better photographer! Now you just have to get that bird to take off from it's perch again...

One more trick

Chances are, your digital SLR will have an option for a 'highlight alert'.

If it does, I reckon it's a great idea to activate it.

What a highlight alert does is this: if your camera senses that part of a shot has been blown out, it will flash that region in alternating black and white on your camera's screen, as a warning, like in the example below:

The moment you see those blinking black bits, you know you've got some blown-out (overexposed) parts in your image. So you get the chance to reduce your exposure and take the shot again.

Beginners’ series on
digital SLR photography

• A beginners’ guide — basics for digital SLRs
• Learning from examples — picking up where the first article left off
• Understanding histograms — making sense of this handy tool
• Preventing washed-out photos — why it happens, and how to prevent it
• Understanding Exposure Compensation — why your photo subject can look so badly exposed, and what to do about it
• Megapixels — how many is enough?

Photo Sales

• Photo Library — pics of Australian critters

An unhelpful guide

• Wildlife photography — this guide will not make you into a better photographer
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