That little graph thing which appears on the back of your camera can help you take better photos
What is the histogram saying?
The histogram is telling you how much black, white and in-between tones are in your photo.
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 your camera. You’d end up with a totally black image like this ‘masterpiece’ shown below.
It’s a simple image, so it produces a simple histogram. On the left side (representing black) the graph would go right up off the top of the scale. Why would it go right up to the top? Because there is such a huge amount of black in the image. And since there are no light tones that’s all the histogram would show.
In the next example (above) the photography’s improving, but not by much. It’s a totally grey image now. The histogram shows there are lots of mid-tones and nothing else.
And of course if the image was totally white (above) the histogram would go off the top of the graph at the right side.
Hopefully this is starting to make sense now. In each case, the histogram is nothing more than a visual representation of how much black, white or in-between tones there are in a photo.
That’s enough of the blank images.
The next example (above) shows a typical real photo of a Noisy Miner. 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.
Actually, this is a good chance to see what the histogram is really saying, so let’s look at the photo again. See all that green grass in the photo? That green is mid-toned, meaning that it is not white and not black but pretty much in between. And there’s a real lot of it in the photo. Which is why the mid-tones part of the graph goes up so high. But see how there’s not nearly as much blacks or whites in the photo? So, sure enough, the graph is much lower where it represents the blacks and whites.
I’ll make the point here that in this shot, the graph is making 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.
You see, as long as the graph does not reach into the limits of the left or right sides, then the histogram is telling you there is nothing in the photo that is either too under-exposed or too over-exposed for the camera to handle.
And just how is this making us better photographers?
Let’s imagine you’ve just taken a photo outside 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 hard to tell 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’d see that the entire captured image is made up of tonal values that sit comfortably in between the extremes of under-exposure (blacker-than-black) and over-exposure (whiter-than-white). 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 for that shot. If everything else kind of looks okay, then most likely the shot is correctly exposed.
But what about the one below?
At first glance that picture might look okay on the back of your camera. But see how on the far right of the histogram, the graph makes a sudden jump up to disappear off the edge? That’s a warning sign. It means part of the shot has been overexposed.
And sure enough, when you look at the resulting photo on your computer back at home,
you’ll see that the top of the bird’s right 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 which has yet to catch on in the camera books)
in your photograph. Even if you try to tweak the image using something like Photoshop,
you’ll be disappointed. Because the histogram is telling you 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 Priority 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 its 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.
