Impress people at parties – Dynamic Range and Histograms:
Welcome back. You are back, aren’t you? (crickets…)
As you saw from the examples I posted in “Exposure – Part 1”, a powerful way to gauge exposure is to use your camera’s histogram, (which is a just big word for “graph”…). By now you should know how to view one on the back of your camera, because you read the manual (you did read the manual, didn’t you?). You’ll also know how to interpret the histogram and the corresponding image.
To put a bit more technical background behind it, the histogram is a graph that shows what tonal detail is being recorded by the camera. At the very left is pure black, on the very right is pure white. Photographers call that tonal range Dynamic Range (DR). I won’t go into it in detail, but a measure of DR is ‘stops’. In camera terms, if your exposure is good, you would expect that the peaks and troughs (tonal points) on the graph will fall between black and white, without being pushed up hard against the right or left side.
But here’s the rub – The human eye has a very good DR, and can register upwards of 20 stops in tonal range. That means that you can look at a tree on a sunny day ( a high contrast scene) and make out the details in the shadows under the tree as well as details in the bright sky.
Unfortunately, cameras have a DR of only around 12-14 stops – they can’t see as much as we can. Let’s think about that for a minute. If a high contrast scene has a DR of 20 stops and your camera can only register 12 stops at any one time, you will have to choose which parts you don’t see in your final image. Stop and re-read that if it didn’t sink in, because it’s fundamental to good exposure. I’ve included a graphic example below to help with that concept:
Your eye might be able to see all the above tonal range, but your camera may only be able to register the orange section. So if your shot has a DR wider than the sensor, which part are you going to forego?
Getting Thirsty Yet?
I know you want to start pushing the shutter button, so let’s move on to the ‘holy trinity’ of exposure in the world of digital photography – Aperture, Shutter Speed and ISO. Using these three controls, we set up the exposure of the picture we are taking. But like driving to the beach, you can take many roads and still end up at the same place…
The reason for this is that we make artistic choices that trade one setting off against another. Let me explain with a popular analogy: If a good exposure was a glass of water, we could fill the glass slowly by turning the tap on a little bit, or we could fill it fast by opening the tap wide open. Both ways will get a glass full of water but we are trading one thing off against another to get the result we want.
Aperture is your tap:
The aperture in your camera lens is a hole with an adjustable size. You can set the aperture to be wide open, which lets in more light faster, or you can set it to be smaller, which means that to get a good exposure takes longer. So the choice you make with Aperture is like deciding how much you are going to turn on your tap.
Aperture is measured in ‘f-stops’. A lens might have a range of possible f-stops ranging from say, f2.8 through to f22. Smaller numbers (f2.8) mean a bigger aperture. Bigger numbers (f22) mean a smaller aperture.
So why would you not always use the maximum aperture on your lens? Because you are trading off depth of field (how much of your image is in focus) and sharpness (how sharp the focal point in your image is). Here are two sample images. The one of the left is shot at f2.8 (wide open aperture), the one on the right is shot at f16 (aperture ‘stopped’ down quit a bit):
You can see that they are very different, and there may be good artistic reasons to choose one or the other. If I am taking a portrait, it’s great to be able to use f2.8 because my background will have smooth, blurry ‘bokeh’ that places the emphasis on my subject (in this case, my very attractive hand). But if I am taking a picture of a beautiful mountain landscape, I will want the whole thing to be in focus so that people looking at the images will explore the whole scene from front to back, left to right. The extent to which an image is in focus from front to back is called ‘Depth of Field’ (DOF).
In additional to artistic drivers, all lenses typically have a ‘sweet spot’ of sharpness that is between the extremes of the f-stop range. If you were seeking a really sharp image then you might trade off aperture for sharpness. So rather than get really smooth bokeh at f2.8 but a slightly soft subject, you might stop down to f4.0, still get pretty good bokeh, but ensuring that your subject’s eyes are really sharp.
Still with me? Good. Let’s take a break and when we return we’ll deal with Shutter speed…
[...] Exposure – Part 2, we addressed aperture, and how it impacts on exposure. We also learned about the tradeoffs that [...]
[...] the past few weeks in Exposure 1, Exposure 2 and Exposure 3 I have written about the subject from a theoretical point of view, but how does all [...]