Let’s start with asking what is a properly exposed photo? And we might reply that it is not too dark and not too light, it’s the Goldilocks photo.
But what about a photo of a street at night with one shop with its lights still on? Now we want or expect most of the photo to be dark, maybe almost black. Or what about a snow scene with a lone tree? Now we want or expect most of the photo to be almost white.
One of the great things about digital cameras is that if you take a photo and the exposure is wrong, you can see that when you review the photo in the LCD in the back of the camera. Then you can adjust your settings and take another photo.
But that’s a bit clunky and it assumes that you are photographing the kind of subject that is going to still be available the next time you press the shutter.
Most of the time the camera gets it right. And you might ask how it does that. How does the camera decide how to expose what it is being pointed at? How does the camera know what it is being pointed at? It could be pointed at a scene with a full range of tones, or it could be pointed at a black cat on a white sheet.
Well, cameras are computers and they analyse the tones in the scene and compare them with a mid-grey reference colour. So if they see that the scene is mostly white, they bump up the exposure to make the photo whiter than mid grey.
So then you can leave your camera on automatic mode and let it make all the decisions.
The end.
Except, as a photographer, you have preferences. You may want more depth of field so everything from near to far is in focus. Or less depth of field to separate the subject from the background. Or a faster shutter speed to freeze the action, or a slower shutter speed to give the impression of movement. And maybe you want the scene to look a bit darker and more moody simply because that’s your style and you are the photographer.
And you can’t do that using the camera’s automatic mode, because the camera makes the decisions, not you.
To exercise more control, you need to use the tools that are at your disposal in the camera , which allow you to control the aperture of the lens, the shutter speed, and the ISO. If those controls still aren’t giving you what you want, you also have access to the exposure compensation dial.
Remember, the camera calculates the amount of light that the camera needs to properly expose the scene. And it has two variables to work with, which are aperture and shutter speed. It is like a simple multiplication table where 2×12 gives the same result as 6×4 or 3×8.
So halving the shutter speed and doubling the aperture will give expose the scene exactly the same a doubling the shutter speed and halving the aperture.
That is because f2 is one stop (twice as much light gets in) compared to f2.8 and 1./200 of a second let’s in half as much light as 1/100 second.
As I explained in an earlier article, imagine if the blades on a lens opened and closed continuously with no intervals to indicate the apertures, then photographers would have no indication of how much more open or closed a lens was as they turned the ring around the lens.
From the early days of photography, lens makers designed lenses with intervals, or stops. As you turn the ring, the blades open one click at a time. And each click opens or closes the aperture by a specific amount. Some lenses have clicks for every full ‘stop’. That means that if the lens is stopped down one stop, then exactly half as much light will reach the film or sensor.
This is very handy for adjusting the shutter speed and the aperture to keep the same exposure because you can double the shutter speed and halve the aperture. Or halve the shutter speed and double the aperture. And so on.
So for example, a shutter speed of 1/100 second and an aperture of f2.8 will give the same exposure as a shutter speed of 1/200 second and and aperture of f2.
Aperture Priority and Shutter Priority
The two most commonly used modes are Aperture Priority and Shutter Priority.
In Aperture Priority, you set the aperture by turning a ring on the lens or a dial on the camera body. That is your variable. You set it.
Once you set your aperture, the camera calculates the correct exposure for that aperture and sets the appropriate shutter speed. That’s all there is to it.
In Aperture Priority you set the aperture and the camera sets the shutter speed.
What you see from this is that you control the aperture but you don’t control the shutter speed. The camera controls that after it has worked out the correct exposure.
But let’s suppose you are photographing a horse race. You are standing right by the track with the horses racing towards you. For that kind of event, you want to be in charge of the shutter speed so you can freeze the action. So you would shoot in Shutter Priority. You set the speed to, for example, 1/1000 second and that is your variable. The camera will calculate the exposure and the camera will set the appropriate aperture. That’s all there is to it.
In Shutter Priority you set the shutter speed and the camera sets the aperture.
I mentioned ISO and exposure compensation. I’ll talk about these in the next articles in the series.
Photograph Works 