This is the first part of a four-part series on exposure.
Exposure – an overview
No matter how manual or high-tech your camera may be, or what kind of photos you like to take, what photography really boils down to is exposure. It’s all about making sure the right amount of light reaches your film or electronic sensor to create the desired effect.
There are three things that affect the final exposure: the shutter speed, the aperture, and the ISO. Each one of these aspects works with the other two to create a perfect image.
It’s impossible to go into any kind of depth on such a huge subject with just this one post, so for today I’ll give a quick overview of all three from both a technical point of view, and a creative one. In the following weeks I’ll write about each of them in more detail.
1: Shutter Speed
The shutter speed is probably the most commonly understood aspect of exposure. This is the length of time the shutter is open for, thus allowing light onto the film or sensor. It’s a very simple idea; a fast shutter speed is open for a shorter time and lets in less light, a slow shutter speed is open longer and lets in more light.
The shutter speed is measured in seconds, and fractions of a second. For instance, 1/250 is a two hundred and fiftieth of a second, 1/1000 is one one-thousandth, and so on, and if you take the speed down you get to full seconds, such as 2”, which is two whole seconds.
Apart from the way they control the light, each one of the aspects of exposure has its own special influence on the image’s style. The effect of the shutter speed is the way it captures motion in an image.
A fast shutter speed will freeze action, which is great when taking photos of children, animals, drops of water, or anything else that doesn’t stay still for long.
I dragged my brother out into the cold garden this morning to juggle so I could demonstrate this for you properly. These photos aren’t works of art, but they get the idea across. The first photo was taken with a fast shutter speed; you can see the juggling balls frozen in midair.
As a comparison, for this second photo I put the shutter speed down as low as I could without overexposing the photo. Instead of freezing motion, a slow shutter speed blurs the movement of what you’re capturing.
This can create some interesting effects, although as you can see it didn’t really work in my favour here, aesthetically speaking. His movement makes him look less in focus than the background (Which, you’ll notice, is more focused in this photo than in the first. This is because I had to close the aperture, which widened the DoF. We’ll talk more about that in a moment). But at least it shows how you can capture movement with slow shutter speeds. I promise to have better examples next time we come back to shutter speed!
Finally, of course, a slow shutter speed allows in a lot more light. With a tripod, or other steady surface, you can set your camera up to take photos where light is in short supply. I took the following photo by resting the camera on the stone steps, and waiting. It was a 16 second exposure time so, while the lights on the water’s surface are completely crisp, you can see some very slight motion blur on the boats.
The aperture is basically the size of the hole that is letting light into your camera. You can open and close the aperture of your camera’s lens, affecting the exposure. The size of the aperture is measured in f-stops – the smaller the number, the wider the aperture, and the more light can get through (at college I used to repeat over and over, as I was trying to work out my old manual Pentax, “small is big, big is small, small is big, big is small” It helped me…), for instance, an aperture open to f/2.8 will allow in a lot more light than an aperture only open to f/16.
When you’re shopping for a lens, you’ll see these f-stop numbers as part of the lens description, this is the widest the lens’s aperture will open. When you have a large aperture, more light is allowed through and you can get away with a faster shutter speed (hence some lenses being known as “faster” than others).
The aperture affects the Depth of Field (or DoF) in a photo. In other words, it dictates how much of your photo is in focus. A small aperture (a large number, such as f/16) means higher Depth of Field. In a photo with very high Depth of Field, everything will be sharp from front to back. High Depth of Field is often used in landscape photography.
I took this next photo with a very small aperture (f/20). You can see a lot of the detail in the woods behind the bluebells, and across the ground as well.
A large aperture (a small number, like f2.8) means low Depth of Field. In an image with very low Depth of Field, there may be only a tiny section of it in focus, with the rest simply a blur of colours.
This is the same set-up as the picture before, but with the aperture opened right up to f/1.8. As you can see, there is a huge difference between the Depth of Field in the two pictures. The background woods now have little more detail than some shadows and lighter patches, and even the foreground is out of focus.
Using a large aperture is an excellent way of bringing attention to a subject, and is commonly used in portraiture.
The ISO is the way a film or camera-sensor’s level of light sensitivity is measured. A low ISO number, such as ISO 100, indicates a lower light sensitivity; it will absorb light slower than a higher ISO setting. Conversely, a high ISO number such as ISO 1600 will absorb light a lot faster than a lower ISO setting.
A low ISO number should provide smooth, even colouring, with low grain. Here is a photo I took with a very low ISO.
And a close up, so that you can see the smooth colours and lack of grain.
A high ISO, will capture light a lot faster, but as it does so it creates grain in the image produced. This grain increases with the ISO setting. At ISO 400 there may begin to be some traces, while at ISO 1600 or 3200, the grain will most likely be very pronounced.
Here is a photo I took right after the previous picture, but this time I bumped the ISO way up to 1600. As you can see there’s a lot of grain all through the image, although in this case I quite like the effect it gives here. You should be aware of how ISO causes grain in your photos so that having grain is a conscious decision you make to suit your subject.
And here is a close up of the high ISO image so that you can see the grain compared to the first of the two photos.
And when you tie it all together…
You get perfect exposures.
Well. Not quite. If only it were that easy.
What I’ve written here today is the most basic of overviews. I will be posting on each of these aspects of exposure separately in much more detail, showing how each one affects the other two, and the choices you can make to get the best results for any subject.