Lens Apertures

Built into a lens is a set of overlapping blades like you can see here. The photographer makes the aperture bigger or smaller by twisting a ring on the outside of the lens. Twist one way to make the blades open up so the aperture gets bigger and twist the other way to make it smaller.

lens apertures shown in a series of stopped down lenses

When the aperture is bigger then more light gets through the lens onto the film or digital sensor. Which begs the question of why one would ever want to stop a lens down. Wouldn’t it be best to use it wide open all the time?

I’ll answer that here, but first did you know that one thing that separates a good lens from a so-so one is how many blades there are in it. The more blades the more nearly circular the aperture is, and the better the transmission of the light onto the film or digital sensor.

Imagine a lens with just three blades. The aperture would be like a curved triangle and light would spill and scatter and reflect off the edges of the blades. So the more blades the better.

Correct Exposure

You look at a scene and press the shutter. The shutter opens and lets the light through the lens and captures the scene on a piece of film or a digital sensor in the camera.

To get the exposure right, the camera needs just enough light and not too much for the scene being photographed.

Exposure is a combination of the size of the aperture and shutter speed. You can adjust the shutter speed and aperture by making one bigger and the other one smaller and it will give the same exposure.

For example, a one second exposure and an aperture open a certain amount is the same as two seconds of exposure and the aperture open half as much.

And that’s exactly how lenses are designed. Imagine if the blades on a lens opened and closed continuously with no intervals, then photographers would have no indication of how much more open or closed a lens was as they turned the ring around the lens.

So right 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.

The question is, when and why do photographers want to use these different combinations of shutter speed and aperture.

Let’s just highlight a few of the more obvious facts.

In low light, photographers need every bit of light entering the lens that they can get, so that means a large aperture.

Of course, they could choose a slower shutter speed, but maybe there isn’t a slow enough shutter speed in the camera. Or perhaps the subject is moving and a slower shutter speed won’t freeze its movement. Or perhaps the shutter speed would be so slow that even if the subject is not moving, the shot would be blurred because of camera shake.

Camera shake just means that the photographer can’t hold the camera steady while the shutter is open.

Now let’s look at the opposite of low light. In bright light there might simply be too much light entering the lens if it is at maximum aperture.

Of course, you could choose a faster shutter speed. But if you have reached the fastest shutter speed in the camera and it’s not fast enough, then you need to stop down the lens to let in less light.

Aperture also affects depth of field. This is just a fact of how lenses behave. With a small aperture, the front to back distance that is in focus is bigger than with a large aperture.

Large apertures are good for separating the subject from its background. But the small depth of field can mean it’s actually more difficult to keep the subject in focus. For example, if the subject is static, then it’s easy to focus and be sure that front to back distance of sharp focus will capture the subject.

But if the subject is moving backwards and forwards, then there’s a chance that the subject will be outside the front to back distance that is in focus when the shutter is pressed. And that answers the question of why one would ever want to stop a lens down, and why it is not always best to use the lens wide open.

One final point. Not all lenses have the same maximum aperture. Lenses with bigger maximum apertures are heavier (because of the weight of the glass at the front end) and more expensive (because of the cost of the materials). Cheaper lenses have smaller maximum apertures, which means of course that they won’t let in as much light. And if you are photographing animals in low light, bigger maximum apertures can mean the difference between success and no shot at all.

Read this for more about aperture and depth of field.