Understanding the Role of Lens Aperture Range in Digital Photography

By Julie Adair King

The aperture is an adjustable hole through which light must pass to reach the image sensor on your digital camera. Aperture size is stated in f-numbers, more commonly referred to as f-stops. A higher number indicates a narrower aperture size. So f/11, for example, results in a smaller aperture opening than f/8.

Changing the aperture size is one way to manipulate exposure. But the f-stop setting also contributes to depth of field, or the distance over which focus appears sharp. The smaller the aperture, the greater the depth of field.

For these variations, the focal length was the same, but the aperture size (f-stop) was adjusted to manipulate background sharpness.

If you’re keeping track, you now know that the lens gives you two points of control over depth of field: the focal length and the aperture setting. In the image above, the same focal length was used for each shot, so the aperture setting is the sole reason for the shift in depth of field.

And why, you’re probably wondering, is the exposure of both images the same, given what was just stated about the f-stop affecting image brightness? This is why: When you open the aperture, you also reduced the exposure time, so the light is able to strike the image sensor for a shorter period. The light sensitivity (ISO setting) was kept the same for both photos.

For the purpose of comparing lenses, you need just a few more bits of aperture information:

  • Every lens has a specific range of aperture settings. Obviously, the larger that range, the more control you have as a photographer.
  • The larger the maximum aperture, the “faster” the lens. Again, the more open the aperture becomes, the less time is needed to expose the image. So if one lens can open to a maximum setting of f/4 and another lens has a maximum aperture of f/2, the f/2 version is said to be faster.

A fast lens is especially beneficial when photographing action, because a moving subject blurs at long exposure times. But it also helps when you shoot in dim lighting, because you can get the shot at a lower ISO setting, reducing the chances of image noise.

  • On a zoom lens, the aperture range may change as you zoom in or out. For example, on an 18–140mm lens, you may be able to open the aperture to f/2 when the lens is at the 18mm position but only to f/5.6 at 140mm.

You can buy zoom lenses that maintain the same minimum and maximum apertures throughout the zoom range, but be prepared to part with more money than for a lens that doesn’t offer this feature.

  • Depth of field at any aperture varies depending on the size of the image sensor and lens. Cameras with small sensors and lenses produce a much greater depth of field at any f-stop than cameras with larger sensors and lenses. The result is that it can be difficult to achieve much background blurring even if you open the aperture all the way. That’s an important consideration if you’re interested in the type of photography that benefits from a short depth of field, such as portraiture. On the other hand, if you’re a landscape photographer, you may love the extended depth of field those smaller cameras produce.