Beginner Tutorial – Maximum Lens Aperture

In the past two posts, I’ve been talking about aperture, and how you’re able to control this setting on your digital SLR camera.

Here’s the high-level overview of aperture:

  • Aperture indicates whether the opening in your lens is wide open or narrow
  • Your digital SLR communicates with the lens via electronic contacts
  • You can control the width of the opening by changing the aperture setting from the camera
  • A small aperture number (2.8) represents a WIDE opening in the lens
  • A large aperture number (22) represents a NARROW opening in the lens
  • Aperture numbers follow a standard scale called f-stops
  • Changing an aperture value by a full stop either doubles or halves the amount of light passing through the lens
  • Aperture values can be changed in either half stop or third stop increments

Here’s the standard set of aperture numbers:

f/2.8, f/4.0, f/5.6, f/8, f/11, f/16, f/22

In this post I’m not going to go into as much detail about the aperture setting itself (that’s been pretty well covered) but instead I’ll explain why all lenses are not created equal, and how aperture comes into play.

There are two key points about lenses that are required to begin with:

  1. Virtually all lenses can be set to the same narrow aperture (f/36)
  2. Not all lenses can be set to the same wide aperture

When it comes to lenses, this second point is a limitation of some lenses and this limitation has a name: maximum aperture.

Let’s compare two lenses that are equivalent when it comes to focal length: both are 18-55mm zooms. Let’s say that lens A has a maximum aperture of f/4 while lens B has a maximum aperture of f/2.8. Now, BOTH lenses can be set to narrow apertures (anything between f/4 and f/36), but the f/2.8 lens can open up much wider than the f/4 lens. In fact, the lens with the f/2.8 maximum aperture can let in twice as much light as the lens with the f/4 maximum aperture.

The practical take-away here is that if you tend to enjoy taking photos in REALLY dim light (cloudy days, building interiors, etc.) then you’ll want to look for a lens with a very wide maximum aperture.

The most common aperture numbers that you’ll see on kit lenses are f/3.5 and f/5.6. The reason that there are two numbers is because the maximum aperture CHANGES depending upon the focal length of the lens. At 18mm the maximum aperture of a kit lens is a wide f/3.5, but if you zoom to 55mm that maximum aperture narrows down to f/5.6.

Now that you’ve got a better understanding of maximum aperture, let’s discuss why using the maximum aperture isn’t always such a good idea.

When it comes to image sharpness, lenses don’t perform as well at extreme apertures as they do with apertures in the middle of the range. Example: a 50mm lens set to f/2.8 or set to f/36 won’t capture images that are nearly a sharp as when you use apertures f/8 and f/11. Now, some lenses are better than others at maintaining sharpness throughout the entire aperture range – this is why they cost well over $1,000 USD.

For anyone who doesn’t have this type of budget for lenses, you’ll just have to live with the fact that your lens probably won’t be ultra-sharp at very wide and very narrow apertures. If you take photos with plenty of light, then you won’t have any problem using apertures between f/8 and f/16 all the time. However, if you enjoy taking shots in dim light, you’ll often find yourself using the maximum aperture of your lens.

When shots don’t turn out quite as sharp as you hoped for, maximum aperture can be one of the culprits.

By kind permission of Chris Roberts of