For many photography beginners, when doing research of which lens to purchase, you may find a lens named “Fixed Aperture”. What is meant by fixed aperture? What is the pros and cons of such lens? This tutorial helps you find out the secret behind.

The Largest Aperture on a Zoom Lens

If you have studied the article 7 Things You Must Know About Camera Lens, you should know that the number on a lens, for example on a 18-55mm lens, f/3.5 – 5.6 means the largest aperture you can set during zooming in and out. When you are using 18mm, the widest aperture is f/3.5. When you are using 55mm, the widest aperture is f/5.6.

Can I use the same largest aperture throughout the whole focal length?

Of course you can! This is the magic of a Fixed Aperture Lens. When you are using a fixed aperture lens. No matter what focal length you are in, you can still use the same largest aperture. That’s why it is called “fixed” aperture. Remember the word “fixed” only refer to the largest aperture. You can still use smaller aperture like f/5.6, f/8, f/16 etc in any focal length.

Pros of a Fixed Aperture Lens

Aperture has two main functions:

  1. Controls the amount of light entering the lens
  2. Controls the depth of field

So when the aperture of the lens is fixed throughout the whole focal length, you can still use a large aperture when zooming in. As a result the amount of light is larger (than using smaller aperture) and a lower ISO is needed to maintain Safe Shutter Speed. The photo produced will then have a lower level of noise which helps make it sharper.

Another benefit of using the same largest aperture during zooming in is that you can generate a more shallow depth of field. This is particularly useful in portrait photography which a shallow depth of field can make the subject more stand-out from the background.

Cons of a Fixed Aperture Lens

In terms of drawbacks, weight and cost are the most significant. Due to more complicated structure and more lens glasses needed, both the cost and weight go up. Usually a fixed aperture lens is double the weight and price tag may go two-folded or even tripled. So it’s your call to decide if a fixed aperture lens worths the extra grams and fortune.


As an experienced photographer, larger aperture can always be useful during practical shooting, especially you always do portrait and star photography. I have seen a lot of students giving up their variable aperture lens and purchase a fixed aperture one after several years of photography. So if you can afford the extra weight and cost, just go for a fixed aperture lens!


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Author: Alex Tam