What Does Fast SLR Camera Lens Mean?

  • What types of situations require a fast lens?
  • Are you considering a new lens, but you're not sure what to buy?
  • What does a fast SLR camera lens mean, and how do you know if you have one?

Whether a lens is fast or not relates to how much light a lens can admit through the opening or closing of the overlapping lens blades, called a diaphragm.

  • What is the advantage?
  • What does this mean for you?

As we go, the chart below will help you understand a fast lens.

Aperture Sizing Diagram
Aperture Sizes

"Fast Lens" describes a lens feature.

"Are you using a fast lens?" has a different meaning than "Can you photograph fast-moving subject?"

A fast lens is about the maximum amount of light a lens can admit. When you look at the above diagram, imagine looking into a lens and observing how much light can enter through the opening.

A fast lens is beneficial in situations such as:

  • Shooting in low light both indoors and outdoors.
  • When you need a faster shutter speed to capture your subject.
  • Fast-moving subjects like birds or sports action.
  • Children or pets at play.
  • Photographing wildlife at dawn or dusk.

How do you know if you have a fast lens?

Look at the numbers on the lens and discover what they tell you. Sometimes it takes a bit of looking to find them, but they'll likely be:

  • On the glass of the lens.
  • On the lens barrel (body).

First, find the number sequence, which begins with 1: The number(s) after 1: are the ones that tell you if the lens is fast or not. 

Close up of camera lens
Note: 1:3.5-6.3 (after 250mm); it is not considered a fast lens.
Close up of camera lens
Note: 1:4 (after 120mm on the lens); it is faster than the first example, but we can do better.

They may vary in range from these examples, and that's okay. So what do the numbers tell you? 

The numbers in the first example, 1:3.5-6.3, refer to the aperture. More specifically, the maximum aperture of the lens.

In the first example, the lens information reads 1:3.5-6.3. 

The number "1" listed first, relates to - "the maximum aperture of this lens is." Therefore, the maximum aperture is 3.5-6.3 in the first example.

In the example, 1:2.8, the maximum aperture is 2.8, so what's the difference, other than the numbers?

Your lens has a diaphragm comprised of a series of blades that form an opening in the middle.

This diaphragm, or opening, is referred to as the aperture.

The diagrams below represent a few of the openings created by the diaphragm, openings you can change for different photography situations.

The chart is an example of a lens diaphragm depicting the aperture. As you can see, numbers refer to each diaphragm opening.

Lenses attaining apertures of f/2.8, f/2, and f/1.4 are considered fast lenses.

Aperture Sizing Diagram
Aperture Sizes

By comparing the numbers on a lens to the lens diaphragm opening, can you visualize the blades closing or opening?

The two lenses below are fast SLR camera lenses because the aperture, or lens opening, can open to f/1.7 and f/2.8. A sizable amount of light!

Close up of macro lens
Close up of macrolens
Close up of fast camera lens

This lens is not considered a fast SLR camera lens because, in comparison, the lens diaphragm cannot admit as much light.

You now know this because the size of the opening can, at best, only open as wide as f/3.5

What else? 

Did you notice that in the example of f/2.8 (above), this lens also has a zoom capability of 70-200mm?

In the second example of f/3.5-6.3 (below), the lens has 18-250mm zoom capabilities.

What happens when you utilize the zoom feature?

There is a significant difference when using zoom:

  • The lens opening stays the same when using the zoom 70-200mm, f/2.8, at a 2.8 setting.
  • However, the lens opening will change when using the 18-250mm zoom if the setting is 3.5 before activating the zoom.

How do you know that?

The information is in the numbers with the dash.

The first example has only one number after 1: 2.8. The second example varies from 1: and the numbers 3.5 - 6.3.

This means when the zoom is NOT utilized, so at 18mm, the lens diaphragm can open to f/3.5. However, when fully zoomed to 250mm, the lens can only open to f/6.3.

And this means less light is available as the lens diaphragm is adjusted and the lens opening size is reduced.

Hopefully, a fast lens makes more sense as you review the chart of your lens and the lens examples.

In low light, using a zoom where the aperture setting can only deliver f/6.3 can be downright frustrating - even when using other camera features to help!

Here is another example for you.

Let's pretend you're taking a drive and you've come across a photo opportunity, but the light is beginning to fade.

You have two lenses with zoom capabilities. One is a 1:2.8, and the other is a 1:3.5-5.6. Which lens will you use to allow more light in as you zoom?

If you said 1:2.8 or the aperture f/2.8, you are correct! 

The 1:2:8 is a fast lens compared to f/3.5-6.3 because of the ability to admit more light.

The 1:4, or f/4, will also remain constant when using the zoom, based on the same principle as the 70-200mm 1:2.8.

In other words, no dash indicates a varying change of aperture when zooming, as in f/3.5-6.3.

The example of the macro lens at 1:2.8 is slightly different because it's a fixed lens at 105mm. Therefore, the focal length remains the same, and there is no option to zoom.

The same applies to the 50mm 1:1.7. A fixed lens means no zoom capability.

We know this because the lens has given us 50mm and 105mm numbers, compared to 70-200mm.

Does this mean you need to go out and buy a fast SLR camera lens? 

It may be unnecessary if you're usually shooting in sufficient lighting and not having a problem. However, fast SLR camera lenses are fantastic because they admit more light.

Additionally, other camera features help with this, such as ISO and shutter speed.

When do I reach for my fast lens?

  • Bird photography.
  • Sports and action.
  • Low light.

Do you want a fast lens?


  • A fast lens admits more light when using a wide-open aperture (small number).
  • You can do this by adjusting the camera aperture settings in camera-specific settings or modes.
  • Extremely helpful in low light situations or when you photograph fast-moving subjects and need a fast shutter speed.
  • You can blur the background effectively in most situations as you open the lens diaphragm.
  • You can use it to help reduce clutter and distractions in the background.
  • It's fun to take photographs with a fast lens. 


  • They are more expensive due to more significant glass elements.
  • They tend to be bigger and heavier.
  • Operating at f/2.8 often results in a shallow depth-of-field.

Depth of field is how much of your scene is in focus from the front to the back, and at times you may want more clarity than a fast lens at f/2.8.

In conclusion.

If you're frustrated with the ability of your current lens, it may be time for a fast lens.

I love my fast lenses for action, low light, creativity, and macro photography, and the zoom lens with a constant aperture (1:2.8 or 1:4) because it will not change when I zoom.

I've taught students who own the 50mm with an f/1.4 or f/2.8, and the feedback is they love the lens for portraiture photography.

Do you know that you can purchase a compatible fast lens from a different manufacturer than your camera?

For example, any Sigma or Tamron lens compatible with Nikon, Pentax, and Canon is possible and often more economical if that helps you.

If it's within your budget and you can see the benefits, a fast SLR camera lens could be the answer for your next photography adventure.

Visiting your local camera store will help you understand fast SLR camera lenses further. 

Ask if they can show you different lenses and notice which are fast SLR lenses.

Finally, fast SLR camera lenses, like all camera gear, can be great fun and help you capture subjects you might otherwise miss. Initially, I thought it would make little difference, but I soon discovered a fast lens is a faithful companion!

Check the specifications.

Please remember to check the equipment compatibility specifications when you purchase photography equipment online, including when purchasing fast lenses. 

For example, confirm whether the lens you buy is not for a mirrorless camera when you have a DSLR. Or when purchasing a brand other than your camera, such as ensuring a Sigma lens is Nikon compatible with your Nikon camera.

To view examples of fast SLR lenses online, please click on Amazon.com to view fast lenses or click on the image links below or  Amazon.ca.

Disclosure: **As an Amazon Affiliate, I earn a commission from qualifying purchases at no extra cost to you.