What Does Fast SLR Camera Lenses Mean?

What does fast SLR camera lenses mean, and how do you know if you have one?

  • What types of situations require a fast lens?
  • Are you considering a new lens, but you're not sure what to buy?

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?

Understanding the numbers on a lens is the first part. The second part is learning how to adjust the camera to take advantage of the light.

In other words, to understand camera aperture settings.

The chart below, as we go, will help you understand a fast lens.

Aperture Sizing Diagram
Aperture Sizes

Let's go step by easy step.

When applicable to a lens, the term fast describes the type of lens one may be using. For example, the question, "are you using a fast lens?" has a different meaning than "are you able to photograph fast moving subjects?"

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

A fast lens is beneficial in certain situations, such as:

  • Shooting in lower 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?

Let's look at the numbers on the lens and discover what they're telling you.

Sometimes it takes a bit of looking to find them, but they'll either 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 the 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.
Close up of macro lens
Close up of macrolens
Note: 1:2.8 (for both lenses) These two lenses are in the category of fast.


Because they can admit, or let in, a lot of light through the lens diaphragm opening at the optimum setting. 

Your lens will have a set of numbers, too. 

They may vary in range from these examples, and that's okay. 

What do the numbers tell you? 

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

What is the maximum aperture?

Simply put, the aperture is about how much light a lens can admit.

It changes based on camera adjustments when shooting in the Auto function.

You can change it when you are not in the Auto function.

What do the numbers mean and how do they relate to fast SLR camera lenses?

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

The number "1" is always listed first. It simply means, in-camera language, 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 but what does this mean?

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.

Let's look at a lens and connect it to the chart.

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

Aperture Sizing Diagram
Aperture Sizes

Disclosure: angiepattersonphotography.ca is a participant in the Amazon Services LLC Associates Program designed to provide a way for websites to earn advertising revenues by advertising and linking to Amazon.com or .ca. As an Amazon Associate I earn from qualifying purchases.

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 what you see relative to the chart above, at 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 Zoom?

There is a significant difference when using the 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.

In the first example, there is only one number after the 1: the 2.8. In the second example, it varies 1: and the numbers 3.5 - 6.3.

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

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

Hopefully, as you review the chart to your lens and the lens examples, a fast lens is beginning to make more sense.

In low light and 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 absolutely correct! 

It's due to the smaller the number, the greater the amount of light, as seen in the example diagram.

Therefore, to confirm, 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, there is no dash to indicate a varying change of aperture when zooming, as there is in f/3.5-6.3.

The example of the macro lens at 1:2.8 is slightly different in that it's a fixed lens at 105mm.

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

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

Does this mean you need to go out and buy a fast SLR camera lens? Not necessarily.

If you're mainly shooting in good lighting situations and not having a problem, it's unnecessary. 

Also, 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.
  • To help blur the background.

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 whenever you want to.
  • Extremely helpful in low light situations or when you photograph fast moving subjects and need a fast shutter speed.
  • You can blur the background quite 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 experiment with a fast lens. 


  • They are more expensive, especially with zoom capabilities.
  • Sometimes you want the extra light but not the background blur that can happen. It is doable if you have enough of your subject in focus.

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 them for portraiture.

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 for yourself, a fast SLR camera lens could be just the answer for your next photography adventure.

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

They can show you different lens models as you notice which ones are fast SLR lenses.

On a final note, fast SLR camera lenses, like all camera gear, can be great fun and help you capture subjects you might otherwise miss. In the beginning I thought it would not make much difference but soon discovered a fast lens is a faithful companion!

Disclosure: angiepattersonphotography.ca is a participant in the Amazon Services LLC Associates Program designed to provide a way for websites to earn advertising revenues by advertising and linking to Amazon.com or .ca. As an Amazon Associate I earn from qualifying purchases.