What do the numbers on your lens mean?

I've covered a fair bit on lenses this past week, comparing different focal lengths and understanding lens compression, and I thought I'd rewind a bit for any new photographers, and talk about what the numbers on the lens mean. 

This was one of my very first questions I had when buying my DSLR and it took me a little while to figure out! Hopefully this post will help break this down if you are new to lenses too. 

So, let's take a look at some lenses and break down the numbers to see what they mean, and what you should look for. 

A breakdown of what the numbers on the lens mean, and what to look for when choosing your own lens!


Let's start with a lens that you will often see sold as a "kit lens" with your camera:  

18-55mm f/3.5-5.6

Lets look at what 18-55mm means first. The numbers before the ”mm” relate to the focal length of the lens. Essentially, focal length means what you are going to be able to see when you look through the viewfinder. (Ever looked through the lens and everything looked closer than it actually was, or further away? That is your focal length - you can see a full comparison of different focal lengths here)  The important thing for you to know is that the lower the focal length (smaller the number) the wider the view - i.e. you will be able to see more in your image.

The higher the focal length (bigger number) the less of your image you will see, in other words it will look more zoomed in. I’m quite a visual person so here are a couple of examples of some different focal lengths in practice (I’m sitting in the same place for all 3)

As you can see the smaller number has the widest view, and the biggest number has the most "zoomed in" view. 

So when you see the focal length numbers of a lens, the first thing you are probably going to want to know how this is going to look compared to ”real” life.  This will be different depending on whether you have a cropped frame camera like the Rebel or a full frame camera like the 6D. I’m going to start with explaining the full frame because it is the easiest to understand.

On a full frame, a focal length of 50mm will ”see” roughly the same view as you do with your naked eye. Remember this, because when you look at the focal length of a particular lens you will be able to tell whether this will give you a view that will include more in the frame or less. If the lens has a higher number than 50mm for the focal length, like 85mm, you will see less through the frame and it will look more ”zoomed” in. If it has a lower number, like 35 mm, the more you will see in the frame.  Look at the images above again for a comparison. 

50mm is a good all rounder, 35mm and under is great for landscapes or lifestyle photography where you want to be able to fit more into the frame (great for shooting indoors) and 85mm and above are great for portraits. 

How This Changes on A Crop Frame

Quite simple so far, but this is where it gets a bit more complicated. If you are just starting out, you probably don’t have a full format camera, you will have a cropped frame, and this makes a world of difference to how the focal length ”performs” on your camera.

For example, if I put a standard 50mm lens (one made for a full frame like most prime lenses are) on a cropped body camera, instead of the scene through my viewfinder being the same as the naked eye, it will actually look a lot closer - more like how an 85mm would perform on a full frame.  This is to all to do with the size of the sensor being different.

So how can you figure out how a particular lens is going to perform on your cropped frame camera?  

All you need to do is multiply the focal length by a ”crop factor” to see how it compares to the full frame. For a Canon Rebel, the crop factor is 1.6 (so if I used a Canon 50mm F1.8 lens, this gives me the around 80mm - so behaving more like a 85mm as I said above)  

What I can work out from this is that in order to get the same view on a cropped body as I would see with the naked eye, I would need around about a 30mm focal length. (30mm x 1.6 = 48mm)

Different cameras have different sized crop sensors, so you need to know your particular model’s ”crop factor” - Canon’s is 1.6, but Nikon’s is 1.5. As usual DPS have a brilliant article which gives you all the information at a glance here. It also includes a table that shows the equivalent lens sizes for the different crops factors. 

And now the aperture numbers...

Let’s move on the next set of numbers, which if you remember, was f/3.5-5.6. This refers to the aperture range of the lens and is lovely and simple to explain. Aperture is basically how big the opening in your lens is when you take your picture. The size of this opening is measured in f/stops. The smaller the number, the wider the opening and therefore the more light gets into your camera.

So, this particular lens has an aperture range of 3.5 to 5.6. If you have two numbers (like this one) separated by a dash, this means that the aperture changes with the focal length of the zoom. Use the lens at 55mm and you will have a maximum aperture of 5.6. At 18mm,  the maximum aperture will be F3.5. The particular aperture range of this lens doesn’t go nearly as low as I want it to go to let lots of yummy light into my camera and get the nice creamy backgrounds I’m looking for (You want at least F2.8)- so I know that this isn’t the lens for me. 

So, now you can apply this knowledge to other lenses to work out what they mean. Lets look at a couple more.....(these are all lenses that are meant for a full frame, just to avoid any confusion!) 

What do the lens numbers mean?

50mm f/1.4

It only has one number before the mm, which means it is a ”fixed” or ”prime” lens - it does not zoom and has a fixed focal length. The 50mm means that it sees the same as the naked eye on a full frame but on a cropped frame, it is going to behave more like an 85mm (50mm x 1.6) and make everything look a bit closer than it actually is, and it’s a good portrait length. There’s only one F number - F1.4, which is the lowest aperture setting this lens can go to, which is nice and low so I can get lots of nice light into my camera! 

24-75mm f/2.8

It’s a zoom lens, with a focal length range of 24 - 75mm. As I said, if I multiply these numbers by 1.6, I get the equivalent for a cropped camera which is 39 - 120mm.  You can see by doing this just how differently this lens will perform on a full frame or a cropped frame camera!  (Again, bear in mind that I am talking about lenses made for a full frame camera, not lenses specifically designed for a crop frame)

You will also note it only has one F number - that is because you can use F2.8 throughout the whole focal range – it doesn’t change as you zoom in and out.  This is called a fixed aperture zoom, and therefore combines the benefit of a zoom, with a lower aperture to let lots more yummy light into the camera. 

It can take a little while to get your head around the numbers, so I hope this helped a bit with learning what the lens numbers mean and how to decide whether the lens is right for you.

Please note there is a lot more technical mathematical stuff to it than that,  but I think as long as we know what we can do with the lens once it’s on our camera, that’s all we really need to know! 

If you really want to improve your photography skills and knowledge, then check out my Auto to Awesome ecourse - it will rock your world! 

Hope some of you just starting out have found this helpful! If you want even MORE photography tutorials, then make sure you sign up to become a Live Snap Love subscriber (it's free!), and as a thank you you'll get a FREE Photography Learning Blueprint. This highly actionable cheat sheet will walk you through everything you need to learn, and in what order, so you can see what to focus on for the next 90 days…