Numbers on a Lens: Explained and Demystified

By on September 12, 2017

Beginners often have a hard time understanding the labeling on their brand new DSLR kit lens (or other lenses). As far as I know, a camera manual doesn’t explain the numbers or what to make of them. Being able to read these numbers becomes even more important for those who intend to buy a new lens for their cameras. Once you understand how to read these numbers and what they mean, you will be in a better position to make an informed decision when buying your next lens.

The Numbers

Nikkor 18-55mm kit lens

Here is the Nikkor 18-55mm, which is the most common kit lens that comes with all Nikon D3xxx and D5xxx series of DSLRs. Don’t worry if you use another brand. The information here is applicable to all lenses regardless of the brand. On this lens, we can notice that there are two sets of numbers on the lens barrel as follows:

The first set – 18-55mm

These numbers show the focal range or zoom range that this lens is capable of. According to the numbers, this lens can capture an image at focal lengths ranging from 18mm up to 55mm. This focal range makes this lens versatile. If you were shooting a cityscape, you would need a wider field of view to be covered and this lens will give you a nice wide angle view at its shortest focal length, that is 18mm.

The second set – 3.5-5.6

These numbers refer to the minimum aperture values across the focal range. In other words, how wide the aperture can open up is mentioned in these numbers. Since this lens is a zoom lens that has a focal range between 18-55mm, it has two aperture numbers mentioned. The first one, that is 3.5 is the widest aperture available on this lens at 18mm. The second number, that is 5.6, is the widest aperture this lens can go to at 55mm.

Now this means that this particular lens doesn’t have a constant or fixed minimum aperture value throughout its focal range. But instead, it has a variable minimum aperture. So at 18mm, this lens opens up at f/3.5 but at 55mm, it only opens up at a maximum of f/5.6. When it comes to aperture, we know that a smaller number is better because it means more light can get through the lens, which is useful in low-light situations. Unfortunately, this particular lens is faster at 18mm but slower (a little over a stop) at 55mm.

Types of Lenses

We just learnt about a common DSLR kit lens, which is a variable aperture zoom lens. There are other types of lenses also. Here are the most common types of lenses available.

Prime Lens (aka fixed focal length lens)

A lens that has a fixed focal length is called a “prime”. In other words, it does not have a zoom and only offers a single focal length. A 50mm lens is the most popular prime.

Nikkor 50mm f/1.8 prime lens

If you see carefully, you would notice that the aperture value mentioned on this lens is just a single number, that is 1.8, instead of a range as we saw earlier with the 18-55mm kit lens above. It’s because this lens has only one focal length which is 50mm. And at that focal length, the widest this lens can open is an admirable f/1.8, which allows much more light compared to the f/3.5 on the kit lens we saw before.

Prime lenses come in a variety of focal lengths. From fisheye, ultra-wide to super telephoto lenses, primes offer the best image quality… period. And they also have wider apertures compared to zoom lenses, which makes them superior for low-light shooting. Here are some examples.

Nikkor 500mm f/4 super telephoto lens

The beastly Nikkor 500mm f/4

Nikkor 24mm f/1.8 wide angle lens

Nikkor 24mm f/1.8

Having wider apertures also make them ideal for portraiture, macro, food, still life etc. Portrait photographers especially love prime lenses with wide apertures like f/1.4 etc. because it enables them to make their subjects stand out from the background and have a magical rendition of the subject. Here are some examples:

A portrait shot with a Nikkor 200mm f/2 lens

Shot using Nikkor 200mm f/2 by Joe McNally


Portrait of a child, shot using a Nikkor 200mm f/2

Shot using Nikkor 200mm f/2. Image by Niki Strbian

Constant Aperture Lens

As the name suggests, a constant aperture lens has a constant aperture throughout its focal range. A constant aperture lens can be of two types:

  • Fixed focal length or “Prime”
  • Variable focal length or “Zoom”

A zoom lens with a constant aperture offers better image quality and is physically larger compared to a zoom lens that has a variable aperture. Engineering and manufacturing constant aperture zoom lenses cost more that’s why these lenses are significantly more expensive than the likes of a kit zoom lens like the 18-55mm (f/3.5-5.6) we saw earlier. Professional grade zoom lenses are almost always constant aperture but in recent times, lens manufacturers have pushed the quality envelope further even on variable aperture lenses. Here are some high-quality zoom lenses that have a constant aperture:


Nikkor 24-120mm f/4

Nikkor 24-120mm f/4


Nikkon Trinity
Nikkor 14-24mm f/2.8, Nikkor 24-70mm f/2.8 and Nikkor 70-200mm f/2.8

And that’s all there is to it. It wasn’t so hard. Was it? Let me know if you still have any doubts or questions regarding the topic. Feel free to comment or send a message to me. Until next time, happy shooting!


Hi, I am Hammad Iqbal, a professional photographer based in Doha. Welcome to the "HI Blog" where I share tutorials, tips and techniques about shooting and post-processing images using Lightroom and Photoshop. Follow my updates by subscribing to my newsletter and discover with me the art of photography and post-processing!

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