Basic Lens Understanding

Basic Lens Understanding

One of the most common questions a person fresh in photography would ask would be what those numbers on those lens mean. Or to put it in a simpler manner – what da heck does the 18-55mm and the 1:3.5 – 5.6 on the lenses mean?

Let’s see if my simplistic explanation is helpful.

Here’s what a typical lens looks like. The one on the image below is the kit lens from Canon, one of the most basic lens you can find and it’s also a lens that’s great to be part of our subjects’ visual aid. 🙂

Basic Lens Understanding

Lenses always have 2 things indicated. First is the focal length and second is the aperture capability. If the indicators are not necessarily found at the front element of the lens, you’ll just have to find it.

Focal Length

To make it simple, 18-55mm shows that it’s a ZOOM lens, covering the focal length from 18mm to 55mm. The manufacturers will always indicate the widest available focal length first, so there’s no such thing as 55-18mm, you’ll see a host of lens out there with indicators such as 18-35mm, 17-40mm, 55-250mm, 17-55mm, 17-50mm, 70-300mm, 70-200mm and so on so forth – they all indicate the widest and the furthest range.

Bear in mind that the stated focal length is always based on a 35mm camera (Full-frame Digital SLR / Film Camera). An 18mm on a typical Digital SLR is not at 18mm to begin with, you’ll have to multiply it with the crop-factor to know the actual focal length. To make it easier, we’ll use 1.5 – the crop factor used on Nikon systems. So an 18mm on a Nikon Digital SLR like the D300 or lower would actually be 18 x 1.5, which is 27mm.

Here’s an example on the range as it goes from 10mm to 300mm. The photos were taken with a Canon EOS 350D with 1.6x crop factor. The 35mm equivalent of this is from 16mm to 480mm.

Basic Lens Understanding

On some lenses you’ll only see 1 figure, e.g. 50mm, 30mm, 85mm and so on. Such lenses are FIXED FOCAL LENGTH lens, often also known as PRIME lens.

ZOOM lenses have the advantage of versatility. PRIME lenses on the other hand, make up for their lack of versatility by having better optical quality and also wider available aperture as the lack of zoom also mean there’s less component to cater for the shift of zoom distance.


The indicator 1:3.5 – 5.6 shows that the widest available aperture for the lens at its widest is f/3.5 and f/5.6 as its furthest zoom.

On some lenses the indicator may only show a single number, e.g. 1:1.8.

Basic Lens Understanding

This is always the case for PRIME lenses as they are without zoom, so there’s not a need to indicate aperture for widest and furthest zoom. The same could be found on some zoom lenses as well, this indicates that the zoom lens is capable of maintaining a constant aperture across the zoom range.

Constant aperture lens are often expensive as the construction of the lens allows the widest available aperture is available all the way, which is a great advantage to photographers as it allows more light to enter through the lens, thus allowing the use of faster shutter, reduced ISO sensitivity (for lesser noise) and also being able to produce better bokeh.

Note : While the lenses may state “1:3.5 – 5.6”, it’s often typed as f/3.5 – 5.6 over the Internet.

Other Details

There are many acronyms or single alphabet indicator on lenses, you’ll see stuff like VR / IS / OS / USM / HSM / VC / DX / DG / DX / L / G / DT and many more. These indicators tell many things, for example whether the lens is usable on full-frame cameras, or whether they are the higher-grade lenses, among many other things.

Basic Lens Understanding

The indicators vary from manufacturer to manufacturer even though they may be referring to a feature of same functionality, for example Nikon uses VR for their image stabilization technology while Canon uses IS and Sigma uses OS.

Planning Purchase?

So before you decide to embark on your lens upgrade journey, try to understand these readings and find out which lens type suits your style of photography. No point buying a 17-50 range when you love shooting subjects from a distance. 🙂

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  1. I think DCM already covered this lens topic with more detail in their last issue (December 08). 😛

  2. Author

    Thanks. Firstly I’m not DCM. Secondly, DCM also have something about lenses in some 07 issue and I think many other issues. Thirdly, I didn’t subscribe to them. 😛

    Anyway thanks for the info. I’m sure many others wrote about it anyway. I just explain them in the most simple way I can. 🙂

  3. Hey Brian, I always look forward to seeing your post on photography. This is a very good post for beginner/noob like me. Thanks

  4. love it!!!… very useful!!!

  5. Ah good I find this article informative!

    I’m learning a semester of Medical Imaging and the preliminaries are those optics, optics, and more optics.

    I have to learn and understand PSF, OTF, MTFs and even many other transfer functions, plus the lens systems!

    Now I need to read up optic-geometry and some of those photo-taking lessons for my extra info! 😀

  6. Author

    After that, you’ll write documents that we can refer to, right?

  7. Well, I wish I could write it, but I have zero experience in my photoshoot skills. That’s why I’m reading those lens and some photography basics right now.

    All I know is basic mathematical transforms of Optical functions and how to manipulate them for ideal cases. Once I start looking at these Holographic and Adaptive Optics, they are even tougher! 🙂

  8. Thanks for the great read. Now I understand the lenses a lot more!

  9. Thanks for the article, a great big help

  10. i have a question here, is a high aperture lens or a flash more important?

  11. Author

    You have asked a question where there’s no answer. There’s no answer because there’s no way to tell you which one MORE IMPORTANT because it all depends on what you’re shooting and what environment you’re at. It’s a case by case thing.

    Put it simple, if you KNOW what you’re doing and if you KNOW your stuff, you wouldn’t be asking that. 🙂

  12. i see, thx.
    i appreciate every piece of ur work.
    fyi, i’m new in photog. =)

  13. good info..thanks 🙂

  14. Excellent article… especially for an amateur photographer like me 🙂 Could’t resist reading all of your related posts. Keep posting !

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