Photo Tips #6 : A Lens Primer (Part 1)

Posted on Sunday 11 September 2011

If you have a DSLR you have a choice of hundreds of different lenses, ranging in price from $50 to many thousands (the most expensive I ever saw was a Canon for $100,000 – used!). So how to choose? A lot of that comes down to personal preference and what you want the lens for, as well as factors like budget, but in this article I’ll try to give you a basic primer so you at least can understand the differences.


One of the first criteria, pretty obviously, is that the lens has to work with your camera. In general, if you own a Canon camera you buy Canon lenses, Nikon for Nikon, and so on. There are, of course, exceptions. There are third party manufacturers such as Sigma and Tamron who make lenses for multiple camera types (usually referred to as “mounts” – the physical design of the connection between the lens and the camera). These third party lenses are typically of decent quality and considerably cheaper than their first-party cousins, but beware that there have been incidences of compatibility issues in the past, as neither Canon nor Nikon officially publish or license their specifications. So there’s a small risk there, and you’ll also find far fewer reviews of the third party lenses so they’re more of an unknown quantity. But if you’re on a budget, they may well be worth considering.

Secondly, even within a given manufacturer there are differences. For example, current Canon cameras use either the “EF” (film and pro level DSLRs) or “EF-S” (consumer level DSLRs) mount. They’re similar, but not identical – an EF lens will fit in either type of camera, but an EF-S lens can only be used in an EF-S camera. Nikon also have a consumer level DSLR format, called DX. Things are a little different here – a DX lens will fit fine on any of their current cameras but might give less than ideal results if the camera isn’t aware of the lens type. There are no physical incompatibilities between current Nikon gear though, so you are a little safer! In general make sure you know the type of camera you have and if in doubt ask the seller or read online reviews.

Focal Length

The key differentiating factor between lenses is the focal length, or just “length”. It’s a number usually measured in millimeters, and it translates to how much the lens magnifies the scene. A “long” lens, with a high focal length, is good for taking shots of things a long way away, you can think of it like a telescope. A “wide” lens, with a low focal length, is good for landscapes as it allows you to get a whole mountain or building into the shot. Lenses are broken down into categories based on their focal length – I’ve listed the most common below.

Focal LengthNameTypical Uses
30mm-50mmNormalPortraits, Pets, Children, etc
50mm-700mm (and beyond!)TelephotoSports, WIldlife

The Normal group is probably the most important. Most portraits, family photos, weddings, and even many outdoor shots are taken with a lens around 35-50mm. The reason for this is that this length gives you a magnification and perspective similar to that of the human eye, so the photos look more natural to our brains. The perspective point is important, but a little complex to explain here. Basically, taking a shot from a long distance with a telephoto lens doesn’t give the same result as getting close to the subject and using a normal lens – the latter will usually look better, but is often hard to do!

The example below shows the same shot taken at 85mm (telephoto), 35mm (normal) and 17mm (wideangle).

Comparison of Focal Lengths
Comparison of Focal Lengths

Crop Factor

Update: I’ve posted a (hopefully) clearer explanation of Crop Factor here.

So we’ve figured out that a 35mm lens is “normal”, anything “wider” or “shorter” than that is wideangle, and anything longer than about 60mm is usually considered telephoto. This seems too simple, so inevitably there’s a catch! It’s called the “Crop Factor”, and it applies to many commonly used DSLRs. It all comes back to sensor size, as discussed in part 3. This way of categorizing lenses came about when SLRs were film based, and most used a film frame that was 35mm in size (diagonally). That’s why a 35mm lens is considered “normal” – the image it produces is the same size as the film. A 70mm lens would be magnified twice as much, and a 140mm twice as much again. Now some DSLRs use a 35mm sensor – these are called “full frame” cameras and typically cost at least $2000. If you have one of these the crop factor is irrelevant, your 35mm lens is normal and everything is simple. For the rest of us, our cameras probably use a smaller sensor, typically APS-C sized. That means that a 35mm lens is no longer normal – it’s bigger than the sensor and so has some magnification. The crop factor for an APS-C camera is 1.6x, which means that to get the “effective” length of a lens when used on that camera you need to multiply it by 1.6.

This means that my 35mm lens, when used on my Canon 7D with a crop factor of 1.6x, is actually equivalent to a 56mm lens on a full frame camera. Why does this matter? Well it matters when we’re at the extremes. A big problem is wideangle lenses with APS-C cameras. Typically 16mm is considered pretty wide, but put that on a 1.6x crop camera and now it’s a 26mm – not really very wide at all. To counter this I have to buy a 10mm lens (equivalent to 16mm with the crop factor) – and 10mm costs a lot more than 16mm!

At the other end it’s an advantage however – my 300mm telephoto is actually equivalent to a 480mm! That’s a huge difference, and means that users of APS-C cameras looking to take wildlife and sports shots can actually buy shorter lenses than their full frame owning friends. Shorter lenses are smaller, lighter, and cheaper.

All in all you need to be aware of the crop factor of your camera when buying lenses, if you read a guide or book suggesting good lengths for certain shots or techniques, remember to apply the appropriate crop factor before buying that lens.

Zoom vs Prime

So far we’ve talked about lenses with a single focal length, for example 35mm. These are called “prime” lenses, and whilst back in the day that’s all you had, these days lenses with adjustable focal lengths are more popular. These are called zoom lenses. To clarify (because it’s a common misconception) – “zoom” doesn’t mean the same the same thing as telephoto – it just means variable length. In fact, that 10mm wideangle lens I mentioned earlier is actually a 10-24mm zoom. Very far from a telephoto (there’s no way you could use it to magnify a distant subject!) but it’s adjustable, so it’s a zoom. Getting out of the habit of using “zoom” to mean long length (as in “zoomed in”) will help a great deal when lens shopping!

Zoom lenses are obviously very convenient. Rather than carry around a whole bunch of different length lenses you could (theoretically) just have one zoom which covers 16mm-600mm and call it a day. The problem is that this is a compromise. A zoom lens is much more complex than a simple prime – there are many more moving pieces involved. That makes them larger, heavier, and more expensive than equivalent primes. What’s more, the image quality is always going to be worse than a similarly priced prime, and they’ll also be slower (see later for a discussion of lens “speed” and why it matters).

I started off, like most people, with a mid range zoom lens covering kind-of-wide to kind-of-telephoto (a Canon EF-S 17-85mm). It’s a great lens, and I still use it a lot. If you only have space in the bag for one lens then something like that is ideal, you’re likely to be able to cover most situations which come up. However, once I got my hands on a 50mm prime I’ve been using that wherever possible. Although the zoom does 50mm just fine, the pictures which come out of the prime are so much nicer, plus it’s smaller and lighter. My recommendation is really to do the same. Start off with a decent quality zoom covering roughly 16-100mm or thereabouts, and over time you’ll understand which primes might be worth investing in. I think a 50mm or 35mm prime should probably end up in everyone’s bag if they’re serious about taking shots of people.

Of course, if you’re taking pictures of animals who don’t appreciate you getting too close to them, you need a long telephoto, and these are almost all zooms these days. Not to worry, they’re very suitable for wildlife shooting – you don’t need to go looking for a 400mm prime!

To Be Continued…

That’s all for now – tune in next time for more lens talk – we’ll cover speed and aperture (“f-stops”) as well as features such as image stabilization, macro and auto/manual focus.

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