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5 Things You Need to Know About SLR Lenses

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Kurtis Kronk
Brian

Feb. 6, 2007

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5 Things You Need to Know About SLR Lenses

There are two things I enjoy most in life. The first is photography and the latter, well!I like to tell people what to do. It makes me feel powerful. Strong. Like a small pony. Keep reading.

One of the most common questions I get as related to photography is "what camera should I buy?" I've already answered that question (sort of) with 10 Reasons to Buy a DSLR and 10 Reasons NOT to Buy a DSLR.

The next most common question (I mean next literally, as in directly after the previous question) is "what lens should I buy?" Well, that's a tricky question because there simply is no one-size-fits-all lens.

My goal in this article is to give you some basic knowledge about lenses that will help set you on the path to lens buying goodness, or if you're like me, indecisiveness. Hey, it's a start.

What's My Lens Mount? Different cameras use different lens mounts. Before you start looking for the perfect lens, you need to find out what lenses your camera is compatible with. This information can be easily found on the camera manufacturer's website, usually listed under "Specifications." For example, I can see from Canon's website that my EOS 5D DSLR uses the Canon EF lens mount and accepts EF (except EF-S) lenses. Likewise, the EOS 30D uses the EF mount and accepts EF (including EF-S) lenses. I should mention that there are lens mount adapters out there if you want to use a Canon lens on a Nikon camera, for example, but that's not really within the scope of this article...

To make things a little more complicated, there is such a thing as a "Digital-Only" lens. In the case of Canon, these lenses are given the EF-S moniker. In the examples above, note that the 30D supports EF-S lenses while the 5D does not. Meanwhile, EF lenses are fully supported by both cameras. So pay attention when looking at a lens' technical specs. This leads me to my next point.

What's My Camera's Crop Factor? Without going into too much detail, the crop factor (Canon refers to it as the Lens Focal Length Conversion Factor) will act as a multiplier to a given lens' focal length as a result of having a digital sensor that is smaller than a 35mm negative. It's common to hear the terms "cropped body" and "full-frame" thrown around amongst photographers; this is what they're talking about. The 5D is a full-frame camera and so it follows that the crop factor is 1.0x, meaning your focal length is as stated on the lens. The 30D is a cropped body with a crop factor of 1.6x, meaning you will need to multiply a lens' focal length by 1.6 to get the equivalent focal length.

Let's say I have a Canon EF 16-35mm f/2.8L USM lens; the important part of the name for our purposes here is the focal length range, 16-35mm. On the 5D, the lens will act as just that, a 16-35mm lens. However, if you mount it on the 30D, what you'll get is a 25.6mm-56mm lens. This multiplying effect has a greater impact with more telephoto lenses. For instance, take Canon's EF 70-200mm f/4.0L USM lens. Again, the important part here is 70-200mm. We already know that the lens mounted on a 5D will give an actual focal length of 70-200mm. But on the 30D the lens give you an equivalent focal length of 112-320mm. It is for this reason that some people prefer to have a cropped body (long telephoto lenses are very expensive).

What's My Subject?Photographically speaking, what is your typical subject? There is no wrong answer here, and you may not even have a single subject that you typically photograph. Below I've laid out some typical scenarios to give you a general idea of what lenses may be a good fit for you.

Family/Friends, Vacation, Journalism: You don't want to be loaded down with cumbersome equipment; you need something lightweight and versatile. It's more important to capture that special moment than to have the best possible image quality, though ideally you want both of course.
Landscape, Interior, City: You're going to want a wide lens the majority of the time when shooting a beautiful landscape, the interior of a home, or if you want to capture NYC in all its glory. There are exceptions, of course, but a wide lens is a must-have for this type of shooting.
Sports, Wildlife/Birds/Safari, Portraiture: Telephoto baby... When shooting sports you may be on one side of a field shooting to the other side, and nobody wants to see a photo of the entire field. To get in close to your subject you need a long telephoto lens. The same holds true for wildlife/birds/safari shooting scenarios, as you usually can't (or wouldn't want to in the case of safari) get physically closer to the subject, so the glass on front of your camera has to compensate. Portraiture may not seem like it fits here, and to some extent it doesn't! While you may use a 400mm lens to shoot football or birds, you'll want something a little shorter for really great looking portraits. 200mm can be great for portraits for a really nice looking out of focus region (bokeh), but even 50mm or 85mm are sufficient for great portraits.

Do I Want Zoom or Fixed Focal Length (Prime)? You might have noticed that I didn't use the word zoom once in the above section, and this was for a reason! As pertaining to focal length, there are two types of lenses: zooms and primes. The difference is basically that zoom lenses zoom while prime lenses, well! they don't zoom. So if you wanted to have similar focal lengths to what you'd get with a 24-70mm and 70-200mm zoom lens combo, you might get a 20mm, 50mm, 135mm, and/or 200mm prime lens.

Whether or not prime lenses are for you is really a matter of preference. Those who prefer primes usually do so because they have a slight edge in image quality, they're often faster (that is, they have a wider maximum aperture), they're lighter, and they can be less expensive than a zoom which covers a similar range. What they gain in image quality, they sacrifice in convenience, however. If you want to get a different perspective you have to either get some exercise (move those legs) or switch lenses. Furthermore, some of the really high-end primes aren't much less expensive (if at all) than a comparative zoom lens. With that said, if you aren't under any pressure while shooting, and you've got plenty of time to move around and switch lenses when necessary (i.e. landscape, interior, city) then you might want to seriously consider the benefits prime lenses have to offer.

Do I Want Constant or Variable Aperture? Variable aperture may be a non-issue for some, but for others it can be the cause of great annoyance. But before I go off on a tangent, let's talk about what it means! I would say that probably the majority of mainstream lenses (that is, affordable for the casual photographer) have a variable aperture. For example, a lens I once owned (and loved) was Canon's EF 28-105mm f/3.5-4.5 USM. Notice how it says f/3.5-4.5 instead of just f/3.5? What that means is that when the lens is at its widest (28mm) the maximum aperture is f/3.5, and as you zoom in it gradually increases until you're at full telephoto (105mm) and your maximum aperture becomes f/4.5. So what, right? Well, for some of you at least, that may not matter.

Let me pose a scenario where you might be able to imagine a variable aperture gnawing away at your rationality telling you not to throw your camera on the ground and pounce on it in a raging fit. Okay so maybe that's pushing it a bit, but I digress. It's a beautiful sunny day out, you've got your camera set on full Manual mode like the other cool kids (you did read my Beginner's Guide to Manual Photography! right?), and you just found exactly the right balance between shutter speed and aperture for what you're shooting. Then you zoom in and you realize your shot is underexposed. So you look up at the sun, and nope, no clouds have moved in! What the hell? Oh yeah, it's that damn variable aperture stopping itself down as you zoom in, thereby screwing with your previously perfect exposure!

Perhaps that doesn't sound so bad, and if you don't think so, then by all means don't spend any extra money to get a constant aperture lens. Perhaps you always shoot in Automatic mode anyways (in which case the variable aperture will effectively result in a slower shutter speed as you zoom, which can also be a problem). Chances are, if you're serious about photography, you'll eventually get sick of variable aperture and move to constant aperture lenses.

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Hopefully you've found this to be a relatively useful article if you're considering buying an SLR lens or two. Given the nature of this article, I tried not to bore you and go too in-depth. If you'd like to learn even more about lenses, I'm currently working on a comprehensive Guide to SLR Lenses which will discuss many more things in greater detail, so keep an eye out for that.

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