Confused by all those numbers? Not sure which lens best fits your needs? Here’s a breakdown of what all those numbers attached to an SLR lens mean. If you’re thinking of adding a new lens to your camera this year, this quick cheat sheet will help you pick the right lens for your needs.

18-55mm f/3.5-5.6

Here’s a standard lens we can use as an example. This is the kit lens that comes with the Canon Rebel T3i. If you’re not sure what you’re looking at, this might as well be written in a foreign language! First of all, here’s how you say it: 18 to 55 millimeter F 3.5 to 5.6. Now let’s break it down.

18-55mm

This is your Focal Length. Focal Length is how far your camera will zoom in and out. The lower the number, the more you can fit in the photo. In other words, this is how much you can zoom out. The higher the number, the more it zooms in. The lens above will zoom from 18-55mm. 18-55 is a great focal length for an all purpose lens. My all purpose lens that hardly ever leaves my camera is 24-70mm. To give you a point of reference for what 18-55mm means, here are some examples of a few other focal lengths.

10-22mm

This is considered a Wide Angle lens. This lens will zoom out more than the 18-55mm lens. I use this lens when I want to fit a lot in the picture, but don’t have a lot of room to “back up” and fit everything in. A wide angle lens is great for traveling. I use mine to photograph hotel rooms, scenery, and city skylines. I can fit a whole lot in the photo without having to be too far away from my subject.

Lens 10-22mm f/3.5-4.5
Focal Length 10mm
Aperture f/3.5
Shutter Speed 1/15 sec

Lens 10-22mm f/3.5-4.5
Focal Length 10mm
Aperture f/8
Shutter Speed 1/15 sec

Lens 10-22mm f/3.5-4.5
Focal Length 10mm
Aperture f/5.6
Shutter Speed 15 seconds

However, due to distortion and lack of background blur, I would not recommend a wide angle lens for portrait or food photography.

70-200mm (and above)

This is considered a Telephoto lens. This lens zooms in more than the 18-55mm. I use this lens for outdoor action photography. Because it zooms so far in, I don’t know that I’ve ever used this lens indoors. With a 70-200mm lens, you would have to be pretty far away from your subject to fit it in the photo, which is why this lens is best for outdoors or indoor arenas with lots of room where you would want to zoom in. I use this lens for outdoor action shots of Miley and Howie.

Lens 70-200mm f/4L
Focal Length 104mm
Aperture f/4.5
Shutter Speed 1/1000 sec

A telephoto lens is great for outdoors, action, and candid portraits — any subject which you can be pretty far away from and still photograph. I do not recommend a telephoto lens for everyday indoor photography.

50mm

If a lens only has one number it’s called a Prime or Fixed lens. This means the lens does not zoom in and out at all. I could not comprehend this until I got my first fixed lens! I did not know how it could not zoom at all. But it doesn’t. Imagine a cellphone camera without a zoom. If you want to get further away from or closer to your subject, you’ll have to move with your feet.

Lens 50mm f/1.4
Focal Length 50mm
Aperture f/1.6
Shutter Speed 1/500 sec

Prime lenses are great for indoor, outdoor, portrait and food photography. Their only limitation is in their lack of zoom.

So that basically covers your focal length. If you already have a lens, look to see what the focal length is, and take note. Do you like the focal length? Do you wish you could zoom in more? Zoom out more? If you want to zoom in more, look for a lens with higher numbers than what you have. If you want to zoom out more, look for a lens with lower numbers. If you’re pretty happy with the focal length, look for a lens with similar numbers. If you don’t already have a lens and you’re not sure what focal length you need, I would consider a lens within the range of 17-100 to be a great focal length for an all purpose lens.

Now on to aperture.

f/3.5-5.6

Let’s go back to our example lens, the 18-55mm f/3.5-5.6. The aperture on this lens is f/3.5-5.6. Aperture controls the background blur, or bokeh, in your photos. The lower the number, the more background blur you can achieve. You’ll notice that this lens has two numbers, 3.5-5.6. That means that when the lens is zoomed all the way out (18mm), you can set your aperture as low as 3.5. But when you are zoomed all the way in (55mm), the lowest you can set your aperture is 5.6. If you have a lens with just one number here, for example, the 24-70mm f/2.8, that means you can set your aperture as low as 2.8 no matter how much you are zoomed in or out. Keep in mind that the aperture numbers on the lens are simply the lowest the aperture can be set. On both lenses your aperture can always be set higher, no matter what your focal length is.

To make things even more confusing, the amount of background blur created by the aperture is directly proportional to the focal length. The higher your focal length (the more zoomed in your are), the more background blur you will have. For example, a photo with an aperture of f/4 and a focal length of 18mm will not have much background blur. However a photo with the same aperture of f/4 and a focal length of 200mm will have a substantial amount of background blur. Notice the two photos below have the same aperture of f/4.

Tons of background blur here.

Lens 70-200mm f/4L
Focal Length 200mm
Aperture f/4
Shutter Speed 1/640 sec

No background blur.

Lens 10-22mm f/3.5-4.5
Focal Length 17mm
Aperture f/4
Shutter Speed 1/2000 sec

If you want deep, soft background blur at an all purpose focal length, I would not recommend getting a lens with anything higher than 2.8. One other important thing to note: If you want any type of background blur, I would not, under any circumstances, recommend a lens with more than one number in the aperture value. Take our example lens above, with an aperture value of 3.5-5.6. At 18mm, f/3.5 just isn’t a low enough aperture to give you background blur. Likewise, at 55mm, f/5.6 isn’t low enough to produce a soft background either. However, if you could set your aperture as low as f/2.8, you could generate plenty of background blur at a focal length of 18 or 55.

Lens 24-70mm f/2.8L
Focal Length 24mm
Aperture f/2.8
Shutter Speed 1/30 sec

Lens 50mm f/1.8
Focal Length 50mm
Aperture f/1.8
Shutter Speed 1/2500 sec

Look for a lens with a versatile focal length with only one number in the Aperture area.

Macro

One other lens to note is a macro lens. If you find yourself constantly wanting to get up close and personal with your subjects, wishing you could zoom in closer for details, or cropping your photos closely once you get them on the computer, you may want to consider a macro lens. A macro lens lets you zoom in close to your subject and focus on the details so you don’t have to crop your photos or zoom in on the computer. Cropping on the computer only degrades the quality of the photo. Macro lenses are also great for portraits, and depending on the focal length you chose, a general all purpose lens.

Lens 100mm f/2.8 Macro
Focal Length 100mm
Aperture f/2.8
Shutter Speed 1/160 sec

What I’m Using

Here’s a breakdown of the lenses in my arsenal, along with how I use each one.

24-70 f/2.8 L

You’ll notice this falls in the all purpose focal length, with a nice, low, and singular aperture number. This is my default lens. I use it for travel, portraits, and food photography. A great alternative to this lens to consider is the Tamron 17-50mm F/2.8. That’s the lens I used for years before upgrading to the Canon 24-70 lens. The photo below was taken in 2006 with a Canon XTi with the Tamron 17-50mm F/2.8 and is SOOC — no editing. At just over $400, this lens is a great deal.

Lens Tamron 17-50mm F/2.8
Focal Length 50mm
Aperture f/2.8

10-22 f/3.5-4.5

This is a wide angle focal length, and since background blur is pretty much nonexistent at such a wide angle, I don’t care about the aperture number here. I use this lens for travel, scenery, city skylines, and hotel rooms.

50mm f/1.4

This is a fixed lens with a very low aperture. This lens produces excellent bokeh! I use this when I want a ton of bokeh, or in low light when I need to use a lower aperture in order to steadily hand hold the camera. I talk about this more in depth in my Quick Guide to Understanding Your DSLR post. Another lens to consider is the 50mm f/1.8. Super cheap and super fun. You’ll get great bokeh with either of these lenses. They’re perfect for portraits, food photography, and because of the middle-of-the-road focal length, even travel.

100mm f/2.8 Macro

This is another fixed lens with a great low aperture. It’s a macro lens, and the background blur is excellent. This lens is fun for portraits and food photography. I use it when I want to zoom in super close on the details or get tons of background blur.

70-200 f/4 L

This is my telephoto lens, and I mostly use it outdoors to get action photos of Miley and Howie. The aperture is not super low, but since the focal length is higher, I’m still able to get great background blur.

Final Notes

The full name of our lens above is actually EF-S 18-55mm f/3.5-5.6 IS. The focal length and aperture numbers described above are the most important factors when choosing a lens, but just in case, here’s what those other numbers and letters mean.

EF or EF-S. This refers to the lens mount. EF is the standard lens mount on Canon EOS DSLR cameras. This is indicated by a red dot on the lens that corresponds to the red dot on the camera where you attach the lens. If your camera also as a white square, you can also accept lenses with an EF-S lens mount.

L. This indicates the lens is a top-of-the-line lens from Canon, also known as a “Luxury” lens.

IS. IS stands for Image Stabilization. Many of the available lenses will come both with and without an IS option. I always opt for the cheaper, non-IS version because I have a pretty steady hand. However, if you have a shaky hand, this might be an option to consider.

With this information you are now armed with the knowledge to confidently purchase a new lens for your camera! If you have any questions, I’ll be happy to answer as best I can. Just leave me a comment below. Happy shopping!

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