How to find out the zoom of your DSLR camera kit

So you ponder the thought of upgrading to a DSLR camera or you just bought one. You’ve looked everywhere: the front, the back, the lens, the manual and still can’t find that one or two digitals followed by a big X. So where’s the zoom ratio then?

Well that’s one of the most confusing things about getting a camera with interchangeable lenses if you’ve only used a compact or super zoom camera before. This consumer cameras state the zoom as a ratio between the largest and the smallest focal distance. For example they might state a focal range of 6.8-68mm which will give you a 10X zoom range.

As far as digital zoom is concerned that’s not even real magnification. It’s just a software magnification of the image, often coupled with significant cropping. So we’ll let that one to rest. While it might seem at first that it’s much more convenient to use such a system, it’s actually a very unreliable one.

In order to understand how zoom works for professional cameras you should get acquainted with the idea of focal length. It sounds scarier that it really is actually. For most of your rooky years as a DSLR user you’ll need to know mostly about wide, normal and telephoto and forget about fish-eye, super telephoto or other such beasts.

Professional digital cameras (full frame) have a sensor size as large as a 35mm film frame (36mm * 24mm). So what are those three focal length ranges about? Well it is agreed upon that a 50mm focal length on a full frame camera (or film camera for that matter) produces an image that’s more or less equivalent to the angle of view of the human eye. Also 28mm is considered classic wide angle, while anything over 70mm falls into telephoto.

That’s why anything ranging from 15mm to 35mm can be easily considered wide angle, while 40mm-60mm is what you’d call normal view. Anything in between those are transition areas.

So how is that supposed to help you finding your beloved X number? Let’s say you just bought yourself a camera kit with a lens whose focal length is 18mm-105mm. You simply divide the largest number by the smallest one and there you have it 5.83X plus change. But at least now you know that up until 35-40mm of that range you’ll get a wide angle effect with those dramatic distortions near the edges, while above that you’ll get a normal image similar to the one you used to get on a point and shoot camera.

Still, since these are probably your first steps into professional land you should have in mind when doing those calculations what’s the size of your sensor. Both Canon and Nikon have professional cameras with smaller sensors. Depending on the camera model, you must apply a crop factor multiplier (again sounds scarier that it really is). That’s usually specified for each camera type (1.5 for Nikon DX, 1.6 for Canon EF-S lenses and entry to mid level cameras, 1.3 for Canon 1D series etc.).

Now let’s get back to the lens example. Suppose that 18mm-105mm range is from a Nikon lens mounted on a DX camera body such as a D90. That means in order to get the full frame equivalent range you have to multiply both 18 and 105 by a 1.5 factor giving you an effective interval of 27mm to roughly 157mm. While you do lose some precious 7mm on the wide end you gain about 50mm on the telephoto end which might help you or not depending on the type of photography you intend to take.

Every once in a while you might find lenses which only feature a single focal length such as 35mm, 50mm, 85mm etc. Those are prime lenses for which you give up any included zoom capabilities but gain on the image quality. That’s why if you don’t mind doing some extra walking, you’ll be rewarded by sharper, more colorful photos.

So as I welcome you to the wonderful world of DSLR photography you better start thinking in terms of focal length rather that X zoom ranges as this will clearly make your life as an amateur photographer easier as well as further your creativity.