A Bit More on Focal Length: Understanding "Crop Factor"

I know, I know, "crop factor" sounds like the title of a really bad horror or sci fi flick (maybe a sequel to "Children of the Corn"), but it affects focal length with digital SLR cameras so I'd be remiss if I didn't throw in a note of explanation.

Here's the deal: most lenses out there (for Canon, Nikon, or other systems) are designed to work with 35 mm film. In other words, they project an image onto the film that is equal to the size of that particular film. The trouble is that most digital sensors--except those used in high-end digital SLRs with what are called "full frame" sensors--are smaller than 35 mm, meaning that the sensor cannot record the entire 35 mm-sized image and effectively "crops" (or cuts off) the outside edges.

This has two implications for the photographer: first, it creates some disconnect between what you see through the viewfinder (a full 35 mm frame) and the final image; and, second, it has the same effect as using a lens with greater focal length. Recall that, during our discussion of focal length, we talked about how higher focal lengths (200 mm, 300mm, 400mm, etc.) had the effect of magnifying the image and, in the process, narrowing the field of view. While crop factor doesn't magnify the image (remember: the lens still projects the same image; only the sensor has changed), it does narrow the field of view. To understand what this means in terms of the field of view, photographers use a "crop factor" or, alternatively, "focal length multiplier."

Arrrggh ... I can hear the groans already, but bear with me. An illustration should help clarify things a bit. The standard crop factor (or focal length multiplier) for the Canon system is 1.6, which means that my effective focal length is 1.6 times whatever is written on the barrel of the lens. So, for example, when I take a 20-35mm wide angle film lens and put it on my digital SLR, I have to multiply those focal lengths by 1.6 to understand what it does to my field of view. So, 20 x 1.6 = 32 and 35 x 1.6 = 56, and, just like that, I have an effective 32-56mm lens, which means that the lens is no longer really a "wide angle" lens when used on a digital camera. Still a perfectly good lens, mind you; it just doesn't offer a true wide angle perspective.

To address this problem, Canon has designed a series of EF-S lenses that work only with their digital SLR cameras and do offer a true wide angle perspective. Please note, however, that you still have to apply a crop factor to these lenses to get the 35 mm equivalent. So, for example, the 10-22mm EF-S lens is roughly the equivalent of a 16-32 mm film lens, meaning that the wide angle (anything greater than 35 mm) is restored. Another advantage of EF-S lenses is that they can use less glass than their film equivalents, so they are in general smaller and lighter. Still, Canon's EF lenses remain their standard series, and probably always will, so I personally don't plan to invest any money in EF-S lenses.

In any event, having a crop factor isn't always a bad thing. While it constrains the wide angle end of thing, it actually helps on the telephoto side, particularly with fixed focal length (prime) lenses, so my 100mm lens acts more like a 160 mm lens.

Pefectly clear? I thought so. If it isn't, take comfort in this: if you put your camera on "live view," what you see is what you get. In other words, while it's helpful to understand focal length and conversion factors and what it means for your final image, you can learn composition and take great photographs even without understanding this stuff. Trial and error will get you there as well, just a little more sloooooowly.

What’s Focal Length and Why Should I Care?

Long, long ago (it seems) I promised one or more posts addressing two critical features of any lens: focal length and maximum aperture. Let’s tackle focal length first. You can find a definition of “focal length” on any number of websites, but far too many of those sites succumb to the usual temptation and the explanation drowns in a bunch of complicated illustrations and technical gobbledygook.

Let’s see if I can do any better here. “Focal length” pretty much means what you see when you look through a camera’s viewfinder using a particular lens. See, that’s not so bad, is it? Different lenses give the photographer a different view and that difference is explained by the lens’ focal length. Most lenses these days are “zoom” lenses, meaning that you can adjust the focal length. The other kind of lens has a fixed focal length (i.e., one that can’t be adjusted) and those lenses are sometimes called “prime” lenses.

The view you see can generally be classed as (1) “normal”—in other words, it gives a perspective similar to what the human eye would see—(2) “wide angle,” or wider than the human eye would see; or (3) “telephoto,” a magnified perspective that means the lens captures a narrower field of view than the human eye would capture at that distance. I’m afraid those terms get a little loosey goosey, though, and you’ll often find disagreement as to whether a particular field of view is normal, wide angle, or telephoto. Generally speaking, however, I think anything wider than 35 mm is “wide angle,” anything between 35-70mm is “normal,” and anything narrower than 70mm is “telephoto.”

So, how do you know what category your lens falls into? Well, if you take out any camera—even a point and shoot camera—you’ll see a bunch of numbers written on the lens. For present purposes, all we need to worry about is the numbers that end with “mm,” for millimeters. Don’t ask me why they measure focal length in millimeters. I really don’t care, and neither should you. What’s important for you, the photographer, are the numbers that precede the “mm.”

So, for example, I’m looking at a squat, fat lens attached to my camera and it says, “CANON ZOOM LENS EF 20-35mm.” What interests me is the “20-35mm” part. That tells me this is a wide angle lens.

But here’s the kicker: you really don’t need to look at the numbers to know it’s wide angle. All you have to do is attach the lens and look through the view finder. I do, and what do I see? Lots of stuff. That’s what wide angle lenses do: they show lots of stuff. And that’s why landscape photographers love them, because a wide angle lens captures a field of view wide enough to capture the sky, the clouds, the mountain peak, and the little cluster of wildflowers at your feet. Voila!

At the other extreme are the telephoto lenses. These lenses, usually measured in big “mm’s” like 200, 300, 400, or even 500—magnify stuff like mini telescopes. They also narrow the field of view, isolating subjects against, frequently, a blurred background. (The background blurs because the lens captures such a narrow slice of the background relative to the subject in the foreground.) That’s why telephoto lenses are the tool of choice for wildlife, sports, and fashion photographers. They need to capture images at a distance, and they want to eliminate elements that distract from their subject, be it a wood duck, a wide receiver, or a hot babe in a string bikini.

So, what does all this mean for the portrait photographer? Principally, two things: first, the choice of focal length determines the kind of portraits you’ll capture. Many portraits are taken with telephoto lenses of one kind or another. Why? Because telephoto lenses crop out a lot of stuff: narrowing the field of view and focusing the composition on a single subject. With a telephoto lens, the subject—a face, a foot, a family—typically fills the frame, as in the following:
But that doesn’t mean you can’t take a perfectly good portrait with a wide angle lens, only that a wide angle lens will necessarily include a lot of the subject’s surroundings. So, I want a portrait of a kid with a skateboard in front of a graffiti-covered wall. What do I use? Well, probably a wide angle lens because I want to capture the kid and his surroundings. Here's a recent example, not involving skateboards:

The second important point about focal length for the portrait photographer is this: it’s hard to engineer a good lens at the wide angle or telephoto extremes, so, as a general rule, the wider the angle or the greater the telephoto, the more expensive the lens. It’s also harder to engineer a quality zoom lens—since the elements have to change shape to accommodate the different focal lengths—so higher quality zoom lenses cost a lot of cold, hard cash.

To illustrate, consider a few lenses from the Canon line. B&H Photo sells the 100mm macro (a superb lens, by the way), for $490. An optically inferior 400mm, on the other hand, will set you back $1190; and the 500mm, $6000. The huge, white, zoom lenses you see at professional sporting events? Try upwards of $10,000.

In case all this only serves to muddy the waters, here are a few summary take home messages:

(1) Nearly every DSLR (non-point-and-shoot) camera sold these days comes with a zoom lens attached. Zoom lenses are convenient because they allow the photographer to change focal length (and thereby perspective) without moving closer or farther from the subject. But remember the point about technology: it’s hard to do right, and doing it right costs a lot of money. So, that 70-300mm zoom that came with your camera likely is … for lack of a better word … crap. Don’t get me wrong: a crappy lens may be perfect for taking family snapshots for the scrapbook, but if you’re serious about making quality images, it ain’t happenin’ with that kit lens.

(2) Use a wide angle lens to add context or scale to an image; use a telephoto lens to isolate the subject and compress the perspective.

(3) Use a telephoto lens if you want or need to stand far away from a subject, like a grizzly bear or toddler deprived of her usual nap and afternoon snack.