Your first DSLR is a big deal. In many ways, it’s a rite of passage from casual photography to serious photography, and it really does open up whole new worlds of creativity and learning. But it also adds some layers of complication to your hobby, and a big one for a lot of new DSLR owners has to do with those interchangeable lenses and their associated focal lengths. Just when you thought you understood what 14-42mm means, you probably noticed that thing in parenthesis: “35mm equivalent 28-84mm.” What the heck?
This is something you didn’t see in film cameras, because all SLRs (the film camera predecessor to the DSLR) were 35mm. In other words, all SLRs used the same film (35mm), so every frame was the same size, so the focal length of every lens was whatever it said on the barrel, end of story.
Digital cameras are different, because they don’t all have the same filed of view. But before we delve into that any further, it's necessary to understand some of the inner workings of your camera, so let’s first talk a little bit about focal length and image sensor, which are the two most important factors you need to understand before you can grasp equivalent focal lengths.
What is focal length?
"Focal length" is not the physical length of your lens, rather, it refers to the optical distance between the point where light rays converge and the image sensor (or film plane, on a film camera). So while the distance that measurement actually represents is meaningless for most photographers, the number can also tell you something about the field of view you'll be able to achieve when you use that lens. Smaller focal lengths correspond to broader fields of view, while larger focal lengths correspond to narrower fields of view—in other words, lenses with smaller focal lengths let you see more of a scene, while lenses with longer focal lengths give you greater magnification, letting you get a lot closer to smaller, specific parts of a scene.
What is an image sensor?
Digital cameras don’t record images on film, so they need some other way to capture light and turn it into a photograph. Enter the image sensor, which is the part of your camera that takes an optical image and converts it into an electronic signal, which can then be written to your memory card. You can thank your image sensor for the state of your bank account, because without it you’d still be buying film and paying for processing—and you’d almost certainly be taking a lot less photos than you do now.
But all digital cameras are not the same—some are big and heavy, like a professional quality DSLR and some are small and compact, like a budget point-and-shoot. And for the most part, you’ll get smaller sensors in smaller cameras and bigger sensors in larger cameras. This is similar to the differences between the 35mm cameras of old compared to those basic Instamatic type cameras most families owned, which shot photos on much smaller 110-format film.
So when you’re looking out at the world through your viewfinder, how much you see depends as much on the size of your camera’s sensor as it does on the focal length of your lens.
Here’s a visual look at what I mean:
The broadest field of view pictured above (in blue) is what you would get if you shot this scene with a 35mm camera, or a digital camera with a “full frame sensor.” Cameras that have full frame sensors are designed to take images that have the same aspect ratio as those old 35mm film cameras, which corresponds exactly to the standard print size of 4x6. Full frame cameras are typically higher-end DSLRs, or cameras you're likely to have to pay a premium for.
The second rectangle (the green one) represents an APS-C image sensor, which is what you’ll find on most budget and consumer model DSLRs. The third rectangle (the yellow one) is a micro 4/3rds image sensor, which is used by a lot of compact system cameras, such as the Olympus PEN mirrorless cameras. The fourth, inner-most rectangle corresponds to a typical point-and-shoot sensor, though sensor size does vary somewhat depending on brand and model.
So when you’re shooting with a full frame camera you’re going to get a field of view that’s pretty close to what you would get with a 35mm film camera. But with any camera with a smaller sensor, you’ll lose the outer areas of the image (you’ll hear this called “cropping” or "crop factor.")
Because of this difference, manufacturers have lenses designed for full frame cameras and lenses designed for smaller frame cameras, and those lens and sensor combinations work together to give you a field of view that’s equivalent to a 35mm film camera (or a full-frame camera). So if you have an APS-C camera with an 18-55mm lens, you’ll get a field of view equivalent to a 28.8-88mm lens on a 35mm or full frame camera.
If you’re still a bit baffled, let’s look at it another way.
Let’s say you want to take a photograph of a sunflower with a full frame camera. You get close enough to the flower so that it fills the frame, and you take the picture. Now, without moving, you take the lens you used to shoot the photo with your full frame camera and you put it on an APS-C camera. You’re essentially using the same lens with a smaller image sensor, so now you’re only able to get the center of the flower in the picture rather than the flower with all its petals. So it appears as if you’ve zoomed in, when in fact you’ve just got a smaller image sensor that has effectively “cropped out” the edges of the photo you would have gotten with the full frame camera.
Sunflower by Flickr user penbentley
What this means
Full frame lenses are designed for full frame cameras, but as in the above example, they can also be used on APS-C cameras, to some advantage. When you put a 100mm full-frame lens on an APS-C camera, you get more zoom—or rather, you get the perception of more zoom. That 100mm lens becomes the zoom equivalent of a 160mm lens, so you can get visually closer to your subject than you would have been able to do if you used that same lens on a full frame camera. Now you might be thinking, why ever use anything but full frame lenses on APS-C cameras (especially if you're the sort of photographer who loves zoom), but there are some pluses and minuses. First, you'll notice how I said "perception" of more zoom—that's because that full frame lens/APS-C camera combination doesn't really magnify your subject more than a full frame lens/full frame camera combination would. Instead, you're seeing a cropped version of the image—just as you would if you opened up that full frame shot in post processing and manually cropped it. How much this matters depends on other factors, like the number of megapixels your camera can shoot at. If you have a high megapixel APS-C camera it may not really matter that the image is essentially cropped, because you'll still be able to capture plenty of detail within that crop.
You can still see this as an advantage if you think along the lines of practicality—with that full frame lens you'll be able to stand further away from your subject and still fill the frame, which means no additional cropping later on and, more importantly, the ability to shoot in semi-stealth mode. There's a reason why bird photographers use ridiculously long focal length lenses—because their subjects are likely to fly away if the photographer is forced to get close in order to fill the frame. The same is true for much smaller creatures like insects—there's an advantage to hanging back a little, and a lens with a longer 35mm equivalent focal length can give you that advantage.
Pompadour Cotinga by Flickr user holyknight33 (is gone! too busy! sorry guys!)
Ultimately, you really only need this information when you're combining a full-frame lens with an APS-C camera, because APS-C lenses are going to operate at their advertised focal length whenever they are used on an APS-C camera, and lenses designed for full fame cameras will do the same thing on a full-frame camera. So unless you are mixing and matching, you can safely ignore that 35mm equivalent number, because it won't apply to you. Nevertheless, it's still good knowledge to keep in the back of your head because there may come a time when it will be relevant, and you won't want to be left scratching your head trying to remember what that equivalent number actually means.
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