If your DSLR camera came with a lens, it was probably a “kit” zoom lens. Kit lenses are great for beginners. The ones that come bundled with most DSLRs typically have a very good range of zoom, usually somewhere in the range of 35mm to 70mm. Most hobby photographers don't need to move much beyond that range of zoom for the majority of what they do with their cameras. But even if you love your kit lens and you never, ever plan to leave it, you do need to know a little bit about how it works and what it is capable of. With that in mind, here is your primer on zoom lenses.
Optical zoom versus digital zoom
This is generally just a concern for owners of point-and-shoot cameras, but since I think everyone should have a point-and-shoot camera in their pocket or purse for emergency photos, I’ll mention it first. A lot of point-and-shoot camera manufacturers will eagerly advertise the digital zooming capabilities of their cameras. To the uninitiated, this may sound as if you can get good shots of subjects at a distance, as well as good shots of subjects that are nearby. Unfortunately, the word “zoom” is misleading. “Digital zoom” basically just means that your camera shoots the image first, and then digitally zooms in on whatever it is that you want to fill up the frame. Now in case you're wondering exactly why this is a problem, here's the reason: you can do the exact same thing in post processing—you don't need your camera to do it for you. And the problem with doing it in post processing is exactly the same as the problem with doing it in camera. Essentially, it's just cropping. Once you’ve digitally “zoomed in” on your subject, you’ve lost a ton of resolution from the image. In fact you’re effectively just cropping out the majority of the image in favor of a lower resolution version of something that fills up the frame. So if you're shopping for a point-and-shoot camera, and the camera of your dreams it says it has 5X digital zoom, consider whether or not it really is the camera of your dreams. You’ll never be able to use it to get zoomed-in shots that are as high quality as the ones you take at the camera’s widest setting—in fact you may not even be able to get good quality prints from the images that it does get at those pseudo-focal lengths.
Look instead for a camera that advertises optical zoom. Optical zoom means that the camera zooms in using the lens, then takes the picture at the full resolution. The result is going to be not only a much higher resolution image, but a better quality, sharper image as well.
Prime vs. zoom
Another thing you need to know is the difference between a prime lens and a zoom lens. If you're still using the kit lens that came with your camera, you may not even be aware that lenses are available in fixed focal lengths. A fixed focal length lens, or “prime” lens, is just a lens that doesn’t zoom. So why would you want a lens that doesn't zoom? Isn’t it inherently going to be a lot less versatile?
The answer is not as black-and-white as it seems like it ought to be. The first reason has to do with low-light capability—unless you sink a lot a lot of money into your zoom lens, the chances are it's not going to be as fast as a less expensive prime lens will be. While your kit lens may only allow you to shoot at apertures as wide as f/5.6, for example, a 50mm prime lens may let you go as fast as f/1.8. That can actually make a big difference, especially if you like to shoot in low light. At those very wide apertures, you can still hand-hold your camera and get a sharp image even after the sun goes down.
It also means, of course, that you will have to zoom with your feet. This isn't always practical or even possible, which is why a zoom lens tends to be the best choice if you don’t want to spend a lot of time swapping lenses, walking back and forth or if you're traveling and you want to pack light.
The 1/f rule
Another consideration that you need to have whenever you use a zoom lens is how focal length affects shutter speed. This information is also true for fixed focal length lenses of longer focal length (such as a prime 300mm lens), but for most beginners it’s really only going to apply to those longer zoom lenses. Let's say you have a 72 to 300mm zoom, and you’d like to use it to shoot images of your kids at play. But you've noticed that many of your photos aren't that sharp, even the ones that feature kids who aren't moving around a lot. What's going on?
The simple answer is that it is difficult to keep a long lens stable even during a faster exposure. So if you're trying to photograph a kid who’s doing something quiet, like playing a game of checkers, and you’re using a shutter speed of 1/125 with a 300mm zoom, you're probably going to get a blurry picture even though your child is still quietly pondering which piece he’s going to move. That's because when you magnify a scene with a zoom lens, you’re also magnifying the movement of your camera. And no matter how steady you think your hand is, at some point the movement in your camera is going to show up in your photograph as camera shake.
Sanderling by Flickr user DaveInman
For most people, the point at which camera shakes start to become obvious is roughly the same. To figure what that point is, you can use a very simple formula. Just take the focal length of your lens (let's say 200mm) and compare that to the bottom number of your shutter speed. If the bottom number of your shutter speed is smaller than the focal length of your lens, you will probably not be able to handhold your camera during the exposure without getting some camera shake. For that 200 mm lens, this means shutter speeds slower than 1/200 are out of the question unless you have a tripod you can mount your camera on.
Now the exception to this rule is lenses that have image stabilization. And here’s the thing: most modern lenses do, especially the better quality ones. If your lens has image stabilization it will be marked right on the barrel, and there will also be some sort of switch that activates it. With image stabilization turned on, you can safely break the 1/f rule—usually by about three or four stops, depending on how steady your hand is.
Zoom related distortion
One final thing to think about when you use your favorite zoom lens is how the change in focal length affects your subject. You may have noticed that when you use your 18 to 55mm kit lens on the widest end to shoot an image of your child or your dog, you end up with a subject that has a really odd looking face. Maybe your dog’s nose looks a lot bigger than it actually is. Maybe your child’s forehead looks enormous. That’s because at very wide angles, you get a lot of distortion on your subject.
The reason why this happens is because wide angle lenses make objects in the foreground look larger, and objects in the background look more distant. When you shoot a human face up close, technically the nose is going to be in the foreground and the ears and sides of the head will be in the background. So what happens is that you get noses and foreheads that look huge, and ears that look small, which can make for a comical if not very flattering photograph.
The most flattering focal range for human beings is generally around 50mm to 100mm. At those focal lengths, the features on the human face tend to look pretty much the same as they do in real life. If your goal is to take a nice portrait-style photo of your subject, a focal length in that range is the way to go.
Once you get past 100mm, you start to get some more distortion. What happens at longer focal lengths is that you get a sort of pancake effect, where the face starts to flatten out a little and the features become smaller. At long focal lengths there is less definition between the tip of the nose and the side of the face, so while this can make the nose look a lot smaller (and more flattering for those who don’t really love their noses) it also makes the face look a lot wider, which is probably not an effect that most subjects will love.
You can apply these perspective distortions to any subject, of course, not just humans. Landscapes shot with a wide angle lens will actually look bigger in scale than landscapes shot with a telephoto lens, not just because you will be including more of the scene in the frame but also because you’ll be exaggerating the distance between those elements. That’s why landscape photographers nearly always shoot wide—it’s the best way to capture that “wow” factor when you’re trying to communicate to your viewer just how big the scene is in real life.
Hnausapollur by Flickr user Lovepro
And you thought “zoom” just had to do with how close you could get to your subject with your lens. As it turns out, there are a lot of subtleties to that zoom lens that you need to understand before you can effectively use it. Now that you know what they are and what they can do, you’re going to be much better equipped to get some great zoomed in shots of your favorite kids and animals and equally great zoomed-out shots of your favorite places.
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