Your camera may boast “8x zoom”, but most DSLRs do not advertise values like these. So how do they compare? The answer is more complex than you may think.
That “8x” value that doesn’t necessarily mean objects in the photo will look 8 times bigger than they do with your eyes. It just means things will be 8 times bigger than its most zoomed-out position—but two cameras in their most zoomed-out positions will not look the same size.
Every lens affects your image in a different way. A wide angle lens warps the perspective in the image so it shows more than you could see with your naked eye. A telephoto lens does the opposite, zooming in like a telescope to distant objects. These things are separate from the actual “zoom” function on your camera, so one 8x zoom lens may not make objects as large as another 8x zoom lens.
So how do we calculate how much bigger an object appears in a photo compared to your eyes, where you’re currently standing? To find that out, you need to know the focal length and field of view of the lens you’re using.
Focal Length and Field of View
In photography, the focal length of a lens is the distance between a the camera’s sensor and the internal components of the lens itself. This focal length determines how close objects look to your camera and what part of the scene actually fits within the picture—otherwise known as your field of view. A massive, telescope-like lens with a 1000mm focal length will make objects look very close. Lenses with smaller focal lengths will make objects appear farther away.
Many lenses can “zoom” to different focal lengths. For example, an 18-135mm lens will let you zoom from an 18mm focal length to a 135mm focal length.
Here’s an example. I shot the following two images with my Canon 650D and an 18-135mm lens.
The first photo was taken at the shortest focal length: 18mm. It’s a pretty wide field of view.
The next photo was taken in the exact same place half a second later. The only difference is that I’ve zoomed in to use the lens’ longest focal length, 135mm.
As you can see, the field of view is a lot narrower in the second photo than the first, because we’ve zoomed in on the mountains.
Here’s the catch, though. Different lenses, at their shortest focal length, will show things differently. Remember that 1000mm telescope lens? Even if you don’t zoom in with it, you’re still seeing things much closer than a camera with an 18-135mm lens. So focal length alone isn’t enough to say “this lens shows objects X times larger than they appear to human eyes”.
For that, we need to know what focal length compares to the human eye.
The “Normal” Focal Length
Comparing cameras to eyes is a difficult task. They both capture images, but do it in very very different ways. This means that it’s impossible to say that human eyes are the equivalent of a 35mm lens with an aperture of f/5.6, for example; the concepts just aren’t interchangeable. Instead, we’ve got to compare properties in other ways: like using the field of view.
A “normal” lens, then, is a lens that approximates the field of view of the human eye. Oskar Barnack, who created the Leica camera, arbitrarily set it to 50mm for a full frame camera, although any focal length between about 40mm and 58mm will appear roughly normal. On a crop sensor camera, the normal focal length is somewhere between 28mm and 36mm.
Calculating the Relative Field of View
Okay, we’re finally ready to answer that original question: compared to how we normally see things, how much more or less of the scene will a photo show? To do this, we’re going to calculate the relative fields of view for lenses with different focal lengths.
The formula is quite simple: just divide 50 by the focal length you’re using for the photo (because a normal lens is 50mm, as we discussed above). If you’re using a crop sensor camera, divide the equivalent focal length on a full frame camera.
Let’s look at an example. A 28mm lens has almost 1.786 times the field of view of a normal lens (50/28 = 1.786). For simplicity’s sake, we’ll say it’s “almost twice the field of view”. That means that an object in a photo taken with an 28mm lens will look about half the size of something in a similar photo taken with a normal lens—or, half the size of what you’re eyes see standing in the same spot.
A 200mm lens has one quarter the field of view of a normal 50mm lens (50/200 = 1/4). So an object in a photo taken with a 200mm lens will look about four times bigger than what your eyes see.
In the image above, you can see that comparison in action. The first photo was taken with my 18-135 at 18mm (equivalent to 28mm on a full frame camera), the second was taken at 35mm (equivalent to a normal-ish 56mm on a full frame camera) and the final shot was taken at 135mm (equivalent to 216mm on a full frame camera). The sunglasses in the first photo are about half the size of the ones in the second, and the ones in the second are about a quarter the size of the ones in the third. Here’s a close up of them side-by-side.
Remember, Focal Length Is Different Than Zoom
Remember when we said “one 8x zoom lens may not make objects as large as another 8x zoom lens”? Now you understand why.
Say you buy a compact camera with a whopping 35x zoom. That sounds better than your DSLR lens with an 8x zoom, but this doesn’t mean that things look 35 times bigger than they do with your eyes. Instead, it means the ratio between the shortest and the longest focal length of that lens is 1:35. Depending on the focal length of each camera, the DSLR may make objects appear much larger, even though it has a smaller zoom. Zoom values are not necessarily comparable from camera to camera.
So don’t be tempted by a camera just because it has a higher zoom level—that doesn’t tell the whole story. And remember: compact cameras are good, but they’re still no match for a DSLR when it comes to image quality, sensor size, low light performance, autofocus, and dozens of other non-zoom features.
Focal lengths and their relative field of views can be tricky concepts to grasp. Most lenses don’t make things appear that much bigger than normal, instead the advantages of DSLRs lay elsewhere. If you want to get really close to something miles away, a super zoom compact camera will do better, but a DSLR will take much better images of everything else.