The Focal Length Conundrum

Take a look at this photo of buildings in midtown NYC. What focal length would you say it was shot with? Given the stacking effect you’d probably guess about 200mm or more. If it was shot with a “full framer” your instinct would be right. But it wasn’t, and the puzzle it poses is the subject of this discussion.

Each lens is identified with a focal length that tells you, among other things, its angle of coverage (angle of view). Think of angle of coverage as an arc that starts at the front of the lens and spreads out to encompass the scene before you. Wide-angle lenses have a wider arc, while telephoto lenses have a narrower one. In short, focal length tells you about the peripheral vision of the lens; in some it is wider than the normal eye, in others narrower. Some focal lengths are dubbed as “normal,” by the way, which simply means they match the peripheral vision of most people.

Telephoto lenses, as their name implies, bring distant subjects closer, while wide-angle lenses tend to make distant subjects seem farther away. Zoom lenses incorporate all the focal lengths within their range; they are often expressed by the ratio between the wide angle and telephoto maximums. A 7–14mm lens is referred to as a 2X lens, for example, while a 7–49mm lens would be a 7X zoom.

The problem with all this, however, is that focal length is expressed in relation to a 35mm format (24 x 36mm image area), thus with that frame size a 28mm is “wide”, a 50mm is “normal”, and 200mm is a “telephoto.” Simple enough, I suppose, and that simple solution is because of “the habit of terminology.” But habit does not often feed utility and, to paraphrase Confucius, if everything was righly named the universe would be in order.

Digital frames, the sensors, are what have thrown all this into a bit of a tumult. You might have an 18mm lens on one digital camera and think you’ve got a “super-wide” going for you when in fact you have what’s known as an “effective focal length” of about 28mm. Or if you have a 200mm on you might be “effectively” working with a 300mm (or a 400mm, or a 320mm, depending on camera.) Because the size of the sensor is smaller than a frame of 35mm film it acts as an image “crop,” if you will, because the image circle projected by the lens (yes, they are circular) gets cut off at different points. But the industry has insisted on expressing the focal length in terms of its 35mm equivalent.

And technically they are right. The reason is that the focal length of the lens is the focal length of the lens. That may sound like specious reasoning, but in fact focal length is the distance from the rear element of the lens to the imaging plane (sensor, film) when the lens is focused at infinity.

For example, say the area of the digital sensor is ¼ the size of a 35mm frame (given the same aspect ratio, which is the ratio of height to width). To find the equivalent angle of coverage you would multiply the focal length by 4. So, a 6mm lens on a snapshot-type digital camera would give an angle of coverage of a 24mm lens on a 35mm camera, and a 20mm lens on that same camera would be equivalent to using an 80mm lens on a 35mm camera. In the “under full frame” world, telephoto lenses effectively get longer and wide-angle lenses get less wide.

So, what to do? Lens makers will not touch this debate with the veritable ten foot pole. Editors in the worldwide TIPA group, to which Shutterbug belongs, have had discussions, but the opinions are, like the group, all over the map. So writers about photography are always forced to use the “equivalent” phrase, which when discussing “full frame” lenses that fit a number of camera types can lead to a paragraph giving those equivalent figures. And photographers are forced to make calculations every time they consider the next lens for their kit.

Of late, we’ve seen more and more “full framers” in every camera confirguration, but I am certain that this will not spread to all classes, so we are stuck with the dilemma. I do have some thoughts on this, making angle of view or at least descriptive terms like “wide”, “tele” etc, a part of a lens’s nomenclature, but frankly I also see the weakness of this approach when applying the “exception proves the rule” standard.

If you have any thoughts on this matter please leave a comment below. I will take these to the next TIPA Technical Group meeting and certainly incorporate them in future discussions of this matter in the magazine.

Share | |

X
Enter your Shutterbug username.
Enter the password that accompanies your username.
Loading