I keep seeing people talking about focal-length of lenses without saying how large the frame is. Is there a more sensible way to talk about how much a lens “zooms”?
(I have seen so many adverts etc that just have the focal-length(s) of a camera so this is a common problem)
Focal length is focal length. Field of view is field of view. Unfortunately, sometimes they're conflated and the former means the latter.
If it's a concern in a particular question, I'd suggest asking that the OP clarify with his/her crop factor.
To put it simply, the field of view of a camera body, which depends on its sensor size, determines the effective focal length of a lens when used on that body. There are a variety of sensor sizes and body depths, and therefor a variety of fields of view, for cameras these days. If we take just Canon, they have three sensor sizes for their DSLR cameras: Full-Frame 35mm (1x Crop), APS-H 28mm 1.3x Crop, and APS-C 22mm 1.6x Crop.
When it comes to lenses, a single lens may be able to be used on multiple camera bodies. Again, if we take Canon as an example, the bulk of their lenses are the EF mount. A single EF mount lens, say the 24-70mm focal length L-series lens, supports all three of Canon's DSLR sensor sizes (and therefor all three fields of view.) One may buy the 24-70mm lens for their first Rebel series 550D body, and later upgrade to a full-frame 5DMkII body. When buying an expensive lens that should have a very, very long life, the field of view of the camera body should not really be a factor.
The focal length of the lens itself is really the key factor, and as long as you know the appropriate multiplier for your sensor, you can calculate the effective focal length for each body it might be used on, and its usefulness on that body. This little fact was useful for one of my recent lens purchases. I have a Canon Rebel XSi (450D), and I needed something in the 24-70 range. Since I know my crop factor (or focal length multiplier) is 1.6x, it was easy enough to calculate that the 16-35mm L would effectively be a 25-56mm lens, which generally fit the bill. I also know that when I upgrade to a 5DMkII (or III) in the relatively near future, that this lens will behave as a very nice, ultra-wide to wide angle 16-35mm zoom lens ideal for landscape photography.
If lenses were rated in their field of view, it would be rather confusing to make such a simple determination as effective focal length when a lens is used on different bodies with differing sensor sizes. Lenses are lenses, and should be rated in focal length. Camera bodies are camera bodies, and there should be a simple way of determining their focal length multiplier due to the field of view the sensor provides. In most cases, cameras have a known multiplier, and if not, the information can be easily gleaned (Canon has 1x, 1.3x, and 1.6x, Nikon has 1x and 1.52x, etc.)
From the wikipedia article on focal length:
The focal length of a lens determines the magnification at which it images distant objects. The focal length of a lens is equal to the distance between the image plane and a pinhole that images distant small objects the same size as the lens in question.
So the focal length of a lens is an optical property - it does not change when attached to a different camera. However photographers are used to what the focal length numbers mean when attached to a 35mm film SLR - which is equivalent to a full frame DSLR.
The "crop-factor" multiplied by the focal length gives you the focal length of a lens that will produce the same field of view on a 35mm sensor. (However it will not be exactly the same image.)
I don't have any particular suggestion of what else to use. I don't find it too confusing myself. You could use the field of view angle in degrees. However that would have to specify the sensor aswell and lots of prime lenses are used on cropped sensor bodies, so it wouldn't make that much sense.
Edit: Just found this article which argues for using the suffix "e" for "35mm equivalent". So for APS-C cameras, you might say "This is a 60mm (96e) lens" - which is succinct and saves mental arithmetic. When it's not clear what sensor size it might be used with, it would be best to go for just the focal length.
I rather like this idea.
In theory you should probably talk about angle of view -- but almost nobody knows that. Most people are accustomed to thinking in terms of focal length X on a 35mm camera, so that's how things like "effective focal length" are usually expressed.
When you look at things more carefully, however, there are still differences that can't take into account. For example, depth of field depends entirely on aperture and distance from the subject. This is why some people say that a full-frame camera gives less depth of field than a cropped-format camera. Technically, it's not really true -- if I mount (for example) an 85mm lens on a full-frame camera and then on an APS-C camera, and shoot at the same aperture and distance from the subject, both will show exactly the same depth of field -- but given the narrower field of view on a crop-format camera, with the same lens you'll be farther away from the subject to get (roughly) the same field of view, and that extra distance from the subject will increase your depth of field.
Depends entirely on their question. If they are talking about what lenses to use in certain situations, I believe we should encourage the use of effective focal length, as personally, I don't memorize field of view or angle of view numbers.
When talking about lens reviews and comparisons, unless specifically about its application and a specific type of environment, it is unnecessary to mention effective focal length.
The practice I adhere to is usually mentioning both.
If you have blue paint and yellow paint and you mix the two you get green paint. It is utterly useless to talk about the “green potential” of your blue or the “green potential” of your yellow until you know which blue AND which yellow you actually have.
