 BetterPhoto Member |
Calculation of Zoom Length I'm just trying to find out how the formula or calculation works which determines the Zoom length of a camera lens. I think it has something to do with the focal length and range but I'd like to know where the 70~210mm or 70~300mm comes from. If anybody could help me with this it would be much appreciated? p.s. ... I spent about an hour on my calulator trying different numbers and couldn't find the correct method ....
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John A. Lind |
Paul, Your frustration is understandable. The "zoom length" is the same as the focal length of your lens. In order to measure the focal length of a practical compound lens, such as your camera lens, you need an optical bench in a lab. The formal definition of focal length is the distance from the rear principle point to the focal plane when the lens is focused at infinity. For camera lenses, the focal plane is the film plane in the camera. This is where it is held across the film gate by the pressure plate in the camera back. If the lens is focused at some distance less than infinity, this moves the rear principle point farther from the film plane than the focal length. For lenses where the medium is the same in front and behind the lens (e.g. air), the rear principle point is also the rear lens node. You will see the two terms interchanged often when discussing camera lenses, although they are technically different. The rear principle point is most likely inside your lens (see last paragraph), and like its focal length, you need an optical bench to find its location, especially with a complex compound zoom lens that probably uses internal focusing (by moving internal glass elements relative to each other). The ranges you gave (70-210mm and 70-300mm) are focal length ranges. When the focal length of a zoom lens is changed by moving the zoom ring (e.g. from 70mm to 210mm), the location of the lens' rear principle point is shifted (e.g. from 70mm in front of the film plane to 210mm in front of the film plane). Zoom range is sometimes expressed as a multiple of minimum focal length to the maximum focal length. A 70-210mm is a 3X zoom, and the 70-300mm is approximately a 4X zoom. If you want to see some diagrams of all this for simple, theoretical lenses, see my web page about lenses (warning: it contains some techno-babble): Now for the kicker. The focal length can be longer than the physical length of a camera lens (even though it's not a mirror lens). This is the technical meaning of the term "telephoto" which is often misapplied to what really is a "long" lens, an old term not used much any more for lenses with focal lengths longer than standard. Keep in mind that the zooms you mentioned are complex, compound lenses, often with over a dozen glass elements, some of which move inside the lens relative to others with when changing zoom or focus. -- John
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