camera lenses.. can some one explain the mm thing please?

Question

Hi, just wondering if anyone can please explain the like 17-85mm thing on SLR and DSLR lenses... what do the numbers mean?? thanx :D

Answer 1

It is a determination of the optical characteristic of a lens called the Focal Length. See the article referenced below and look at the section on photography for more details.

The answer that says it is a measurement from the Optical Center of the lens to the Image Plane is CORRECT! The one that mentions the "Angle of View" has identified one of the key factors in comparing lenses for use on a camera, because that determines the "magnification" of the lens.

The answer that says -

"(35mm Film Camera) the lenght from the film plane to the frount element on the lens is measured in millimeters..."

is WRONG!!!. The measurement is a useful guide for comparing lenses for ANY type of camera and has nothing to do with "35mm" "film" or "cameras" per se. And it is made from the center of the lens to the image plane (on most cameras with interchangeable lenses there is a circle with a line through it similar to this Ø to indicate the image plane). Any lens with a specified focal length, say 50mm, will produce the same size image on the same size camera (assuming it can be mounted on that camera).

The reason the measurements are in MM, or on rare occasions CM, is because the majority of top quality lens manufacturers for many years have been located in Germany and Japan where they used the metric system. Also because many lenses are 2" (51mm) or less in length measurements are more precise and significantly easier to understand & compare in metric terms.

So basically the MM length is used as a means of comparison between various cameras or lenses for cameras. Because 35mm cameras were a "standard" for many years specific range of sizes are described for that format as follows (these ranges are somewhat arbitrary especially at the low and high ends)-

"Fisheye & Super Wide Angle" 6-8mm - 20mm
"Wide Angle" up to about 35-40mm
"Normal" about 45-60mm
"Telephoto" about 85-500mm
"Super-Telephoto" greater than 500mm

For digital cameras, manufacturers no longer need to adhere to a standard to accommodate the film so these ranges can be significantly different. Each design can use a different size image sensors. For example: many super compact digital cameras use very small sensors so that a 12 -15mm lens might be called normal (because it has an Angle-Of-View like a 48-60mm lens on a sensor the size of a 35mm film frame). This is called a "crop factor" or CF, in this case CF = 4. So a 60mm lens on this camera would be a moderate to large telephoto lens (4 * 60 = 240, in the list above). Each time you double the length the "magnification" is increased by 1.

Zoom lenses are, as described in other answers, lenses with an adjustable range of focal lengths.

Answer 2

The mm thing is the focal length of the lens. This is the distance from the optical 'centre' of the lens to the plane of the film or sensor when the lens is focussed at infinity.

For zoom lenses, the first figure is the shortest focal length the lens can be set at, and the second the longest.

You will find people will talk about equivalent focal length. This is where they translate the actual focal length of a lens on a digital camera to a 35mm equivalent. This is to allow you to have a common comparison of what you can achieve with a particular lens. Digital SLRs have a x1.5 or thereabouts factor. So a 17-85mm zoom on a digital body will have a 35mm equivalent of 25.5 - 127.5mm. Compact cameras have a much higher factor, up around x4 to x6 or more, depending on the size of the sensor.

Answer 3

It's the focal length of the lens- in real world terms it determines how wide or narrow your field of view is when looking through the lens. Technically it has to do with how much the lens causes light to converge or diffuse when passing through.

When you see a lens advertised with a dash between two mm numbers, that means it's a zoom lens, one where you can adjust the focal length. So 17-85 means at the widest setting it's a 17mm lens and at the farthest setting, an 85mm lens

This assumes you have a film camera, for digital cameras the effective focal length of any lens you attach is affected by the sensor size.

Here's a nice explanation of focal length that doesn't get too involved in the physics:

Answer 4

maximum suitable answer from final Anon, as commonplace, yet there is one small component he did no longer point out. you're able to ask your self why the aperture ameliorations from extensive perspective to telephoto focal lengths. i won't supply you a lesson on lens layout, (through fact i won't be able to... lol), yet i will inform you it somewhat is time-honored for many zoom lenses. SLR cameras do have zoom lenses attainable that are consistent aperture, meaning they carry an analogous aperture for the duration of the zoom selection. regardless of the undeniable fact that, those lenses are plenty extra advantageous, heavier, and so plenty extra high priced. for that reason maximum shopper grade lenses have the variable aperture. it is purely a miles less complicated lens layout to construct. i understand of no bridge digicam that has a persevering with aperture zoom lens. steve

Answer 5

17-85mm is the focal length of the lens, its the universial way of measuring lens focal distance for example some lens' may go up to 800mm or as little as 8mm.

SLR means Single Lens Reflex.

Answer 6

(35mm Film Camera) the lenght from the film plane to the frount element on the lens is measured in millimeters


(digital cams have conversion factors)


so MM means millimeters, photography is meteric not imperial so if you want to learn more learn a bit about meterics

I know England and USA use inchs and such but the world uses meterics (even the photographic world)

Go hard thumbdowners - its a big world out there


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Answer 7

go to howstuffworks.com
to find out