lens measurement ? how so..

[Dirt_Bike_Ryda]

i used my first dSLR the other day at a Motorbike event, it wasn't mine, not my families, i'm not actually sure who owned it, but some dude said it was his and that i could use it. anyway. i loved it ( Cannon 40D ) it had the kit 17(18?)-85mm lens on it.

anyways, after using it for a while it occurred to me that when i was at the widest (17mm), the lens were physically longer then 17mm. Sooo... how do they call it 17mm.. i just don't understand that. could someone please give me some explanation, don't worry about being too technical and all (im kinda good at understanding technical jargon).

thanks

[sybren]

It's the effective focal distance of the objective. Due to different lenses that make up the objective the actual length of the objective may be longer or shorter than the focal distance. An extreme example is Canon's 70-300mm DO lens, which uses diffractive optics (like a Fresnel lens) to dramatically reduce its length.

[Dirt_Bike_Ryda]

ok.. soo.. where do u measure this? like what is being measured?

[sybren]

It's measured using the viewing angle, a.k.a. the field of view. More is explained on http://en.wikipedia.org/wiki/Photogr...d_focal_length

Another way is by using the magnification of the objective, as explained in http://en.wikipedia.org/wiki/Magnifi...ptical_systems

I link to Wikipedia articles because they can explain it much better than I could ;-)

[OsmosisStudios]

Most IF (internal focusing) lenses work like this, as do many other kit lenses.

The focal length is a weird sort of bouncing measurement taken from the front of the lens to the focal point in the lens. There was another thread discussing the basics of it, but with this kind of lens it moves about and is somewhat more complicated. The concept's base, however, remains sound.

[mainak_ch]

My understanding in this matter is that , that if the lens is focused at infinity at a particular focal length (say 55 mm), then the same distance (55mm) measured from the focal plane (it should be marked in the camera, and should be directly above the sensor) would give the position of the effective lens to produce the same FOV. However, as all SLR lenses are composite ones, with a number of elements, the distance from the focal plane to the front element of the lens will be greater than the focal length, with the effective lens position somewhere in between the foremost and rearmost lens element.

[dcclark]

Yup... modern lenses are crazy, with all kinds of moving parts and lens tricks going on. The effective focal length is based on the field of view, typically. This is why most telephoto lenses have a physical length which is nowhere near their marked focal length -- optical engineerls are very tricky.

[Doug Pardee]

Like horsepower ratings for automobile engines which bear no real relationship to the power of horses, the focal length is a metaphorical value, not a physical measurement. The focal length is used to specify the magnification factor of the lens.

A lens with a focal length of 18mm gives the same magnification as a hypothetical infinitely-thin single-element lens positioned 18mm from the sensor/film would. Specifically, an object "x" feet across that is located "x" feet away will produce an image that is 18mm across on the sensor/film (when the lens is focused at infinity).

"Telephoto" or "retrofocal" lens element groups are included in lenses in order to get to them to focus at the sensor/film without needing to have any specific relationship between focal length and lens length.

[Dirt_Bike_Ryda]

ohh.. well... thats that then. ill just accept that some superdooper dude somewhere has said that is 18mm. and ill just accept that it is 18 mm. lol hahaha. i honestly didn't think it would be that complicated

thanks all the same

[Doug Pardee]

Originally Posted by Dirt_Bike_Ryda

i honestly didn't think it would be that complicated

It's not really that complicated. The focal length tells you the magnification of the lens. It tells you how big the projected image (on the sensor/film) of an object will be.

Let's look at some simple examples, for various distances to the object, size of the object, focal length, and the resulting projected size of the image:

DISTANCE  OBJ.SIZE  FOC.LEN  PROJ.SIZE
--------  --------  -------  ---------
1000'     1000'     18mm     18mm
1000'     1000'     50mm     50mm
1000'     1000'     100mm    100mm
1000'     1000'     200mm    200mm
1000'     1000'     500mm    500mm

See how the focal length directly tells you the size of the projected image?

It's not just with 1000 feet. It's for any object whose size is the same as the distance that it's being imaged from:

DISTANCE  OBJ.SIZE  FOC.LEN  PROJ.SIZE
--------  --------  -------  ---------
10'       10'       50mm     50mm
100'      100'      50mm     50mm
1000'     1000'     50mm     50mm
2000'     2000'     50mm     50mm
5000'     5000'     50mm     50mm

The projected image size is directly proportional for objects of different sizes (photography would be messy if it wasn't):

DISTANCE  OBJ.SIZE  FOC.LEN  PROJ.SIZE
--------  --------  -------  ---------
1000'     100'      50mm     5mm
1000'     1000'     50mm     50mm
1000'     10000'    50mm     500mm

Similarly, the projected image size is inversely proportional to the distance to the object:

DISTANCE  OBJ.SIZE  FOC.LEN  PROJ.SIZE
--------  --------  -------  ---------
10'       100'      50mm     500mm
100'      100'      50mm     50mm
1000'     100'      50mm     5mm

To put it into mathematical terms, the ratio between distance to the object and the size of the object is identical to the ratio between the focal length and the size of the projected image of the object. Or:
PROJ.SIZE = FOC.LEN x (OBJ.SIZE / DISTANCE)

In practice, this isn't exactly true at close distances because the process of focusing at close distances changes the focal length of the lens. The focal length that is marked on the lens is measured when focused at infinity, and can be considerably different when focused at 5 feet.