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Lens width/length ratings

System for rating wide angle and telephoto lenses on particular cameras
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For a given lens focal length, the field of view can very substantially depending on the size of the sensor or film frame in a camera. For example, a 75mm lens for the Pentax 6x7 (60mm by 70mm film frame) medium format camera is a wide angle lens, but a lens of the same focal length on a Sigma SD10 camera (20.7mm by 13.8mm sensor) is a telephoto lens. There are several film size standards, 110, APS, 35mm, 645, 6x7, and even more CCD sensor sizes in use. In fact Kodak recently announced that a new 45mm by 34mm sensor will be used in a new Pentax medium format digital camera, which will use the same lenses and accessories as the 60mm x 45mm Pentax 645 film camera. This leads to awkward ways of describing the characteristics of lenses that are used on the new system, in order to put it into terms that can be understood by someone who is used to using the older film version of the camera. We get phrases like "Your 85mm Pentax 645 lens becomes a 110mm lens on the new D645" which is factually incorrect as focal length is actually a physical property of a lens and does not depend on the size of the film or sensor behind it. To clear this up, I propose to define a characteristic, called "length", for how tele a telephoto lens is on a particular camera as the ratio of the focal length of a lens and the focal length of a normal lens on that camera. Thus, a 100mm lens on a 35mm film camera would have a length of 2, 50mm being the normal focal length on that format. I propose to define "width" as the reciprocal of the "length", that way, a 25mm lens on a 35mm film camera would have a "width" of 2.

Examples:

Canon PowerShot A95: "length" of 0.77 to 2.3. (built in 7.8mm to 23.4mm zoom lens, 7.1mm wide sensor)

Polaroid x530: "length" of 0.72 to 2.2, or you could say: "zooms from as wide as 1.4 to as long as 2.2". (built in 7.3mm to 21.9mm zoom lens, 7.1mm wide sensor)

Sigma 10-20mm zoom on Sigma SD10: "width" of 3.0 down to 1.5. (20.7mm wide sensor)

Sigma 30mm on Sigma SD10: "width" of 0.99. (pretty darn close to normal).

Pentax SMC 165mm 6x7 lens on Sigma SD10: "length" of 5.58.

Pentax SMC 165mm 6x7 lens on Pentax K1000 SLR: "length" of 3.3. (35mm film)

Pentax SMC 165mm 6x7 lens on Pentax 6x7 SLR: "length" of 1.65. (60 by 70mm film)

Note: higher "width" numbers indicate wider effective field of view, higher "length" numbers indicate narrower effective field of view (and higher effective magnification).

[edited for clarity 3/17]

JakePatterson, Mar 11 2005

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       This is what I've been saying for quite some time in relation to digital camera lenses that can't be interchanged - don't tell me the focal length, tell me the angle of view.   

       If people got to know the angle of view of a 50mm lens on a 35mm film system, or an 80mm lens on a 6x6 system, they'd be able to compare this information with any given device that has a lens.
Ian Tindale, Mar 11 2005
  

       I've seen blurb that describes a lens as "equivalent to a 80mm lens on a 35mm camera", but it's by no means standardized. If you know how the figures are derived, and you have the required background data, this isn't required, but generally people don't, so please focus on the 210mm pastry (Rollei fit).
angel, Mar 11 2005
  

       [210 mm pastry]   

       Is this an example of the new enviro-friendly muntions? [see [scout]'s environmentally friendly mortar shell]
normzone, Mar 11 2005
  

       //don't tell me the focal length, tell me the angle of view.// I have three Tamron lenses (28, 70-200 and 500mm) from the 1980's and 90's and each is clearly marked with the angle of view. Baked.
aov = 2tan^-1 (s/2f), where "s" is the sensor dimension (usually the horizontal one) and "f" is the focal length - simple mental arithmetic.
Other than that, this idea assumes that people are familiar with 35mm focal lengths, which may not apply in all cases, where people may have started using APS-C digital cameras, or simply used point-and-shoot in the past.
AbsintheWithoutLeave, Mar 17 2005
  

       Absinthe, the 35mm examples were just examples. The idea is that a "1" always means normal, and a length of "2" always means a telephoto lens with twice the focal length of a normal lens. The probelm with the angle of view being printed on the lens is twofold, first, many lenses actually are designed to be used with multiple image sensor sizes, and secondly, the angle of view specifications for lenses are advertised as if they will be used with a sensor that is exactly as large as the image circle of lens. The sensor is always smaller then that, and in any case is not related to that number, except that it must be smaller.
JakePatterson, Mar 17 2005
  
      
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