What's the big deal? We're talking, what -- another 14 degrees angle of
view over the 18mm lens?
The formula for determining angle of view from focal length is:
angle = 2 * arctan(width / 2 * length)
where "length" is the focal length, "width" is the width to be covered at
the film plane -- about 43mm diagonal on a 35mm frame. (BTW: you can use
this formula to figure the angular width and height coverage by plugging
the proper numbers in for width.)
The movement of the arctan function is asymptotic, so changes in
millimeters make less and less difference in angle of view as you approach
zero. In order to have a rectilinear 180 degree angle of view, "width / 2*
length" would have to be infinity.
Of course, we all know lens makers fudge on specs. That Sigma is probably
actually more like 14.4999mm, but plugging the numbers for 14mm, we get 114
degrees. Cutting Olympus the same slack, the 18mm lens has a 100 degree
angle of view. To contrast, the Vivitar 17mm covers a paltry 103 degrees.
Those last 90 degrees (above a 90 degree lens) have an asyptotically
increasing cost, in terms of precision and materials. No wonder there are
relatively few, very expensive lenses down there -- or that cheap
alternatives, like the Sigma or Vivitar, don't measure up!
: Jan Steinman <mailto:jans@xxxxxxxxxxx>
: 19280 Rydman Court, West Linn, OR 97068-1331 USA
: +1.503.635.3229
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