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2. Important Angles (continued)
2.2 Field Angle

    fig.: Field Angles (10kByte)

Look at figure 2.2. Between the rays from the topmost object point and from the lowest object point to the principal point   H   the entire object angle   2w   is included.

Between the rays from   H   to the topmost image point and to the lowest image point the entire image angle is included.

With the very most lens systems, object angle and image angle are the same and you can call both "field angle"   2w .

From figure 2.2 you can easily see that
        (field angle) = 2w = 2 * arc tan ((G/2) / g)         (8a)
        (field angle) = 2w = 2 * arc tan ((B/2) / b)         (8b)

You feel that, with increasing field angle, the task of producing undistorted, well-defined, and well-illuminated images gets more and more complicated.
That's why lens systems are specified for some "maximum allowable image angle".

If you want to rely on lens data, you'd better keep within this angle.

For some lenses, image diameter is defined instead of image angle. Use this diameter at the image distance which is defined or which belongs to the scale factor (magnification) defined. There you obtain the image angle.

Regarding certain domains of field angles, certain lens properties typically tend to become critical. To get a coarse classification, you can use the image angle denominations of the classic 24mm*36mm snapshot camera:

focal lengths (mm)
field angles 2w (degrees)
60 ... 200
12 ... 40
30 ... 50
47 ... 72
10 ... 28
75 ... 130

Focal lengths ONLY apply with   24mm * 36mm   image format!

In the wide-angle domain, it is difficult to keep the image (especially at its rim) undistorted, well-defined, and well-illuminated.
The narrower the field angle, the less critical the lens.
But: Tele-lenses need huge diameters to achieve good lens speed. And they need very long object and image distances.

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Last modified Nov.28th, 2002 17:49