The Pentax fish-eye zoom lens has a diagonal FOV varying between 180° at the wide end and 90° at the long end. The Birds-Eye lens forms a circular fish-eye image with FOV of 180° across the image circle. All other Fish-Eye lenses form full-frame images with a diagonal FOV equal to 180°.

Diffuse lightingexample

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Until now all Pentax DSLRs use a sensor smaller than the standard 24 x 36 mm film frame. The lens still produces a 24 x 36 mm image, but the sensor only captures the central portion of that image — your wide-angle is not so wide and your tele is even longer. The calculator below takes the 1.5 x crop factor into account.

The calculator below converts between the focal length f and the field of view (FOV) of a rectilinear lens. The formula that it implements is FOV = 2 arctan (x / (2 f)), where x is the diagonal of the film. The FOV is measured across the frame’s diagonal, and is therefore smaller across the horizonal dimension, and even smaller across the vertical dimension.