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The image produced by an eyepiece is a virtual image (unlike the one projected onto a CCD sensor or screen). The "apparent field of view" refers to the apparent diameter of the field stop (the black border surrounding the image in the eyepiece) compared to an actual circle drawn on a wall with a given diameter viewed from a given distance.
A Super-Plössl with an apparent field of view of about 52˚ shows a field stop that looks like a circle with 1m diameter viewed from 2m distance. A 100˚ eyepiece has a field stop that looks like a 1.2m diameter circle viewed from 1m distance.
The actual amount of sky shown depends on the magnification of the telescope/eyepiece combo.
The true field of view on the other hand is a circle projected onto the sky. If two stars have an angular separation of one degree, and they sit on opposites edges of your field of view in the eyepiece then the telescope/eyepiece combo has a true field of view of one degree.
Cheers
Steffen.
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