Eyepiece Field Stop Question

Hi,

I'm hoping someone can clear this up for me.

I've noticed the 30mm Widescan III eyepiece has an advertised 44mm field stop which would seem to provide the potential for a generous TFOV for any given focal length.

However, what happens if the field stop of this eyepiece (or any eyepiece for that matter) is not fully illuminated by the light cone of a given focal system? What role does off-axis light play in all of this?

The way I understand it, a 30mm eyepiece focuses 30mm beyond the focal plane of the converging light cone for a given focal system, say f/5, but if thats the case, the diverging light cone 30mm beyond the focal plane does not fully illuminate the field stop of an eyepiece like the Widescan III.

What am I not getting here?

Thanks in advance for any help,

Eric

The eyepiece as a complete system has a focal length of 30mm, but the focus is (should be) always at the field stop of the eyepiece, which is made to COINCIDE with the focal plane of the telescope in the act of focusing.

Having said this, it is often the case that the telescope itself doesn't fully illuminate every point in the view of a wide-true-field eyepiece, and yes, indeed, some light can be lost as it begins to diverge after passing the focal plane at the field stop. Most people don't notice this effect unless it is very pronounced, but you can detect it even if you are not very sensitive to it by defocusing a star slightly and noting how much of the diffraction pattern is "bitten away" on either side of focus.
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Mike