Power contained in a square area of an image formed by a diffraction-limited imaging system with a centrally obscured circular pupil is calculated and compared with the power contained in a circular area. It is shown that, regardless of the amount of obscuration, the difference between the corresponding ensquared and encircled powers is less than 9% of the total image power. Approximate expressions are obtained for the power lying outside a large square or a circular area.
© 1978 Optical Society of America
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