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LENSES AND MIRRi
Equivalent Focus
(21) A term (commonly abbreviated as E. F.) used
only with relation to lenses having two or more elements. of which the projection lens is an excellent example. In effect it means that the lens, viewed as a whole, has the same power of magnification or reduction imple
lens having the same focal length as the E. F. oi the compound lens.
(22) Although the simple, uncorrected lens possesses such faults as spherical and chromatic aberration and cannot project the same sharp image that a corn compound lens does, these faults do not, in any dq alter the focal length of the simple lens.
Critical Angle
(23) Light passing through a transparent medium of one density will, upon contacting a transparent medium of greater density enter it, but with a certain percentage of reflection loss. If, however, the angle of incidence is too great, the light will not leave the lighter medium and enter the heavier. Instead it will be reflected from the surface of the heavier medium in its entirety. The critical angle varies with different mediums. It is approximately 41° between air and polished glass.
Focal Length
(24) Distance between optical center of a lens and the object when its image is in sharpest focus.
Free Diameter
(25) The diameter of a lens that is free and open (<> the passage of light.
Incident
(26) Optically "incident" means contact. "Light is incident upon" means it is in contact with some surface.
Lens Correction
(27) The grinding of lens surfaces and the combining lenses of glass of different refractive indices so that