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the
_{T}£tf part of an inch in diameter, then the eye perceives the image as a point sharply defined; and this has given rise to the term in question. Several rules are in existence for finding out the nearest point in focus, but one of the best is that by Mr. W. Cheyney, in the "Journal of the Franklin Institute," which is as follows: - Multiply the diameter of the aperture of a lens by the equivalent focus thereof, divide the product by the greatest allowable error, and to the quotient add the equivalent focus. The sum will be the distance of an object upon which the lens should be accurately focussed in order that all objects beyond a point one-half of the above distances shall be apparently in focus. Thus - Let
f = the equivalent focus.a = the diameter of aperture. e - the greatest allowable error. Then d = the distance of an object, upon which if the lens be accurately focussed, all objects beyondwillapparently be in focus.
Or,
point in focus.
Examples : - Thus with a lens of 8-in. focus anddiaphragm we haveAgain, if we use a lens of 4-in. focus andaperture we have
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