The Dictionary of Photography for Amateur and Professional Photographers baixar o livro de graça

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1902 edition. Excerpt: ...there never has been a fixed-focus lens yet made and never will be. But what we mean to state is this--a fixed focus is a theoretical impossibility, but with every lens there is a point beyond which objects are in sufficient sharpness to satisfy our requirements as to definition, because when the image of a point is less than the 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 beyond d/2 will apparently be in focus. Or, fg V f----+ / = d, then d-j-2 = point in focus. Examples: --Thus with a lens of8-in. focus and //8 diaphragm we have 1x8--j--+ 8 = d = 2,008 in. = 167 fi. 4 in. Then d-7-2, or 167 ft 4-in. 4-2 = 83 ft. 8 in. Again, if we use a lens of 4-in. focus and f/16 aperture we have i x 4 + 4 = d = 254 in. = 21 ft. 2 in. Mr. Cheyney gives the following table, tested by various rectilinear, wide-angle, and single landscape lenses: --Fractions of an inch are unnecessary. On reading through the....