# The Dictionary Of Photography

### A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

 Focal Plane Shutters                                           Focimeter Note. - In case the above may not be clear^to some photographers, the following rules may be better understood : - To find the principal focus of a lens (p), focus a near object in the camera, and measure the distance between it and the ground-glass (D) ; next find the proportion which any dimension in the object bears to the same dimension on the ground-glass (r). Thus, if the original dimension be four times as large as its reproduction, we say that r equals ( = ) 4. Multiply D by r, and divide the product by the square of a number greater by one thanThis rule is due to Mr. Debenham. To find the lesser conjugate focus (/) (if p and r are known) multiply p by the sum of r + I and divide the product by r. Or divide D by r + I. To find the greater conjugate focus (F) multiply p by r + I. Or multiply / by r. To find D (the distance which the ground-glass should be from the object to be copied in order to get a given value for r) multiply p by r,« the sum of To find r divide F - p (the difference between F and p) by p. Or divide p by f - p. Or divide F by/. To find x divide the square of/ by 16 times the square of a (the diameter of aperture to lens). For example : focus an object which is five inches high, so that it is one inch high on the ground-glass; thus we know that r= 5. Next measure the distance between the object and the ground-glass (D), which is found to be 45 inches. Under the heading Spectacle Lens will be found a table showing the correction to be made on account of chromatic aberration. Focal Plane Shutters. See Shutter, Instantaneous. Focimeter. An instrument of M. Claudet's for determining the difference between the visual focus of a lens and the chemical focus. A convenient form is a series of small cards, each bearing a number, and placed step-fashion one behind the other, so that each one is at a different distance from the camera. The middle card is accurately focussed and a photograph taken with the full . 334 •