Focal Length
which will be in focus (all measurements to be in inches, and the distance of object to be measured from the optical centre of lens)  (i) Multiply the focal length by the diameter of the stop, and the result by the difference between the focal length and the distance of the object. (2) Multiply the focal length by the diameter of the stop, and add _{T}^ part of the distance of the object. (3) Divide the first product by the last, add the focal length, and subtract the result from the distance of the object, when the result will be the distance sought for in front in inches. To find the depth of focus behind a given point  (1) Multiply the focal length by the diameter of the stop, and the result by the difference between the focal length and the distance of the object. (2) Multiply the focal length by the diameter of the stop, and subtract x^ part of the distance of the object. (3) Divide the first product by the last, add the focal length, and deduct the distance of the object; the result is the distance behind in inches.
Example:  Find the depth of focus when focussing an object 15 ft. distant with a lens of 7in. focus, working at 







33^{2} 
