Table showing distance of object upon which to focus with any stop in order to secure the greatest depth (hyper focal distances).

To Find the Hyper Focal Distance the formula is : - Focal length squared, - ratio of stop X circle of confusion. Now when making these tables it is generally assumed that a point represented as a circle of confusion not exceeding 1/100th inch in diameter will be considered satisfactory as regards definition in the negative. Then by the above formula suppose we have a lens of 5-inch focus with stop f/8 the hyper focal distance will be

If we, therefore, withsuch a lens and stop focus on 26 feet we shall have objects from half that distance (13 feet) to Infinity in good focus.

Mr. Welborne Piper, in his useful book on " The Lens," has pointed out the interesting fact that if we divide the hyper focal distance by 1,2, 3, 4, 5, etc., we shall get a set of distances such that, if we focus on any one of them, the next two distances on the scale are the distances of the furthest and nearest objects in focus. Say the hyper focal distance is 30 feet, then 30/1, 30/2, 30/3, 30/4, etc., will represent 30, 15, 10 and 7 1/2 feet, and if we focus, say, on 10 feet then the 15 and 7 1/2 ft. will be the near and far depths, or on 30 ft. then on Infinity and 15 ft.

To Calculate Near Depth when Focussing on near objects.

Formula. - Distance of object in focus X Hyper focal distance. Distance of object in focus + Hyper focal distance.

Example. Hyper focal distance 26 feet object at 8 feet,

For far depth the formula is

Distance of object in focus X hyper focal distance. Distance of object in focus - hyper focal distance.

If the sharp focus is secured at Infinity, then, with the stop indicated, all objects are in focus from the above distances up to Infinity. If, however, the pointer is set to the above distances then, with the stop indicated, all objects are in focus from half the distance focussed up to Infinity. These figures are based on a circle of confusion of 1/100th of an inch, and the distances must be reduced if greater sharpness is desired.