sharpness, but this aperture is calculated from the equivalent focus. Therefore, as the diameter of the aperture of the dia-phragm remains constant, it is obvious that it cannot now be
The Virtual Focal Aperture System. Recent optical research has shown that the above-mentioned modes of estimation and calculation, although sufficient for most practical purposes, are subject to many slight sources of error. The chief of these errors is referred to above, and is due to the concentration of the light upon the diaphragm aperture by the front lens of a doublet, whereby the virtual or effective aperture of the diaphragm becomes greater than the measured or absolute aperture. To make this clear we may quote a few lines from an editorial which recently appeared in The Amateur Photographer : -
" Our readers must excuse us if we go back to the very ground-work of illumination, as measured by a focal fraction, in order to well introduce one important question - distinction between effective aperture and absolute aperture, a matter of very great importance in connection with modern lenses of the stigmatic or anastigmatic type.
"Let us imagine a fixed focus camera with a circular aperture or window instead of a lens, and for our present purpose we will call the distance from the window to the screen the "focal length ": let this distance be 10 inches, and the window or circular aperture be I inch in diameter. The window will then correspond to a diaphragm of. If now we support a con-
densing lens, that is to say, a convex or magnifying lens, at a suitable distance in front of the circular aperture or window, we may so concentrate the light on the window or diaphragm that the illumination of the screen will be increased, as more light will pass through our i-inch window. If, on the other hand, a distributing lens, that is to say a concave lens, is suitably adjusted in front of our i-inch window, the amount of light passing through the window will be diminished. Thus the window without a lens in front will have an effective aperture identical with the actual aperture, while in the second case (convex lens