International projectionist (Jan-Dec 1954)

THE LENS: Key to Projection Quality Problems that result from the use of fast modern lenses and lenses of short-focal length are described in this third and concluding article of a series. Considered also is a step-by-step procedure for the care and cleaning of projection lenses. IF WE double the width of a screen-image by halving the focal length of the lens (using a 21/2inch lens in place of a 5-inch lens, for example), the area of the projected picture is increased 4 times. This means that 4 times more light is needed to get the same picture-brightness with the 21/2-inch lens as was obtained with the 5-inch lens, the speeds of both lenses being the same. This consideration is often overlooked: the exhibitor who buys new short-focus lenses fails to buy new lamps of suitable power to go with them. And the purchase of new lamps usually also entails the purchase of new generators or rectifiers. Brightness Comes First This reminder is not meant to discourage the use of shorter focallength lenses, but adequate screen illumination should never be sacrificed for mere picture-bigness. And as slow, rather than fast, lenses in the extremely short-focal lengths are recommended for better image-definition, the need of suitable lamps to accompany the new lenses is all the more urgent. Specifically, slow and medium lensspeeds (F:3 to F:2.5) are best in the lV2-inch to 3%-inch focal lengths, fast speeds (F:2 or F:1.9) in the 4inch and greater focal lengths. Theatres contemplating a change from 5inch to 4-inch lenses (to increase picture-width from 14.9 feet to 18.5 feet at a 90-foot throw, for example), should obtain coated 4-inch lenses of F:1.9 speed. If the old 5-inch lenses were slower than this, and not coated, new lamps may not be necessary, as the new picture-area is only IV2 times the old area. It depends wholly on whether the lamps gave adequate light with the old lenses. Coated Lenses Coated lenses transmit much more light than uncoated ones — 15 to 25 per cent more light. This substantial By ROBERT A. MITCHELL gain is augmented by the increased speed which can be obtained in modern lenses. All too often, however, the advantages of coated projectionport glasses are overlooked. When both surfaces of these glasses are coated, a gain of about 10 per cent in screen-illumination is obtained. In addition to this positive boost of picturebrightness, coated port glasses assist in the production of more sparkling and lifelike image-contrasts. The "F-number" speed of a lens is found by dividing its equivalent focal length (E.F.) by the diameter of the "pupil," or clear opening through which light may. pass. As an example, a certain lens has an E.F. of 4 inches and an opening of 2.1 inches in diameter. Its speed is F:4/2.1, which equals F:1.9. Arc Mirror Speed Theoretically, a lens will intercept all of the light passing through the projector-aperture when its speed matches that of the arc-lamp mirror or condensing lens. We say "theoretically" because the "theory" usually fails to include a mighty important factor, namely, the size of the aperture. The true optical speeds of arc mirrors and condensers are extremely rapid (in the neighborhood of F:0.4), and are not used in lens-matching calculations. Instead, the mirror is regarded as a lens which images upon the aperture a luminous positive crater placed infinitely far behind the mirror! The "working distance" (distance from center of mirror to aperture) then becomes virtual focal length. On this assumption, the virtual speed of a 12-inch mirror removed 30 inches from the aperture is F: 30/12, or F:2.5. We are allowed to make this assumption because the distance from positive crater to mirror-center ("geometric focus") does not affect the angles of the light-rays when the system is in focus (that is, when the crater is accurately imaged upon the aperture as a brilliant "spot"). All this is simple enough; and such a mirror (virtual speed of F:2.5) will be perfectly matched by an F:2.5 projection lens if the aperture be a FIGURE 2. The mirrors and lenses illustrated here have the same "speed" — F:2. The lens in (A) matches the mirror perfectly, but not the lens in (B). The difference is due to the different sizes of the apertures — the larger the film aperture, the faster the lens must be to capture all of the light and thus match the mirror. In actual practice, therefore, an F:2 lens is too slow to match an F:2 mirror. INTERNATIONAL PROJECTIONIST APRIL 1954