Motion Picture Herald (Apr-Jun 1931)

May 9, 1931 Motion Picture Herald 65 SPEED OF LENSES I HAVE a letter from T. L. Donovan, projectionist of Salisbury, Mo., who asks me to explain just what is meant by 'speed of a lens.' He says he has seen the term used in print many times, but has never either heard or read an intelligible explanation of its meaning. The thing is very simple, Friend Donovan. The "speed" of a lens really means the speed in which it will make what may be called a photographic impression of acceptable density upon a photographic plate or film carrying a certain emulsion. Applied to projection, it means the power of a lens to illuminate a screen of given area and type with a light source of given power. In other words, Donovan, it means the amount of light a lens will admit and transmit. It's possible you know, for a photograph to be made by using a pin hole, but since it would transmit but little light, naturally it would require a lot of time for "exposure." Similarly you might, theoretically at least, illuminate a theatre screen by means of a pin hole, but the illumination would be totally invisible, since only an infinitesimal amount of light would pass through the tiny opening. For the minute pinhole we therefore substitute a lens, each point upon the surface of which is, in ef¥ect, a pin hole, since each point will do what a pin hole will do ; namely, admit, transmit and focus light rays. We thus may pass light up to a certain limit in proportion to tahe lens diameter. It will therefore have a "speed" in illumination proportional to its diameter. While there are some other things mixed in with it, broadly the speed of a lens, in projection at least, means its free diameter. In photography the focal length of the lens has a lot to do with it, but that does not, I think, apply so directly in projection. PUZZLING CASE H. w. MCFARLIN, projectionist of the Capitol theatre, St. Petersburg, Fla., comes across with a very well put case which seems to be not altogether uncommon. He says, "Dear Friend Richardson: For a long time, with the help of your articles in the Better Projection Department and, of course, the Bluebook, I have been able to correct or understand all of my problems. I have one now though, that I can find no explanation for, either in the Bluebook or from my co-workers. "An exchange has been supplying the Capitol with prints which are buckled. This much we all agree on. These prints when projected on our own equipment give us a very pronounced in-and-out of focus effect — so bad that some of our patrons walk out on us. Thin film stock, along with the processing liquid used, is my £uess for the reason the prints were buckled. "The exchange refuses to admit that its prints are buckled, and because of my argument with it, I decided to screen one of the prints at the Florida theatre, expecting End Your Back Stage Worries by Installing the Nationwide Endorsed Stage Equipment AUTOMATIC SCREEN ADJUSTER STAGE RIGGING AND HARDWARE COUNTERWEIGHT SYSTEMS ASBESTOS CURTAINS STEEL CURTAINS SOUNDPROOF CURTAINS HAND AND ELECTRIC CURTAIN HOIST DRAW CURTAIN TRACKS ELECTRIC DRAW CURTAIN CONTROL ORCHESTRA PIT LIFTS ORGAN CONSOLE LIFTS STAGE LIFTS MOVEABLE STAGE BAND CARS TALKIE HORN TOWERS CONTOUR CURTAINS MECHANICAL STAGE EFFECTS REVOLVING STAGES ° Information on any of these products will be gladly furnished on request "Stage Equipment with a Reputation" PETER CLARK, INC. Stage Equipment for Over 25 Years 544 West 30th St. New York City OO LO ^ COVES COLOR HOODS A 5,000 seat Theatre deliberately threw out 4,000 painted lamps and replaced them with Reco Color Hoods. That is convincing proof of Reco superiority and economy. 2651 W. Consrress St. Chicago, III, ESTEROPTICON A lot more effect for a lot less money. Best in name r-nd best in perforfmance. Complete with cord, switch, slide holder and lens, $26.00. Any focus lens from 12 to 2\ inches furnished. 500 -watt lamp. $3.00 extra. AT YOUR DEALER. Brry devices LO I COMPANY 200 FILM BLDG. CLEVELAND, OHIO