The Moving picture world (July 1924-August 1924)

August 23, L924 MOVING PICTURE WORLD 659 Projection (Continued from previous page) not jagged or cracked. If it is the latter you will get either a blurred image or several fuzzy images. PUNCHING A HOLE WON'T DOi It will be too big, not round, jagged or cracked. The hole MUST be round and small and have perfect edges. School Helps A. L. Fell, Collingswoqd Theatre, Collingswood, N J., has the following to say with regard to the Bluebook school. Dear Friend Richardson: With regard to continuing the Bluebook School, I sincerely hope you will keep it up. There are many times when a fellow does not get just the right meaning from something he reads, and is therefore misled. But when questions are asked, as in the school, and the right answer finally appears, one sees where he was wrong, if he was, and is thus put back on the right path. Moreover, it certainly is a most excellent way to get the boys interested in study. Personally I want to thank you for the good the Bluebook School has already done me. (Continued on page 662) Bluebook School— Answers to Questions 74 to 78 Note: When I published the answer to ■question No. 67 I expressed some doubt as to the injury to lens correction being the whole reason for injury to the screen image. As I told you, I wrote Dr. Kellner, of the Bausch & Lomb Optical Company, who was, I think, tickled pink, darn 'im, at the chance to "set down" on me. He says : Dear Richardson: If I had to answer your question No. 67 I would say that the result on the screen would be a poor image. The image would be poor because there would not be good definition. The definition would not be good because the correction of the lens would be spoiled. This really is all one can say about the subject. This is one occasion, at least, where I can get rid of my optical feelings in a simple manner. Well, that settles it. Friend Kellner and I have many friendly battles, but in a matter of this kind when he says that's that, why so far as I am concerned, that's that ! Question No. 74 — Can you alter the E. F. of a projection lens by altering the length of the lens barrel, and thus the distance between the front and' back combinations? W. E. Lewis, Endicott; H. C. Spence, Charlottetown, P E. I.; Win Bennett, Newton, Iowa ; Karl H. Sommermeyer, Marietta, Minn.*; Harry Dobson, Toronto, Ontario*; G. W. Bennewitz, Sioux Falls, S. Dak .* and Charles Oldham, Norwich, Connecticut, answered correctly. I note that this is the first (so far as I remember) our friend Constantino has missed answering at all since the school began. He apparently, doubtless by some error, seems to have skipped question 74. Brother Sommermeyer's answer seems best suited for publication. Here it is : A highly corrected lens, such as is used for projection, is a. very delicately constructed and adjusted instrument, and to alter any of the adjustments made by the manufacturer (of which the distance between front and back factor is one) will result in disturbing some of the corrections, as well as changing the focal length of the lens. It therefore would be a foolish procedure. To this Brother Dobson adds this pertinent comment : "But in an emergency it would be possible to thus alter a lens and get what might be termed 'fair' results," to which I would myself add that while this is correct, still the emergency should be a very genuine one, because the lens could only be brought back to its original excellence and efficiency by restoring the original distance between the combinations. Question No 75 — Can the same projection lens be used to project different sized pictures ? Oldham*, Burnett*, Daniel Constantino, Easton, Pa.; Sommermeyer, Dobson*, Lewis, Spence* and Bennewitz made correct replies. The reply of brother Spence has been selected for publication He says : Any projection lens may be used to project different size pictures, but the size of the picture will vary as the projection distance is changed. The longer the projection distanc the larger the picture, and vice versa. In altering the projection distance you also alter the working distance of the lens. The shorter the distance from lens to screen the longer the distance from lens to film. . One otherwise excellent reply was rejected because of the fact that the good brother did not let well enough alone, but went on to say that as the picture size was reduced the screen illumination would be more intense and vice versa. Now this might or might not be true. It would be, or might be affected by several things. Probably it would be literally true, but in the answer the man gave no evidence that he gave consideration to or knew of but one thing. Suppose you have a piano convex combination condenser and a lens which just picks up the beam at 3 inch working distance. You now bring the screen closer until the picture, which was 18 feet wide, is only ten feet wide. How much of the light would that lens pick up? Maybe I did wrong to reject the answer. What do you think? Question No. 76 — How would you calculate size of picture a lens would project at a given distance, if you knew size it would project at another distance? Lewis*, Burnett*, Spence*, Sommermeyer*, and Bennewitz*, were all who came through. It is a bit surprising how some men got balled up on this rather simple problemDobson, for instance, I know to be about as good as they make them when it comes to putting high grade results on the screen, and putting them there efficiently, yet his answer to this question — well, Dobson, I think when you wrote that you must have just got up and had only one eye partly open, or else you'd just returned from a joy ride into Quebec ! Constantino, too, fell in over his head, kersplash ! But here is the classic of them all. It comes from California: To find the size picture a lens will project at a different distance when you know what size it will project at one distance, proceed as follows: Project the picture at the known distance, say ninety feet. Then subtract the other distance from the known distance. Suppose the new distance is 70 feet. Then you subtract 70 from 90, which leaves 20. Then you measure twenty feet from the screen and hold your tape line up in the light and you can see what size the new picture will be. There, by gum ! You know all about it now. Only if the new distance were a longer one than the old — oh well, why worry over trifles! Anyhow he had sufficient ambition to try, which is a bit more than any other California man has had up to this date. The reply of Bennewitz and Spence have equal value as to correctness and brevity, but I think that of Spence is a bit the best, in that the average man will perhaps understand it a bit better. Spence says: To find size of picture at a given distance, when size at another distance is known, as a 16 ft. picture at 70 ft.: 16 ft x 12 = 192 ins., therefore our picture is 192 inches wide and the light beam spreads as much per foot as the width of the picture in inches divided by the projection distance in feet amounts to, or In this case, 192 70 = 2.742857. Now if the beam spreads 2.742857 per foot, it is only necessary to multiply that number by the number of feet in any desired projection distance to ascertain the width of the picture at that distance, as at say 35 ft. the picture would be 2.742857 X 35 = 95.999995 inches, or practically eight feet. Question No. 77 — How would you measure the focal length of a lens? Dobson*, Lewis*, Oldham*, Burnett*, Sommermeyer, Spence* and Bennewitz*, gave excellent answers but the question was meant to apply to simple lenses only. I shall publish two of the replies for reasons you will readily understand. Bennewitz says, applying his answer to both questions 77 and 78: To measure the focal length of a simple lens, or the E. P. of a compound lens, proceed as follows: Make a stereo slide from tin. In its center cut a horizontal slit EXACTLY half an inch long. Next measure the exact distance from center of screen to inside wall of projection room, along the optical axis of the stereo lens. Using the lens to be measured, project the aforesaid slit to the screen and measure its length exactly, In inches. Next divide that measurement by .5, which is precisely the same thing as multiplying it by two. Next add the exact distance from center of lens you are measuring to inside of projection room wall to the known distance from inside of front wall to screen. Now reduce the total lens-to-screen measurement to inches and divide it by the result obtained when you divided the length of the projected slit by .5 (multiplied it by 2). The final result will be the focal length of the simple lens, or the E. F. of a projection lens. Spence replied thus : In measuring the focal length of a lens it is best to have a frame, or optical bench with which to hold the lens in true position. It is best to have it so the lens may be moved back or ahead on a measurement scale. Attach a sheet of paper to the end of the frame, or to a wall if no frame is used, opposite a window or other opening in a dark or semi-dark room, outside of which is a tree or" other object not less than 100 feet away, with the flat side of a piano convex lens (if that is what is being measured) next the paper screen, bring the object to as sharp a focus as possible. Take the measurement flat side of lens to screen and then reverse the lens — flat side away from screen — and again focus the object and measure from flat side of lens to screen. Add the two measurements together and divide the result by 2. This will be the focal length of the lens. The method is not accurate, but is close enough for the measurement of condenser lenses. The bi-convex is measured with a single focusing of the object, measuring from central plane of lens to screen. A meniscus lens cannot be measured the same as a piano convex, as its optical center is outside the lens. The meniscus may be best measured by the method described on page 153 of the Bluebook (same as Dobson's answer.— Ed.). Question No 78 — This question is pretty thoroughly covered by the replies to question 77. However, I will quote the reply of Brother Spence, which is: If convenient use an optical bench. If you have none, then remove the projector mechanism (Correct nomenclature! — Ed.), and in its place mount a sheet of metal having an aperture .75 of an inch square. The center of this aperture must be on the optical axis of the condenser. Next, on the screen side of this aperture, support the lens to be measured at about twice its supposed equivalent focus, and in such way that it may be moved ahead or back along the optical axis. Next, with the light projected, support a screen of dark colored cardboard in front of the lens and move the lens and cardboard until the image of the aperture is exactly the same dimensions as the aperture itself, whereupon measure the distance from cardboard to aperture and divide by 4. The final result will be the exact E. F. of the lens under consideration.