Home Movies and Home Talkies (Jun 1932-May 1933)

HOME MOVIES & HOME TALKIES 143 iris of the eye. The eye is a niininture ■camera and you will notice that in a bi-isht light the aperture of the iris is a mere pin point, whilst it becomes ■quite large when the light is dull. The iris diaphragm is actuated by a knurled ring, on which is scribed an indicating line. On the lens barrel are figures such as 3.5, 4, 5.6. 6.3, 8, and 11. When you turn the indicator to 8 you are actually adjusting the lens to a speed of //8. Since the focal length of the cine lens is 1 inch this means *hat the aperture is 1/8 inch. What•ever the make of lens or camera //8 will always have the same illumination value or speed. The same is true of all the other figiu'es marked on the lens barrel. You will see how convenient this is, for it enables exposure meters to be ~ iip£^l>Tuf!£: /"O/y^M. lu the drawing the positions of the focal points are very much exaggei'ated for the sake of clearness. You will see, though, that with a simple lens there must be considerable blurring at the edges of the image, since owing to the bending of the rays which pass far from the centre of the lens these parts of the picture are c^uite appreciably out of focus. As we "stop down" the lens by using smaller and smaller ajiertiu-es there is less bending of the raj's farthest from the centre ; the actual focus becomes more nearlj' a true point and the image is .sharper. This unequal bending by the different areas of a lens of the light rays which reach it is known as spherical aberration. In a lens of high quality its effects are practically Figure 1. An interesting experiment in definitions made which show the correct ajjerture for any light conditions. In the next article I shall have something to say about the most ■desirable speeds for home cine lenses «nd I am going to exjalain some of the difficulties that confront the optical expert when he desires to turn out a perfect lens. I will conclude this article by showing why it is that we found in the experiment with the iblack paper discs that the definition improved as the aperture was reduced in diameter. If you look at Fig. 2 j'ou will see that if a ray of light jsasses through the centre of a lens it is not bent at all. The exact focal point corresponding Figure 2. eliminated by correction. But this is a somewhat expensive process if the usable aperture is to be large. You will now see why a very rapid lens such as an //1. 8 is so much more costly than a slower //5 or//'4 lens. [To be Continued) MAKE YOUR OWN ANIMATED TITLE! (Continued from page 131) it for a few exposures and then uncover the title of the film. The complete shot can be held as long as you desire. Although the foregoing instructions suggest a lot of trouble for so short a How the iigint is bent to the ray marked A is A*. But when rays such as those marke:l D pass near the edge of an ordinary convex lens they are very considerably bent and the focal point (D*) lies much nearer the lens than does that of rays passing through the centre. Rays like those marked C are less bent and the focal length C* is rather rearer to A*. In the case of rays like those marked B which pass quite near the centre, the bending is comparatively small and their exact focal point B* hes very •close to A* c" 3"^' subject, the idea was to provide an exiDeriment which amateur operators might use as a basis for developing the idea of making moving titles with the use of cut-outs. At any rate, we think that it will stimulate a desire to acliieve something a Uttle out of the ordinary. NEXT MONTH : How to Photograph Animated Cartoons. By ERN. SHAW. For Your Home Talkies RECTATONE With Rectatone any system of sound production can be balanced so as to give faithful reproduction of bass and treble. When Bass and Treble are correctly present, RECTATONE preserves them. When the higher notes are missing, RECTATONE restores them. RECTATONE is the ideal LF.couplingfor every L.F. amplifier, since the degree of compensation is variable and may be exactly suited to the particular tuned circuits in use or employed to correct deficiencies due to the loudspeaker or to the acoustics of the room. Moreover, Rectatone's high step-up ratioof 1 :7 makes it possible to obtain the required tone correction without using an extra stage of L.F. amplification. Compensation is controlled by a variable resistance connected externally between two of the transformer terminals. If this resistance be omitted or put out of circuit RECTATONE becomes a nornnal straight-line transformer giving high and even amplification. Price 15/ List No. D.P.33 FOR FREE BOOKLET post the coupon below To Messrs. Varley, Kingsway House, 103, Kingsway, London, W.C.2. Please send me, free and post free, the Book of the RECTATONE Date.. Name , Address. .W.r.l