The Optical Magic Lantern Journal (March 1893)

>f our Hand Camera Pamphlet. jand Camera, write for the Pamphlet. W. I. CHADWICK, 2, St. Mary’s Street, MANCHESTER. 3 (which involves a longer exposure), the easier it would be to accomplish what has been stated. Opticians have done a good deal, but they cannot do impossibilities. Here are a few figures which may be taken as the best that can be done. A 38-inch focus lens working at F 10 can be made to give good definition of objects at 8 or 9 feet and beyond—but a 3-in. focus lens wozking at I* 10 would not cover a 3-plate sufficiently well. Nowa 4-inch lens working at F 10, would require at least 14 feet to give the same definition, and even this is considered too short in focus for a i-plate; so that we must now consider a 5-inch focus lens. The working at F 10 would not define objects at less than 20 feet and beyond, though if we stop it down in the diaphragms to F 15 we may get the same definition at 14 or 15 feet; and to obtain the same sharpness at 9 feet and beyond we must stop down the lens to F 25, and this would be to slow for Hand Camera Work. Thus it will be seen that a 5-inch focus lens—which is about as short as would be useful for a 4-plate, embracing an angle of 44°—if used as a ‘fixed focus” for objects at 20 feet and beyond, no larger stop than F 10 could be used, and if we wished to take nearer objects, say a country market group, or children at play, or hundreds of other subjects at say 14 or 15 feet, the picture would not be sharp, and, as already stated, to be able to take objects so near as 14 or 15 feet with a fixed| focus lens, it must be adjusted for that distance and stopped down to F 15, or else objects beyond 15 feet will be out of focus, and this reducing the aperture for near objects is the very opposite to that which is required ; for it is well known the nearer we approach an object the longer must be the exposure, or the larger must be the stop for the same exposure that would be necessary for more distant objects; then, of course, for near groups, figure subjects, &c., we require a large stop, and are generally willing to sacrifice sharpness in the distant objects. Therefore it now becomes manifest that by employing an adjustable focussing instrument we could leave the stop at F 10, or even increase it to F 8, and by a slight adjustment of the focus, obtain results which with a fixed-focus instrument would be quite impossible. The means of focussing by rack and pinion and the method of using it ig novel. The focussing button attached to the pinion (shown in Fig. 3, under the strap handle) is about the size of an ordinary overcoat button, and is provided with a heart shape aperture forming a little pointer ; through this aperture may be seen a white ivorine tablet or disc, on which is marked an index for the various distances to suit the particular lens in use; the Camera being available for any suitable lens of from 3 to 54 in. back focus. The user should make his own index, which he can do in three minutes, by means of the focussing screen supplied, afterwards to be described ; for assuming that I do not supply the lens, it is quite impossible to provide an indes—not knowing the exact focus. On the other hand, assuming that the lens is supplied with the camera, and the index marked in feet or yards, I venture to say that very few Photographers will use it correctly ; not one man in half-a-dozen can estimate distances Telegraph Address— STEREO, MANCHESTER.”