Amateur Photographer & Cinematographer (1935)

tw AMATEUR PHOTOGRAPHER o 6 CINEMATOGRAPHER a June 12th, 1935 J etters to the Editor The Editor is not responsible for the opinions of his correspondents. DEPTH OF FOCUS. Sir, — May I suggest a useful extension to Mr. Colbourne’s recent article in " The A.P.” under the above heading ? He takes 1 /500th in. as the permissible circle of confusion, setting up this rather high standard of definition in view of the need for extreme sharpness in miniature negatives. Since small negatives are enlarged more greatly than larger ones, his adoption of a common circle of confusion, irrespective of negative size, amounts to stating that a half-plate enlarge¬ ment from a small negative ought to be less sharp than a half-plate print made by contact — -which is not quite what we want. It is quite easy to make a correction for negative size, simply by using a formula which sets a higher standard of definition for small negatives than large. By so doing a half¬ plate enlargement from a miniature negative and a half-plate print from a half-plate negative will be absolutely identical, provided that the stops chosen in making the two negatives are such as to make the hyperfocal distances of the lenses the same when calculated by the “ corrected ” formula. The formula in question is : Hyperfocal Distance _ (Focal length of lens) X K (in feet) Stop value x diagonal of film. The focal length of the lens and the diagonal of the negative must both be measured in inches. K is a constant by choice of which the standard of definition is set to suit the taste of the user. For a circle of confusion of i/iooth in. on an enlargement 15x12 in., K = i6o. For the same circle of confusion when enlarging only to 10x8 in., K = 100, or for 1 /100th in. on a half-plate print, K = 70. (Round figures.) If K is made 140 (double 70) the circle of confusion on a half-plate print will drop to 1 /200th in. Messrs. Leitz, in their depth-of-focus tables for the Leica, take K as 100, which suggests that this figure is suitable for all-round work of a high standard of sharpness. Mr. Colbourne’s formula, with its choice of 1 /500th in. as universal standard of definition, sets an extremely high stan¬ dard (K=335) for half-plate size and rather a low one (K = 7o) for Leica size. By using the “ corrected ” formula for hyper¬ focal distance, the standard of definition ceases to depend on the size of the original negative — which, I submit, is as it should be. The construction of a depth-of-focus table from the hyper¬ focal distance calculated by the formula I have suggested is done exactly as explained by Mr. Colbourne ; the substitution of my formula for his alters nothing in the rest of his article. But I should like to draw attention to 9 slight ambiguity on one point ; he does not make it quite clear that when focus¬ sed to true infinity the depth of focus extends only to the hyperfocal distance. To get sharp focus from half the hyper¬ focal distance to infinity it is necessary to focus accurately on the hyperfocal distance itself. — Yours, etc., A. L. M. SOWERBY. WHY NOT AN “ A.P.” CLUB ? Sir,— I have read with considerable interest Mr. Willatt’s account of his “ brainstorm ” in " The A.P.” of May 29th, which has aroused my enthusiasm. Mr. Willatt’s idea (with or without beard) is excellent and deserves the attention of all serious amateurs, whether ex¬ perienced or not. Perhaps my opinion in response to Mr. Willatt’s request may help in this respect — anyway, here goes : — In addition to the number of useful objects suggested I think that the idea would be for the benefit of the photo¬ graphic world in general. By such an easy means of intro¬ duction, formalities would be overcome within a few minutes of meeting, and experienced workers and beginners alike should welcome the opportunity to " get together ” which such a club would afford. Again, members of " The A.P.” ilk would have an easy means, when among a number of fellow-citizens equipped with cameras, of 'distinguishing between readers and non-readers of “ The A.P.,” and by making acquaintance with non-readers would thus be able to bring our admirable journal to the attention of many and so enlist new readers to our ranks. This suggestion is perhaps a little beyond Mr. Willatt’s actual intentions, but I think it illustrates another possibility of the idea. In conclusion may I congratulate Mr. Willatt on his humorous as well as practical letter and, should my remarks evoke sufficient interest to appear in print, endorse his remarks in full. Here is the opportunity for readers of ” The A.P.” to show their appreciation of its good work and practical hints it contains. — Yours, etc., B. SUGGITT. Sir, — I read with pleasure, “ Why not an ‘A.P.’ Club ? ”, the letter by F. E. Willatt. I think it a great idea and agree with all the lines laid out, especially the badge. I am a member of the C.T.C., and the friendly “I. I.” as members meet on the road stimulates one, making you feel like one family, bringing about closer friendship. Yes, good luck to the suggestion. — Yours, etc., THOS. HOWE. HINTS AND TIPS— MOUNTING. Sir, — It is sometimes difficult for some people to place a print squarely on the mount. Even Salon exhibitors experience this difficulty, to judge from some of the exhibits. Some people rely upon a good eye, others upon elaborate measure¬ ment to ensure accuracy. I venture to suggest the following : It is assumed that one is using standard mounts, say, 15x12 in., on which a wholeplate print is to be mounted. A trimmed print is usually 6 in. wide. There will therefore be side margins on the mount of 3 in. The idea is, therefore, to cut a piece of card 15x3 in. (say, from a spoilt mount), care being taken to ensure that the width is absolutely uniform. If this strip is placed with its outside edge exactly flush with one side-edge of the mount, all that is necessary is to place the print confidently with its adhesive up against the strip and fix it there with pressure. A pencil line drawn across the strip, about 2f in. from the top, will show how far from the top of the mount the print should be. This proportion looks about right and leaves 3! in. at the bottom. Other sizes, e.g., half-plate on 12 x 10 in., can be worked out in the same way. Slight variations in the width of a trimmed print can be allowed for. If it is a little narrower, experiment will show how far from the edge the strip should be placed, very little judgment being required for this. The idea will obviate those cock-eyed specimens one occasionally sees. — Yours, etc., MAURICE WYRILL. A BEGINNERS’ POSTAL PORTFOLIO. Sir,— In the Novices’ Number of “ The A.P.,” D. G. H., of Manchester, wrote in the “ Readers’ Questions Answered ” section, asking for postal clubs that gave special attention to the beginner. May I point out that the P.M.P.P. has a special circle for beginners, known as Circle Five, and in this circle they are given every help and instruction possible to give to a beginner through the medium of the post. Mr. George A. Slight also gives a criticism on all the entries, and generally " mothers,” or should I say " fathers,” the beginner till he has advanced sufficiently to take his place in one of the more advanced circles of the club. In addition, each member has to criticise prints, and in this manner is encouraged to use his powers of observation and very soon comes to an appreciation of the many requirements of the picture-maker. Although only an infant club, the P.M.P.P. has already found the demand for membership so insistent that five circles of thirty members each have been set up, one reserved for Leica users, and one, Circle No. 5, for beginners. Mr. George A. Slight, of 18, Ewelme Road, Forest Hill, S.E.23, is the General Secretary. — Yours, etc., R. C. LEIGHTON HERDSON. 26 557