Early Work In Photography
A Text-book For Beginners

68 Early Work in Photography. (carrying the lens) is extended as far as it will go, without using the second extension which is actuated by the milled focussing screw. It is just possible that the lens is too far from the screen to give an image of correct focus, so we must release the clamping screws and move the lens-board nearer to the ground glass, at the same time keeping the head under the foe ussing cloth and closely observing the appearance of the image. If this does not become quite clear and distinct, we must return the lens to its original position and clamp it. We must next resort to the milled screw and turn it towards the lens; this will bring the “double extension” into action, and move the lens still farther from the focussing screen. As the image is gradually seen to become sharper and better defined, it must be watched closely until it finally begins to go out of focus again. At this stage the focussing screw must be turned in the opposite direction until some object in the centre of the screen is at its sharpest possible focus. It will be well for the student to move the lens back and fortli till this one par- ticular object is absolutely sharply defined. Upon now turning his attention to the marginal objects he will notice that they are all more or less blurred, or out of focus. The student must now try and divide his attention between the central object and an object in one of the margins of the screen while he very gradually turns the focussing screw first one way and then another: He will now observe that, by the time the marginal point is sharply focussed, the central object is slightly blurred. He must now focus (as nearly as possible) an average between tliese points and then try the effect of using a “stop.” Insert the stop, marked (say) //n, in the place provided for it in the lens; or, if the stops are not so marked, insert the one having the largest opening but one. Then (without disturbing it in any way) again care- fully examine the focussing screen. Owing to the action of tlie stop (which, of course, intercepts a certain amount of light) the image will appear much less brilliant than before, but the general focus will be more clearly defined. The marginal and central objects will appear almost equally sharp, but the student had better try the effect of again using the focussing screw. After racking the lens back and fortli several times (letting it slightly pass the point of extreme definition each time) lie may try the effect of using a still smaller stop. By tlie time he has tried several experiments with each stop, he will note that the smaller stops reduce the lumin- osity of the image, while they increase its definition. When