Early Work In Photography
A Text-book For Beginners

Facts about Lenses. 57 of our light (and rapidity of exposure) for extra sharpness of image. And as circumstances vary, so that at one time sharpness of image, and at another time shortness of ex- posure is the more important, the lens-makers provide a series of “ stops,” so that the photographer may use a large or a small one, as circumstances require. This form of lens is exceedingly useful for all ordinary landscape work, groups, and portraits, but it has certain de- fects which become very apparent when we use it for copying plans, or for photographing buildings and other objects, in which there are straight lines near the edge of the picture. Curvilinear Distortion is the chief difficulty in this direction, and though it is rather difficult to explain, we may be able to make it fairly clear by an exaggerated example. We Fig. 16. may say that the defect is introduced by the stop or small aperture that we provided to remedy the aberration just mentioned. This stop cuts off the rays proceeding from the corners of the house to the centre of the lens, so that the image of corners and outer lines is formed entirely by rays passing throught the outer portions of the lens, while the image of the central part (say, the top of the door) is formed almost entirely by rays passing through or near the centre of the lens. The outer part of lens deflects the rays most strongly, so that the width and height of the house become diminished in proportion to the width and height of the door which is near the centre; while the corners are de- flected more strongly than the centres of the walls, because tliey are more distant from the lens centre. Curvature of Field is another difficulty with lenses. In our diagrams of the candle-flame we have represented the image as coming to a focus on a plane surface, as a flat ground-glass screen. As a matter of fact this does not happen, but the image comes to a focus on a curved field. The line C C represents such a curved field, and it will be it once seen that if the plate (or ground-glass) is placed at A, the central part of the object will be sharp, while the top and bottom of the candle-flame will be out of focus. If the