Early Work In Photography
A Text-book For Beginners

Facts about Lenses. 59 single lens is - which it forms a two rectilinear or the symmetrical, are the two main types of which all photographic lenses may be said to be modifica- tions. As the rectilinear lens consists of two single lenses, mounted at opposite ends of a tube, it is possible to screw out one of the singles and use the other. In this case the is about double the focal length of the doublet of ch it forms a part, so that the rectilinear can be used for distinctly different purposes. If the front combination is used it will have a slight tendency to ‘pin-cushon dis- tortion,’ while the back will give slight distortion of the « barrel " type. ,. : There is little more to say of the general construction of he lens, until we come to a more advanced hand-book than .his, but there are one or two points in working that must Cristis the question of focus and FOCAL LENGTH. WESEX with the pin-hole, that the image was, to all intents and purposes, equally sharp, whether the ground-glass was three Inches or six inches distant. In this case the rays of light from the object passed through a hole so small as to be practically a point. With the lens it is different, as we have been; for, in order to get the image sharp, we must place the ground-glass at the point where the rays from the centre and from the sides Of the lens meet, or focus (see fig. 17). The distance between the centre of the plate, and th centre of the lens when in this position, is called the focus, or the focal length of the lens. Though this is not a scientifically correct statement, it is near enough for practical purposes, and is what is commonly used. It is not quite so easy to measure the focal length of a doublet as of a single lens, though for practical purposes it may be obtained focussing on some distant object, and measuring the dis- from the diaphragm aperture to the ground-glass, opticians do it in a different way, and speak of the EQUIVA- I ENT FOCUS. This means the focal length that is equal to that of a single lens giving an image of the same size as is given by the doublet. We have seen that, with a pin-hole and screen greatly a the lens If we have (nages RO-KI) In the same way with the lens. If we nave a lens of (say) one inch diameter, throwing an image on a a lens eight inches distant, the linage will be just four times as brilliant as it would be if thrown on a screen sixteen inches distant. And as the brilliancy of the image directly affects the length of exposure necessary to the plate, the relative size of the lens-opening to its focal length pin-hole, the’ distance between ftects the brilliancy of the image If we have