25 MARCH 1938, Page 62

Photography

WHY PICTURES ARE NOT SHARP By W. R. AYLING IN most cameras of the folding type and in some of the others provision is made for altering the focus. This

in effect means moving the lens panel nearer to or farther

away from the sensitive surface, of the film.- ur plate. By making such a movement the user secures a sharp image on the film of objects which may be only a few feet distant or many miles away. In the case of cameras having a fixed focus, with no facility for altering the distance from lens to film, the setting is one which will permit of every object being sharp, if over a certain distance away. Folding and other cameras having an adjustable focus usually bear a printed scale which shows distances in feet, yards or metres and a sign like a figure eight turned on its side. This latter stands for " infinity," and all objects over a certain distance are included in the ability of that particular lens to render them sharply or in focus. On one of my own cameras there is a scale fixed to the baseplate, on which are the figures in feet 5, 7, 1o, 15, 3o and then the "infinity" sign. In this case all distances over about 4o feet can be called " infinity." Optically there is only one distance at which objects will be dead sharp on a perfect surface, but photographically. there are other factors to be taken into consideration. The lens will only record as sharp, points of light down to a certain size,: termed the- circle of confusion;.smaller than which would be indis- tinguishable in detail to the unaided eye. The emulsion surface of the film is also unable to_ render detail smaller than a certain size because it is composed of grains of silver salts and other chemicals -held by ,a coating of gelatine. When developed these grains have a slight scattering effect and cause diffusion. The permissible amount of diffusion depends , upon the distance at which the finished picture is to be viewed. It would be most unfair, for instance, to look at an enlargement 24 inches by 36 inches from a distance of two feet. Many defects including diffusion would be apparent.

Here then are the two main causes of unsharp or " fuzzy pictures." The first camera-shake, and the second wrong focussing.

. The latter often comes about through the user taking one picture at say ten feet correctly focussed and the next at -3o feet, where he or she has forgotten to alter the .focussing to suit the altered arrangement. Remember always to return the setting to " infinity " after making each exposure, and also remember to set for correct distance before making an exposure.

It is now necessary to consider another factor, namely, the size of the aperture of the lens itself when the exposure is made. With very fast lenses, marked fir.5 f/2 or 1./2.8 (these are just typical examples) the latitude in focus—termed the depth of focus—is very little. If such a lens should be focussed on an object five feet distant, other objects in a plane, say, ten feet distant, will be fuzzy or " out of focus." Simi- larly objects only three feet distant will be out of focus. If the aperture be decreased in diameter (this would be indicated by a higher figure), for example, from f,/.2 decreased to f/8, these .o'3jects in other planes would be sharp. The depth of focus would have been increased by the action of making the effective diameter of the lens smaller. This is a constant in photography. The- larger the aperture -the smaller the- latitude- in focussing and vice versa. This particular property of fast lenses is rather valuable. When making a picture of the head and shoulders of a person it is often noticed, with annoyance, that details of the background intrude. If the lens aperture is opened as widely as possible-ind the head andhoulders- of the person 'to be portiayed are far removed from the background, the latter will be out of focus when the object is sharp. BY this means a slightly stereoscopic effect will be gained and the principal object in the picture will stand out from the remainder. This applies only when the aperture of the lens is opened wid. When it is partly or fully closed, the depth of focus is increasA and all objects within a' certain area will be sharp. ' 'The siec of this area is dependent upon the f/ number to which the aperture is set. To repeat, the higher the f/ number the smaller the actual aperture, and the greater the depth of focu' available. (Example ,F/22.) The loWer the f/- number ti larger the aperture of the lens and the less latitude available I 1 focussing. (Example F, 3.5) _ _ .