A photographer who owns a number of lenses and wants to determine which one to use for any particular view, may do so when visiting the spot beforehand without lenses or camera, and with very little trouble. We only require to know what size of plate each lens will cover, and its equivalent focus. The method of determining the equivalent focus of a combination is, in brief, this: Measure the distance of the focus of a distant object from any definite point on the mounting of the lens, and again from the same point with the lens versed. One - half the sum of the two measurements will be very nearly the equivalent focal 16 to suit the subject and the angle of view required. There are rules for calculating these, and they are useful when it is really particular that a certain and gives amount of subject must be included in a view. Generally, a look into the camera will soon satisfy the operator whether or not his picture, as a whole, is what he desires to secure.
307. There are other methods of satisfying yourself as to what amount length. To find out what lens to use, take any small stick or twig and break it off equal to the horizontal width of the plate to be used, or the twig may be shaped to the outline of the plate. This is to be placed upon a rule, one end of which is pressed against the lower eyelid. Then move the twig along the rule until its ends cover the extreme portions of the view to be taken, and read off the number of inches, distance at which it must be placed from the eye, and this will be the focal length of the lens required. In case of having only one lens at disposal and we wish to find what sized plate must be used to include a certain view, provided it is in the capacity of the lens to cover, mark off from one end of a stick, by placing the thumb-nail upon it, a length that will include the view, at the same time holding the stick at a distance from the eye, measured along the rule, equal to the equivalent focal length of the lens to be used. In practice, a folding foot-rule in four parts is a convenient pocket size. To make this answer for a focal length, for instance of twenty inches, reduce everything to half-size, calling the focus ten inches, and when the size plate is found for a ten-inch focus, multiply by two to get the size actually required. Again, if, in case of stereo views, you expect that a lens of three or of two-and-a-half inches focus will be required, to avoid a chance of error that might occur from not being able to measure from the true centre of the eye, double or treble the length of the twig or stem of grass that you hold up to include the view, and you will then get accordingly two or three times the focal length to be determined with sufficient accuracy for practical purposes. - John M. Blake.
The following calculations have been made to show what angle of view is included in any picture when the equivalent focus of the lens by which it was taken is known. When the base-line of the picture measures the same as the equivalent focus of the lens, the angle of view will be fifty-three degrees. If it measures a quarter part more than the equivalent focus of the lens, the angle will be sixty - four degrees. If it measures half as much again as the equivalent focus of the lens, the angle will be seventy-four degrees. If three-quarter part more than the equivalent focus of the lens, the angle will be eighty-two degrees. If double the length of the equivalent focus of the lens, the angle will be ninety degrees. sufficiently accurate for all practical purposes. A B is an object in a horizontal position; L, the lens; CO, the image on the ground-glass; E L, the distance from the lens to the object; and F L, the focal length of the lens. It is easy to see, or at least to feel, that the given dimensions are in some relation to the proportions of the triangles alb and cld.
307. If the standpoint is not a given one, and it is desirable to have more or less of the view in the field, alter your distance from the object, until you find a suitable position for placing your camera. Again, the position, or distance, and size of plate are given, and you want to know the focal length of the lens necessary to give the required image: Place yourself at the given standpoint; hold the respective plate-holder before your eyes; move it to and fro until the plate - holder frames just what you want; measure the distance between the plate-holder and your eye; the distance is equal to the focal length of the lens required. Thus you are enabled to pick out the position to suit your taste and purpose. Frequently, view photographers are puzzled about the problems: How much of a view will my lens give at a given distance on a certain size plate? or, What distance is required for my lens to give of subject you may expect to secure from any given standpoint. It is well to understand than all, for it will save many a climb.Inexperienced parties known to expend an hour or two in trying to so much of a view on a given fixe plate? or further, if the distance is given, what is the focal length of the lens necessary to accomplish what I wish? All this, and for the thinking photographer a great deal more, may be answered by the following simple method.
Now, suppose you turn your camera around the lens
L until the ground - glass, on which you previously traced the image with pencil, takes the position of Gh; take off the lens, and put your eye where the lens was placed, and the image on the ground - slats will just cover the object
I l, then, will be equal to F L, which is the focal length of the lens. Consequently, we come to the conclusion:
The image formed by a lens on the plate is precisely the same as the one which is seen by one eye through the plate-holder, or the glass plate itself, at a distance equal to the focal length of the lens from the same standpoint. Suppose you have a twelve-inch lens, and you want to take a view on a 10 x 12 plate: Hold your 10x12 frame twelve inches from your eye, and all that which you see through the frame with one eye your lens will produce on the plate, provided the angle of your lens is wide enough. - J. Zentmayer.