But while large lens apertures have the advantage of permitting shorter exposures, they have some disadvantages. In the first place, to get a large aperture we must have a large lens, and this means an expensive lens; also, the errors of definition, which are called the "aberrations" of lenses, increase very much as the apertures increase, so that only the very best types of lenses in which these aberrations are removed to as great an extent as possible, can be made of large aperture and still give good definition. Large aperture lenses are therefore costly.
But even when we have a lens with a large aperture we shall have to regard this as a reserve power for use in special circumstances, and we shall not by any means be able to use it at its largest aperture all the time.
From the construction of a lens it follows that only the rays from a mathematical point can come together in a point again, and that the rays from any point nearer or farther than the point focused can not meet in a point image on the film, but must produce a small disc of light instead of a sharp point of light. (See Fig. 21.)
The disc is termed the circle of confusion. If the circle of confusion is small enough we shall not be able to distinguish it from a point, and the picture will appear to be sharp.
With what are known as "fixed focus" cameras, such as the Vest Pocket Kodaks and the Box Brownies, no attempt is made to secure a wholly sharp focus for objects at all distances, but the cameras are sharply focused on the nearest point to the camera which will still enable distant objects to appear approximately sharp in the pictures, and in this way objects in the middle distance are perfectly sharp, and near objects are also sharp, provided they are not too near.
The following table of these distances, beyond which everything is sharp when the largest stop is used, may be useful:
Vest Pocket Kodak ................... .
No. 0 Brownie ...........................................
No. 2 Brownie ..........................................
13 1/2 "
No. 2A, 2C and No. 3 Brownie
If we are using a No. 0 Brownie, for instance, as long as everything is farther off than nine (9) feet we can rely on getting a picture with everything focused sharply.
With the focusing Kodaks we must judge the distance of the object on which we wish the focus to be sharpest and set the scale to that; then we shall find that objects somewhat nearer, and also objects a good deal farther from the camera are also sharp, and the distance from the nearest to the farthest objects that appear sharp in the negative is called the "depth of focus." This depth of focus depends on the focal length of the lens and on the size of stop used in the lens; the greater the focal length the less the depth of focus, and the bigger the stop the less the depth of focus. Thus in Fig. 28, we have a lens focusing near and far points at full aperture and producing large circles of confusion. In Fig. 29 a smaller stop is used in the same lens, and the circles diminish in size in proportion to reduction in the size of the stop.
Sometimes we have to focus near objects at the same time as distant ones, so that it is necessary to "stop the lens down" to some extent.
Stops are marked in two different systems, though both are based on the fundamental ratio of the diameter to the focal length of the lens. In the one system the stop is expressed simply as a fraction of the focal length; thus F./8 (commonly written f.8) means that the aperture is one-eighth of the focal length of the lens f.16, one-sixteenth, and so on. The rectilinear lenses fitted to Kodaks are, however, marked in the "Uniform System" (U. S.) in which the numbers are proportional to the exposure required, f.4 being taken as unity, so that the scale is as follows:
Focus of Distant Object
Best focus For Both Together
Fig. 28. Depth of Focus with Full Aperture.
Fig. 29. Depth of Focus with Smaller Aperture.
The U. S. numbers give the relative exposure that is required with the f. system stops, the exposure varying as the square of the f, value, so that f. 11 requires twice the exposure of.8;f. 16 twice that of f. 11 and so on.
Kodaks, Premo and Brownie cameras are listed with several different kinds of lenses, the smaller cameras being listed with either Meniscus, Meniscus Achromatic, Rapid Rectilinear or Anastigmat Lenses. The larger cameras have either Rapid Rectilinear or Anastigmat Lenses, while the Special Kodaks and Graflex cameras have Anastigmats only. The Box Brownies are equipped with Meniscus or Meniscus Achromatic Lenses, while with the Folding Brownies there is a choice between Meniscus Achromatic and Rapid Rectilinear lenses.
Many people do not understand the meaning of these terms, and while it is a safe rule to choose the best lens which can be afforded, certain that the better lens is worth the extra cost, it is still better to understand the properties of the different kinds of lenses and what advantages can be gained from the use of the higher grades.
The simplest lenses which can be used are made of a single piece of glass, the form of the lens being of the type which gives the best definition; that is, a Meniscus or crescent shape, and the lenses are called Meniscus (not Meniscus Achromatic) lenses. Such a Meniscus lens can only be used in a fixed focus camera where the maker of the camera has put it in the correct position for forming a sharp image upon the film, but if such a lens were used in a focusing camera we should find that however carefully we focused the picture on the ground glass the negatives would not be sharp, unless the difference between the focusing point of the visual rays by which we focus, and the chemical rays which affect the film, was provided for.