116. If a lens be found faulty in this respect, it should be returned to the seller, Indeed, in these days there is no reason why photographer should "manage" with a Lens in the least bit defective. Many pages could be filled describing the various methods of testing lenses; their varied constractions and grades, and abilities. That seemso much to be the business of the dealer in them a- to be uncalled for here, and it is omitted. If you want a new lens,you are supposed to know what you want it to do, to begin with. The dealer in them should be able to sup-ply you with what you want, and, as usual thing, is willing to exchange if your desires are not me1 with in the first trial. There are so many maken now that one has the righi to expect every opportunity to select the best, or at least what he believes will answer his purpose best. The excellence of your results and the enjoyment yon will have in producing them, depends so much upon your decision in this matter, that yon ought to use even means to secure a good instrument nnd amateurs not to listen to the very natural desire to try their skill on large apertures, and giving higher magnification, if they do not wish to he disappointed, and lose time and money But the school of experience seems to be the only one to cure this desire. - .Joseph Zentmayer.
There are two causes, from either of which such a defect might arise. It may he that the sensitive plate does not occupy precisely the same plane as the ground - glass. This you can only certainly ascertain by very careful measurement, although it is possible that a slight examination will suggest the existence of such a defect. Examine the dark-slide, and see if the corners on which the plate rests are all firm and in their proper position. If these are all right, proceed to measure. Take out the lens, and measure carefully from the aperture in the front to the ground - glass; then remove the ground-glass, place the dark-slide containing a plain plate of glass in its place, and measure again; if the distance coincides, all is right there; if not, make the proper correction. If the fault is not in the position of the plate and ground-glass, it may be in the lens. In some lenses the visual and chemical foci are not incident - that is, the rays which come to a focus, and give a sharp image to the eye on one plane, come to a chemical fed on another plane. In this case, there is no remedy but obtaining a better lens, or, else, always making an allowance for the defect in focussing. - Unknown.
116. I give below a resume of tests made in selecting lenses for a friend. I started with no fixed ideal which the performance of the instruments was to equal. I did not expect a mammoth to " cut " in the same proportion as a 2/3 or 2B. Knowing something of the rapidly increasing ratio of the imperfections that have to be equalized and corrected (the spherical aberration increases as the square of the aperture, the chromatic as the cube), I resolved to be contented with the best that leading opticians offered: and here I will say that, as far as I can, I shall refrain from indicating who the makers were The lenses will be designated as A and B. A was made to order, subject to approval. B was one of a number of the same size ready made.
Test first, of price. A cost over fifty per cent, more than B.
Test second, for whiteness of glass. The Unset were unscrewed, and placed side by side on white paper. B proved to be of much clearer, whiter glass than A, which latter had a rather yellowish - green tint.
117. The use of the diaphragm now demands your consideration. Some idea of its mission and value has already been given. The dia-phragm is a piece of metal, wood, or card-board, with an aperture in the centre, varying in size, and is placed before or behind the lens both to stop or prevent the action of certain rays upon the lens, and to neutralize their effect upon the negative, even though they do pass through. The diaphragm is, therefore, also called a "stop."
Test third, for comparative ratio of aperture to focal length, B was by measure a fraction of an inch larger in diameter than A. The focal lengths were obtained by measuring the distances of the image from the object (dots twelve inches apart on white card); the image was in each case made the same size as the object when focussed with the full opening. Had a small diaphragm been used there might have been an error, as in this case there is some latitude in focussing. The distances when quartered gave pretty nearly the equivalent solar foci. B had a somewhat longer focus than A, but in each case there was about the same ratio between the squares of the apertures and the focal lengths respectively, from which it might be expected that they would work in nearly equal time, while the superior whiteness of B would be an element in its favor.
Test fourth, for actual time of working. This test was very carefully conducted, repeated several times on different days, always about noon, with a clear sky. A still subject in halftone was used, and care taken to distinguish between an increase of intensity, which might make the detail in the shadows more apparent, and actual amount of detail obtained with similar and dissimilar exposures. These trials showed that A with forty seconds equalled B with forty-five seconds, a difference of but little practical value.
Test fifth, for definition, made by copying an engraved head of four inches length to the same size with full apertures. Very decided in favor of B.
Test sixth, for flatness of field, made by copying a newspaper to actual size, and by pushing in the ground-glass till the centre letters were barely sharp. Much in favor of B, which had a field of sharp letters almost double the linear extent of A, though it was surprising how small the extent of apparently flat, well-defined field was in either case. When, afterwards, the lenses were tried on standing figures the comparison had the same result.