860. During many years of productive activity, Voigt-lander & Sohn have not only furnished an endless number of lenses, all of the highest type of their times, commanding the highest prices of any photographic lenses on the market, but they have built up an organization and accumulated an experience in the manufacture of optical instruments which few can equal. The following data regarding Voigtlander lenses has been supplied by the manufacturers:

861. As a guarantee for the quality of lenses which we offer to the public our record of one hundred and fifty years of leadership in lens making is of particular interest and value. It is a record which is maintained by every individual instrument that leaves our works, and by every new construction which we place before our patrons. Most notable of our recent achievements is the production of two series of extra rapid lenses - the Heliar, mainly for use in the studio, and the Dynar, principally for outdoor use - both styles marvelously fine productions.

862. Focal Length

Focal Length. The focal length of the lens determines the size in which the image of an object will be reproduced. The greater the focal length the larger the image will appear on the ground-glass, other conditions being equal. The size of the diaphragm has no influence whatever on the size of the image. The focal length is, to a certain extent, the measure of the size of a lens and of its capacity, making due allowance for the superiority of one style over another. Thus, a lens of 6 inch focal length is comparatively small, covering a small or medium picture only, while a lens of 24 inch focus will be called a large size and would be used for making large pictures. While most manufacturers designate the different sizes of their lenses, by numbers, it is quite evident that such designation is altogether arbitrary, while the focal length carries meaning and description with it. All Voigtländer lenses have engraved upon them their focal lengths in inches, so that inferences as to the size of the image, etc., can at once be drawn, and a general comparison with other lenses instituted, without the necessity of referring to the catalogue.

863. While there are certain ideas prevalent as to the proper relations between the focal length of the lens and the size of the picture for which it is used, a most noticeable feature of the progress of photographic lens construction lies in the shortening of the focal length in comparison to the sizes of the picture produced. The latest and most perfect lens constructions are the anastigmats.

864. A fine anastigmat like the Collinear or Dynar, of say 6 inch focal length, will produce as large a picture as can be covered by ordinary rapid rectilinear lenses of 8 to 9 inch focus. Of two styles of lenses having the same aperture, and both covering the same plate, that one is the better which has the shorter focus and a greater depth of field than the longer one at full opening. A comparison of high grade lenses with the usual hand camera lenses on this score alone will at once demonstrate the superiority of the former.