881. Astigmatism

Astigmatism. We divide our lenses into anastig-mats and non-anastigmats. In view of the great superiority of anastigmats, we should explain that we continue to make and list non-anastigmat lenses, such as the Unsymmetrical Portrait Series I. and Series IA, and the two series of Eury-scopes only for very excellent reasons, justified by an experience of many years. The Series I and IA are used exclusively for portrait photography at a very short distance where, in view of their exceedingly small depth, anastigmat flatness of field is of no advantage in comparison with the advantage of their high speed. The two series of Euryscope are of exceedingly simple construction, and at so low a range of price, that it is desirable, for these reasons, to retain them in our list. With the exception of those mentioned, all our lenses are perfect anastigmats.

882. In outward appearance the non-anastigmats are distinguished by their much greater length. The field of sharp definition of the non-anastigmats is very much smaller than that of anastigmats of equal focal length and aperture, and even when used with a very small diaphragm their work still bears no comparison with that of the fully corrected lenses.

883. In selecting a lens the amateur photographer should consider only the anastigmats. In the first place, because he requires a large sharp image at full aperture, and in the second place, because the unwieldy shape and size of the non-anastigmats would render instantaneous work impossible.

884. In our list of lenses the size of the image is always mentioned in connection with each lens. In the case of anastigmats these plate sizes are covered to the very edge with absolute sharpness and perfect marginal definition. In the case of non-anastigmats the sizes indicated are those which general usage among photographers has adopted.

885. Angle Of View

Angle Of View. Stopping down the lens increases the sharpness at the edges of the plate and gives the lens the power to cover sharply a larger plate than that which it covers at full opening, and it is on this larger plate that the greater angle of view is obtained. On the other hand, the increase of sharpness resulting from stopping down has its limits. Briefly stated, the limit lies between the openings f.64 and f.128. We advise against the use of diaphragms smaller than f.64. In fact, in several of our lenses we do not extend the scale below f.45 or even f.32.

886. Choice Of Lens

Choice Of Lens. The choice of a photographic lens involves, in accordance with the foregoing, a decision as to the angle of view and focal length to be selected. It is a generally conceded fact, that for artistic results an angle of 45 degrees should not be exceeded. This would necessitate the use of an 8 inch lens on a 4 x 5 plate, which seems rather long. There is no doubt that the work of the painter seldom comprises a larger angle, and that this produces the most pleasing proportion between the foreground and background. For portrait photography, likewise, a lens of 45 degrees has the advantage of rendering the figure of the sitter in its proper relative proportions. In many cases, therefore, the lens of small angle and long focus is desirable, but there are many other circumstances influencing the choice, and the photographer will have to decide for himself according to the conditions surrounding his work. If the photographer makes use of a camera which permits of exact focusing, as, for instance, the "Reflex," the best results will be obtained by using a lens of long focal length. On the other hand, the shorter focal length lenses being easier to focus, are better for instantaneous exposures, in which there is little or no time for accurate focusing.

887. Landscape and group lenses used on tripod cameras should in no case be chosen with a larger angle than 55 degrees. For portrait exposures experience has demonstrated that the smallest suitable focal lengths are: for small best pictures 8 inches; for cabinet bust pictures, 12 inches; for Paris panels, 16 inches. In short studios, however, shorter focal length lenses frequently have to be used; for wide angle work, on the other hand, the shortest possible focal lengths can only be used.

888. Although our lenses may invariably be used for larger plates than listed to cover, we are opposed to the straining of lenses, and advise against it except where absolutely necessary to secure the greatest angle the lens is capable of producing. The mistake of straining a lens becomes only apparent when the shifting or rising front is used in connection with a lens whose focal length is small compared to the size of the plate. There occurs at once, in such cases, a loss of light toward the edges, and very often the plate is not completely covered. If, for no other reason than this, a focal length should be chosen, which on a given plate would correspond to an angle of about 60 degrees for the lens working at f.6.8; an angle of 55 degrees for lens working at f.5.6, and 45 degrees for a lens working at f.4.5.

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Illustration No. 76

889. Voigtländer's Collinears - Collinear lenses are a new type of anastigmatic doublets, consisting of two symmetrical anastigmatic halves. Each of these halves is made up of three glasses, the contiguous surfaces of which are permanently cemented together. The new Jena glass is used in the construction of Collinear lenses. The outside glasses of the combinations are of a durable, hard quality not effected by the atmosphere. The lenses have no air spaces which might introduce false reflections. The sharpness of definition of the image produced with the Collinear lenses is remarkable for detail and precision, and is the same on the edges of the plate as in the center. The covering power and the flatness of field are unexcelled.

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Illustration No. 77

890. Collinear lenses are achromatic, rectilinear and free from distortion. Their speed, especially of the Series II., is of a very high order. The lenses of this series are the most rapid symmetrical anastigmats. They work with an aperture of f.5.6, and are suitable for focal plane shutter work. (See Illustration No. 76.) The Series III., working with an aperture of f.6.8, is also a very rapid lens, and is a true universal lens to be highly recommended for all general work. The rear combination of a Collinear lens can be used alone as a landscape lens, giving double the focal length of the complete instrument. The general appearance of this lens, also its optical construction, is shown in Illustration No. 77.

891. Voigtländer's Dynar. - The Dynar is an entirely new lens of recent construction, the advantages and attractive features of which are its compactness and lightness, its speed, its careful anastigmatie corrections, and its comparatively low cost. The Dynar consists of five glasses, two sets of each firmly cemented, and the fifth glass placed separately between the two sets. This construction is entirely different from the symmetrical form of the Collinear lens.

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Illustration No. 78

892. Owing to its compactness and speed the Dynar is highly recommended for hand and pocket cameras - Kodak, Hawkeyes, Centuries, Anscos, Premos, etc. The dimensions of the lens mounts are such that they will fit directly into the standard sizes of modern shutters. The speed of the Dynar is f .6, which means that at this aperture

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Illustration No. 79 it is one and a half times as rapid as a lens at f.8. The speed is sufficient for all between-the-lens shutters and for focal plane shutters. This lens is thoroughly corrected - achromatic, rectilinear and anastigmatic; the field is flat, the definition excellent. Although primarily constructed to be a hand camera lens, the Dynar will be found available for producing excellent indoor and outdoor work, views, groups, mechanical work, etc. The optical construction of the lens will be clearly seen on referring to Illustration No. 78.

£93. Collinear and Dynar Lens Cells. - Both the Col-linear and Dynar lenses may be procured in the cells alone for folding pocket kodaks. (See Illustration No. 79.) These lens cells may be screwed, without any fitting, into Automatic or Volute shutters. Any one can send for a set of these cells, and without any assistance remove his original lens cells and replace them with the high grade Collinear or Dynar cells.

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Illustration No. 80

894. Voigtländer's Heliar. - This lens, working with a speed of f .4.5, is one of the most suitable all around lenses on the market. It is particularly adapted to portrait work, yet cannot be excelled for high speed instantaneous exposures with focal plane shutters, enlarging, projection and telephoto work. The lens cells are mounted in two styles of mountings, as shown in Illustrations No. 80 and No. 84. The hand camera mount is used for the first four sizes, and the portrait mount used for sizes 5 to 9. (The advantages of this lens for portrait work are thoroughly explained in the Chapter on Portrait Lenses.) The Heliar lens may be attached to the Graflex, Reflex, and other high speed cameras, supplied with a focal plane shutter.

895. Telephoto work requires a quick and accurate positive lens. Either the Heliar or Collinear Series II. lens used as a positive and combined with a Voigtländer tele-photo attachment forms undoubtedly the quickest telephoto constructions obtainable. (The telephoto attachment is described in the Chapter on Telephoto Lenses.) For enlarging and projecting the speed of the Heliar and Collinear lenses and their sharpness of definition are highly desirable qualities.

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Illustration No. 81

896. Voigtländer's Euryscope. - The Euryscope lenses are intended for the making of portraits. (The Portrait Euryscope is described in the Chapter on Portrait Lenses.) The Extra Rapid Euryscope (See Illustration No. 81) is by far the most popular of the various series of Euryscopes, owing to the fact that it is an all around lens for both portraits and groups. It has a medium speed, the aperture being f.7. Its comparatively flat field with an angle of about 55 degrees makes it especially suitable for group work, and it is frequently preferred, on account of its depth, to the more rapid Series III. Portrait Euryscope or the Heliar. The construction is a symmetrical one, the lenses are of Jena glass, and the iris diaphragm is furnished in all lenses of this series.