lundi 18 juillet 2005

Lenses I've Looked Through Dallmeyer 3B Soft-Focus To Victorian portrait photographers the principal requirement of a lens was that it had a long focal length and a large aperture for greater speed. The original type of portrait lens was invented by Petzval, in 1841, and was manufactured by Voigtländer. For some time this lens type determined the construction of portrait lenses. Dallmeyer Patent Portrait lenses were introduced in 1866 and were found to be supreme in the studio. For a long time there was a strong tendency toward using anastigmat lenses for portrait work, but for studio portraiture, pure and simple, the Dallmeyer Portrait lens possessed an indefinable "something" which placed them ahead of any other type. Dallmeyer Lenses were constructed on a different principle from the Petzval portrait lenses and excelled them in sharpness of definition, in freedom from distortion and flare, and in equality of illumination... in addition, they afforded the means, by the simple sliding lever, of obtaining greater equality or depth of definition. The lens' construction was such that with its cells screwed home it produced the sharpest possible picture of objects situated in one plane. By unscrewing the barrel a turn, or parts of a turn, the previous intensely sharp definition was modified... that is, the contrast of excessive sharpness in one plane, compared with lessening of sharpness in other planes, was balanced, producing the impression of a general distribution of depth-of-focus. Nothing was sacrificed with this feature, and it could be used or not, at the discretion the photographer, who rapidly became proficient in judging what was necessary to produce any desired effect. I still use a Dallmeyer "Soft Focus 3B" Portrait Lens... shown here on an equally old Kodak Studio Camera for which I made an adaptor to take modern Lotus Rapid View 5x7 and 4x5 inch backs and film holders. It's quite a hunk... although the lens' glasses are comparatively thin, to transmit a maximum amount of light, and the body of the lens is largely made of aluminium, so as to reduce its weight... with an approximate focal length of eight inches or so, and being fast with an f/3 maximum aperture (the front glass is 9 cms across) the weight is quite a handful at 1.76 kilos... requiring both a strong camera and rigid stand in use! One remarkable feature of the lens is the iris diaphragm which has a total of 24 blades... so forming a near perfect circle helping to produce remarkable "bokeh" - or rounded out-of-focus background detail - so talked about nowadays in photo circles and tested for in manufacturer's lens catalogues. Method of Diffusing Changing the relative position of the lens' glasses creates more or less definition... you have to do this first and then refocus afterwards. The separating of the individual lenses alters the focal point making it essential to refocus after the desired amount of diffusion had been selected. This method is also necessary when using, for example, the 85mm and 120mm Soft Focus lenses for the Pentax 35mm and 6x7 systems... so nothing has really changed after 100 years! One problem experienced with a portrait lens is that at full open aperture it is more suitable for head and shoulder portraits, not full length figures and groups. For the latter it has not got the necessary flat field and so requires stopping down... and this means loss of speed with longer exposures and less diffusion. Many older types of portrait lenses used "diffusing" disks, but to adjust this system it was necessary to remove the lens from the camera, or to remove the camera's ground-glass, in order to get at the rear of the lens. Either of these methods took time and didn't permit accurate adjustment of the amount of diffusion to the subject. However, later types of portrait lenses, notably those manufactured by the Bausch & Lomb Optical Company, used a diffusing system operated by an adjusting knob which projected outside the lens barrel, and on which a scale of values was engraved. Modern portrait lenses, the professional's choice being the 150 mm and 180 mm Variable Soft Focus lenses for the Mamiya RB67 and RZ67 systems respectively, use special diffusion disks for very rapid use and predictable results in the studio. From that page there's a "pop-up" with an excellent technical brief on the Mamiya RB 150mm Variable Soft Focus Lens by photographer John Woodward. The "speed" of portrait lenses of the same equivalent focal length varies directly with the diameter of the lens. It is, therefore, possible to preserve approximately the same focal lengths in portrait lenses, and obtain greater speed by increasing the diameter of the lens to let in more light. But this increase in diameter materially increases the cost... a graph plotting the increase in money spent per extra stop of lens speed gained would show a truly exponential curve! However, in Victorian times photographers found it very much to their advantage to purchase as physically wide a lens as could be afforded... it has even been said that the possession of a rapid lens added several hours to the photographer's working day. Distorted Perspectives Few photographers realise the disadvantages accompanying the use of a lens having a short focal length as the three small illustrations above show. For the left image the lens was one of short focus (Nikkor 20mm wide-angle), and the distance from the figure to the lens was less than 12 inches. The result in this photograph shows the extreme distortion of the features, completely changing the expression. The centre image was made with a Nikkor 50mm standard lens and is much better... but the right image shows the best "drawing" and was made with a longer Nikkor 85mm medium telephoto lens, the distance from the camera to the sitter being about 48 inches... much more relaxing for all concerned! There's much more information and discussion of portrait lenses on the internet... two sites of interest being Soft Focus Lenses and Techniques by Ernest Purdum... ...and you could browse or join the Large Format Photography Forum for entertaining discussions and opinions on "classic" lenses of all names, types and applications. 9:25:28 PM    comment []