vendredi 7 octobre 2005

Cameras I've clicked with... Pentax 6x7 Why I bought and sold three outfits... Part 3a : Pentax 120mm Soft Focus Lens I'm jumping ahead in recalling my joys and trials using three Pentax 6x7 outfits over a period of many years because I'm re-examining images taken using the dedicated Pentax 120mm Soft Focus lens. This could be a bad move because it may draw me back (for a fourth time?) to that big camera which makes big transparencies with ease. When it comes to "soft focus" images romantic notions take over... whether photographing people - young or old - landscapes, nature, animals, flowers, still-life, nostalgia or glamour, degradation of the image is an interesting effect for an alternative result. Many photographers have at some time felt that their images are too sharp and added a diffusion filter, piece of nylon stocking, scrap of cellophane from a Gauloises packet (a famous trick from Sarah Moon, French model turned fashion photographer) or a smear of Vaseline to a clear filter over the lens. In the 19th century wet collodion photographers sometimes "tapped" the camera during a long exposure to add a touch of softening to the image. But in the last hundred years higher-speed lenses and films have meant shorter exposures... which changed the game. It became necessary to use special portrait lenses which introduced softness to the image. There have been a number of soft-focus lenses commercially made, the Leitz Thambar and the Rodenstock lmagon being notable. Generally, optical designers aided by powerful computers make the sharpest possible lenses for photographers... so the demand for "softer" optics is very much a niche market. However, from niches grow fashions. As photography has embraced "fashions" the demand for images using soft-focus lenses has been revived. One interesting adaptation of soft-focus lenses - indeed the most popular amongst photographers in Japan - has been the meniscus lens from the 1920s Vest Pocket Kodak which uses a cemented double-element achromat. Compared to modern lenses it is not really sharp, but when it's hood is removed it becomes amazingly soft. This effect has been known since the 1930s, and nowadays many of these simple lenses are being stripped out of the VP Kodak, mounted in a helical close-up ring to fit any camera and used to wonderful effect very inexpensively. The Pentax Soft 85mm f/2.2 lens for their 35mm SLRs is the same simple type of VP Kodak lens and it's soft image effect is very beautiful when used for flower subjects, close-ups and portraits. Optically, there are two causes for lens aberrations. Firstly the lens' material, the glass, and secondly the spherical curvature of the lens surface. The aberration that creates the soft-focus image belongs to the spherical aberration. As with other soft-focus lenses, the Pentax lens puts an emphasis on this aberration. But to make a beautiful image with a soft-focus lens you have to learn how to use it. The Pentax Soft Focus 85mm f/2.2 lens for that company's 35mm SLRs was very popular for its distinctive image effects. Pentax then developed a soft-focus lens for the 6x7cm (and with an adaptor for the 6x4.5 cm) systems. A big feature of the lens is its fine soft-focus effect at the picture edges... but this time it is not through a simple VP Kodak type meniscus lens. The new lens has a Tessar configuration of four elements in three groups, compensating for the image blur at the picture edges. The lens produces gentle softness over the entire picture area, controlled by the aperture stops... the smaller the aperture the sharper the image. Thus the lens has two characteristics producing a soft-focus effect at f/3.5~5.6 aperture settings and sharp images at f/11~22. The subtleties are too gradual to show in small images on screen so I'm only showing comparisons between the minimum and maximum "Soft" settings with the tree branches image. The Tessar-type construction is used for working aperture exposure metering. The optical designers deliberately retained an amount of spherical aberration in their design and product. Spherical aberration occurs because a lens element is spherical and the light beams passing through the lens' centre and edges don't come to a single focus point. The soft-focus effect is achieved by using this aberration. The amount of softness is controlled by changing the aperture... being greatest at large apertures and less on stopping down. The point is reached where the soft-focus effect is just perceptible at around f/8 and then the lens reaches very good sharpness at f/11~22. The aperture has eight blades producing a circle at f/4 with fine out-of-focus "bokeh"... there's a good technical article on bokeh, reproduced from the American magazine "Photo Techniques" on the Luminous-Landscape website. All this wonderful, romantic soft-focus imagery is not that easy to achieve without practice though... because the correct focus is slightly behind the in-focus point seen on the focusing screen of the camera. It's best to use a Matte Screen and if necessary correct the focus distance on the special scale around the lens barrel. With the lens on the camera and in your hand it is easier to do than describe... but Japanese professional photographer, Mr. Tanaka, described it accurately in an issue of "Pentax Family" magazine from the 1990s... Focus Correction Method 1 After focusing, turn the focusing ring to the left to align a dot coincided with the Standard Line to the Focus Correction Line. The correction line at left is for a maximum of resolution (larger flare with sharply focused core) and the line at right for a maximum of contrast (the shadow around the image and lesser soft-effect). Focus Correction Method 2 Stop down the aperture to f/11 and focus on the (matte area of the) focusing screen. Then, set the aperture at the picture-taking f/stop (f/3.5~5.6). The corrected position of focus for the aperture range of f/3.5~5.6 coincides roughly with the on-finder focus position at f /11. Then, turn the focusing ring to the correction line indicator. There's a third method... stop down the aperture to f/11 (the finder field will darken) and return to the brighter working aperture which should be around f/3.5~f/5.6. This method is used by many photographers for it's speed of use with live subjects. The degree of softness is not only dependent on the minimum and maximum "Soft" settings, but on the lens aperture used for the exposure. Here you will have to juggle between shutter speed, aperture and soft setting to achieve the best result... and this only comes with practice - a lot of it! The advantage of the Pentax 120mm Soft Lens is that it can also be used on Pentax 645 and 35mm SLR cameras with appropriate adaptors... and by using x1.4 or x2 teleconverters, or Helical Close-up tubes the focal length can be extended or minimum focus distance reduced. That adaptability provides the icing on the cake when considering whether to acquire such a specialised optic for a heavyweight system... and that could become part of my latest photographic dilemma... To be continued... For an article on various soft-focus lenses and related techniques check this direct link to the Large Format photography site. Part 1 of Pentax 67 System is the system compatibility. Part 2 of Pentax 67 System is a comparison with Hasselblad. Part 3b of Pentax 67 System is the 35mm Fisheye lens. Part 4 of Pentax 67 System is being written. Part 5 of Pentax 67 System reviews the two Spotmeters and other bits. 11:58:09 PM    comment []