Soft focus

In photography, soft focus occurs when a camera lens forms images that are blurred due to uncorrected spherical aberration. A soft focus lens deliberately introduces spherical aberration which blurs fine texture in the image while retaining sharp edges across areas of high contrast; it is not the same as an out-of-focus image, and the effect cannot be achieved simply by defocusing a sharp lens. Soft focus is also the name of the style of photograph produced by such a lens.

Photography

Effect Soft focus has been described as "an image that is in focus but has a halo of out-of-focus images around it." The first deliberate use of undercorrected spherical aberration, resulting in halos around highlights (also known as "pearly" highlights), is thought to have been by French pictorialists around 1900, spreading to the United States, where these lenses were most popular between 1910 and 1930. Noted practitioners of soft focus photography include Julia Margaret Cameron, Bob Guccione, and early Edward Weston, though Weston was later credited with moving photography away from soft focus pictorialism. The soft focus effect is used primarily in glamour photography, because it eliminates blemishes. In general, soft focus photography produces a misty, dream-like image, sometimes characterized as romantic.

Technique Special focusing techniques may be required to use a soft focus lens. For example, a front-focusing technique was suggested for the Kodak Portrait, in which the point of focus was placed closer to the camera than the actual subject. Unlike typical camera lenses, which have a generally symmetric depth of field characteristic extending both in front of and behind the point of focus, the uncorrected spherical aberration results in a depth of field which extends past (behind) the point of focus, but not in front. Physically, the effect of a soft focus lens may be approximated by the use of diffusion filter or other method, such as stretching a nylon stocking over the front of the lens, or smearing petroleum jelly on a clear filter or on the front element or even the back element of the lens itself. The latter is less recommended because successive cleaning always introduces a risk to damage the lens's surface. It can also be approximated with post-processing procedures, either during photographic printing or through digital manipulation. Specifically, highlights in an image are blurred, but the bokeh effects of soft focus cannot be reproduced.

Design

Because soft focus results from what are considered technical flaws, typically spherical and chromatic aberration, many older lenses had soft focus built in as a side effect of their construction. For example, the two-element cemented meniscus lens fitted to early Vest Pocket Kodak cameras had a dish-shaped hood which controlled spherical aberration by reducing the effective aperture to f/11; when the hood is removed, the resulting uncorrected images have a strong soft focus effect. After this modification, the lens enjoyed significant popularity in Japan during the 1970s, remounted to modern cameras. Photographers called this lens the Vestan (ベス単), referring to the camera's name and single-group lens construction, and the technique was championed by several Japanese photographers, including Shōji Ueda. The basic design of this lens was revived in 2016 as the Yasuhara MOMO 100. Some lenses designed and sold during the heyday of soft focus lenses (c. 1910–1930), including the Pinkham & Smith Visual Quality series and Busch Nicola Perscheid, were designed intentionally to take advantage of these flaws. As color films became available, well-managed spherical aberration became more desirable than chromatic aberration.

Newer lenses are optimized to minimize optical aberrations, but starting from the 1970s, manufacturers began releasing specialized contemporary lenses which are designed with adjustable levels of spherical aberration at wide apertures. The effect can be disabled entirely as well, in which case the lens is sharp. These modern soft focus lenses and their effect on the images should be considered distinct from the effect of lenses designed to render smooth bokeh using an apodization filter, such as the Minolta STF 135mm f/2.8 [T4.5]. As described in U.S. Patent 4,124,276, realized as the Minolta Varisoft Rokkor, a modern example with variable spherical aberration is a six-element, five-group lens which can be divided into three composite lens groups, marked A-I, A-II, and B. The first four elements (A-I, closest to the object being photographed) are moved as a unit to focus the lens, increasing the meniscus-shaped air gap between A-I and A-II as the lens is focused closer. The air gap distance between A-II and B is used to control spherical aberration; spherical aberration increases as the distance between A-II and B grows. In the patent summary, the inventors noted the object side lens group (A-I) was a Tessar design, although they added that any suitable lens would do, such as a Cooke triplet or Double Gauss lens; the preferred embodiment uses two meniscus lenses on the image side to control spherical aberration.

As an alternative to variable element spacing, some soft focus lenses such as the Rodenstock Imagon use interchangeable sieve aperture "grid" or "diffusion" discs which have a perforated annular shape to control the balance of light recorded between the relatively well-corrected center of the lens and the uncorrected periphery. As light from the periphery is restricted, the soft focus effect becomes less pronounced. Effective aperture values (for computing exposure) range from f/5.8 to f/11.5.

Examples Some examples of soft focus lenses, including modern (after 1950) lenses with controllable amounts of spherical aberration, include:

Notes

Thambar

In 1935, Leitz released a soft-focus lens, the Thambar 90mm f/2.2, for the Leica rangefinder cameras. It was supplied with a special filter to block light through the center of the lens, resulting in the image being formed by relatively uncorrected aberrations through the periphery. It was made in small numbers, no more than 3000 units, and is a rare collector's item today.

See also Rodenstock Imagon Seibold's Dreamagon Aberration in optical systems Lens (optics) Special effect Bokeh Convolution Depth of field Low-pass filter Gaussian blur

References

External links

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