[rollei_list] Re: Hello again, and a lens question

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Fri, 11 Dec 2009 13:48:34 -0800


----- Original Message ----- From: "Stephen Attaway" <stephen@xxxxxxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, December 11, 2009 12:48 PM
Subject: [rollei_list] Re: Hello again, and a lens question


Hi Frank:

That elusive 3-d quality used to be called 'plasticity'. I believe it can be caused by a lens which is well corrected for aberrations in the center but shows increasing aberrations near the edges. The effect is to overlay a sharp central image over a softer one covering the entire field. If you think like a designer who could not fix this, due to limits of materials and computational power...you might try to optimize the results. The effect of unsharp masking was known and in common use at the time.

To quote from Ivor Matanle: "The portrait of Mr Robert Cox was taken in 1974 to mark his fifty years of service to his company ... with a 50mm f1.5 uncoated Zeiss Sonnar, and shows perfectly the curious characteristic of defined yet slightly unsharp detail that makes the optical quality of classic lenses so attractive."

I believe the tele-xenar was a relatively simple four element design. I'd bet its pleasing optical quality is due to the designer aiming for plasticity, using experience and a few ray tracings as a guide.

Most soft focus lenses make use of uncorrected spherical aberration. Spherical exists everywhere in the field but is more pronounced as one moves away from the optical axis. It is obvious that light travelling exactly through the axis of a lens is not deviated so is not aberrated. Spherical is the main aberration in a simple magnifier and comes from the inability of a simple spherical surface to form a sharp image. The effective focal length of the lens varies from center to edge, becoming shorter as one moves toward the edge. The result is a continuum of images all superimposed on one another. So, such a lens has no definite point of sharp focus. As the lens diameter is made smaller by stopping down the image becomes sharper because the difference in angles through which the light is bent is reduced. Spherical aberration is corrected by using combinations of lenses to give the effect of a non-spherical surface. Of course photographic lenses and other lenses such as telescope and microscope lenses must also be corrected for other aberrations. There are seven basic aberrations, sometimes called Seidel aberrations after the mathematition who first enumerated them. They are all corrected by the arrangement of surface curvatures, element thickness and spacing, and choice of glass types. The Cooke Triplet lens, invented by Harold Dennis Taylor, is the simplest lens in which all of the fundamental aberrations can be simultaneously corrected. There are orders of aberrations, a simple lens like the Triplet can be corrected for the first order aberrations but still have higher order ones. These require a more complex structure to correct. The use of aspherical surfaces can also help with some corrections but is not magic, that is, the effect of an aspherical surface can duplicated by the use of several spherical ones, so that the effect of their use is to simplify a lens with a given level of correction. Spherical aberration gives the effect of a soft focus image with a core of sharpness. It also gives the effect of a slight fogging due to the unfocused light. A good soft focus lens produces a halo effect around bright highlights. Other aberrations can be quite unpleasant. The most common one is coma. Coma is a sort of spherical aberration but one which is proportional to image angle and produces a tear drop shaped smear from a point of light. Tail of the smear can point either toward the center or to the periphery of the image depending on its "sign" and where it is coming from in the lens. Coma, like spherical, is proportional to the stop. Some lenses have a tendency to have coma or another aberration called oblique spherical aberration, which looks similar. Such lenses can be quite sharp in the center at large stops but have smeared images as one moves away from the center. A Tessar is such a lens. Other types have less coma, an example being the classic Dagor. The symmetry of the Dagor reduces the coma considerably. OTOH, the Dagor has lots of spherical. This is far from a comprehensive discussion of aberrations, I just wanted to point out what was considered useful as a soft focus lens.

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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