I don't know the exact meaning of that sentence, but I do know that soft focus lenses are very different one from another in the way they render the subject.
Under diffused light the difference is not always easily visible, but under the right lighting it becomes quite apparent, even to an untrained eye.
This is due to the simple fact that the various lens makers used completely different approaches.
The SMC Pentax 2.2/85mm is an achromatic doublet, in practice it's an Imagon that has been reversed (diaphragm is behind, not in front of the objective) and stripped of the "tea strainer".
The Fuji soft focus lenses have a more elaborated design, and the strainer disk has to be inserted inside the lens. Unlike the Imagon, the disk is not adjustable to two different values. All the Fuji SF optics have the same design, across formats.
The Pentax F and FA 85mm share the same optical design, but the FA has a smarter way of dealing with the difficult task of correctly focusing an optic that is by definition "soft focus". The optical layout is rather complex: 5 elements in 4 groups.
I also own a very strange bird, the Pentax-FA 2.8/28mm Soft Focus. It is made of 5 elements in 5 groups and has an optical design that reminds of some very early retrofocus objectives of the fifties, revised, more complex and faster.
The 120mm for the Pentax 6x7/67 is a slightly simpler design (3G/4E), but has a completely different optical approach from the Imagon and the various triplets. There is an interesting university degree thesis that centers around the design and the peculiar rendering of different soft focus projects. For the record, the author criticises Pentax's approach and concludes that the simple Imagon design is still the best. If anybody is interested, I can provide the link or at least the title. It's in english.
All the various Pentax SF objectives control the amount of halation through the aperture. The closer the diaphragm the sharper the result.
The Rodenstock and Fuji approach is more refined, there are both the strainer disk and the diaphragm, and both can be used to control spherical aberration. Some very old SF lenses used chromatic aberration instead, but the advent of panchro film made them obsolete. Photographers who shot with those lenses had to juggle a lot to get correctly focused images. I spare you the details....
Those wanting to experiment on the cheap would find that even a simple 2-elements diopter lens is an achromat... and can be used as a simple SF lens without getting too crazy. The larger the diameter of the diopter, the better it is.
Many legendary soft focus lenses of the past had a specific ring that moved one element inside the lens, affecting the degree of spherical aberration.
One example is the pre-war Meyer Trioplan 270mm, that has the same basic Cooke triplet design of modern Trioplan's, but the central element could be adjusted to dial in the right amount of softness. The same way, all the legendary Cooke Portrait lenses had a similar softness ring, which controlled either the central or back elements of the triplet.
The only modern time objective with a dedicated softness ring is the Tamron 70-150mm Soft Focus. It's rare, expensive, and has the greatest level of control of the level/nature of halo. The use of both the SF ring and the diaphragm allows for a very fine adjustment of the rendition.
I have two of them, cause yes, I like that optics as much! :)
The last one sold for $735 on eBay, and when I did the search I couldn't find a single one on sale.
It would make sense to sell one and dedicate the money to some other vintage lens I dream of... but I am hesitant because of course I have only one example with me, and for some reason I didn't use it at all recently, so at least before selling I want to use it for a while.
Soft focus lenses have never been cheap, but some were exceptionally expensive. Karl Struss, the same man who won the first Academy Award for movie photography, designed some years before the Struss Pictorial Lens. It was a hand-ground SF aspherical meniscus lens for portrait photographers.
Grinding by hand an aspherical profile from large glass blanks was a very complicated affair. Any lens was slightly different from the other, and many were rejected. It was so expensive to produce, that the quartz (yes, from a huge crystal of quartz!) version had almost the same price of the optical glass one.
With the same amount of money you could either buy a large Struss, or 3 (three!!) Ford Model T :) :)
Btw, the second wave of american pictorialism, and the birth of the Hollywood movie industry, called for more high quality, crazy soft focus lenses, so after the Struss, a new quartz lens was made, the Hanovia Kalosat.
These lenses, along with the slightly cheaper Pinkham & Smith, were my objects of desire when I was actively collecting large format soft focus lenses.
When I definitively turned to digital-only I realised it was a shame to buy lenses that I would never use, so I gave up. Btw, most of those I don't have are hugely expensive and I would never, ever have enough funds to buy them :)
I hope my little digest sparked some curiosity. If you want to learn more about the fascinating world of SF lenses, Internet can provide much food for thoughts, and many gorgeous portraits of Hollywood starlets of the twenties, shots with the kind of objectives I just mentioned, and many many others.
The level of technical competence, ingenuity and finesse of the photographers who actually shot those portraits frankly puts to shame the vast majority of present time professionals.
I have been so amazed by those masters of black and white that I checked on Google Books the availability of photographic books of the time.
Well, photography required a good level of wizardry, and the iconographic level of the images was impressively high.
Hats off.
EDIT:
I often find on the Web the theory that at present time soft focus lenses have no reason to exist, because the original image can be tweaked in post production.
I don't agree, and I think that most of those who back this opinion have never seen a beautiful print of a picture shot with a good soft focus lens.
A nice soft focus image has nothing to do with a blurry or poorly focused one.
A proper SF lens provides a sharp picture superimposed with a certain amount of halo, that "bleeds" from highlight areas into mid-tone areas.
It would take plenty of digital wizardry to get the same results in PP.
