Blur Radius vs Blur Falloff ~ ISCO Iscotar 50mm f/2.8

The ISCO Iscotar 50mm f/2.8 is a Cooke formula, first element focusing, triplet lens.  I'm adapting these kinds of lenses to miniature format for soft focus work by using the first and second element separation control of the focusing ring to manage softness level.  Mated to a helicoid and setting the softness level on the focusing ring, the helicoid is used to effectuate the focus itself.

To show how first and second element separation effects softness level, I graphed Blur Radius against Blur Falloff.  Further, to control highlight "softness", I added a number of different Soft Focus filters to the investigation see to what degree they do their job. 

Here's how to read the following graphs. 

 

Comments ~

These illustrate how the focus ring on the ISCO 50mm can effect the level of softness in an image.  And we can see how Soft Focus filters add softness over the base optics.

Cooke triplets are bog simple, but very well corrected lenses.  I wondered if the German 35mm SLR triplets were derived from the same detailed design specifications, but they are not.  The ISCO is contrasty where the Steinheil is lower contrast but sharper.  

There are other German Cooke triplets, but they are unit focusers and it might be difficult to adapt them to soft focus work.  At this point I'm unable to find any Japanese Cooke triplets.  The closest I can find in first element focusers are tessar formula lenses (such as the Cosina 55mm f/2.8).  If someone knows of a cheap Japanese triplet first element focuser let me know, please.