In looking at adapting a 1906 photographic lens blur calculation to small format lenses I came across something I feel worth sharing.
At equivalent blur radius, all blur is not created equal.
I shared a couple 18pixel blur radius images with friends. They noted that one looked softer than the other. So I went back and reviewed images I made when calculating blur radius under varying conditions.
Consider the following photographs. Both were calculated to have 18pixel blur radius using a 24mpixel digital sensor enlarged to 6.5inch x 8.5inch Whole Plate format.
Recall that 18pixel blur radius is what an old brass 450mm f/6 meniscus "portrait" lens is calculated to have. See: 1906 Biblioteque de la Revue de Photographie by L. de Pulligny - C. Puyo "Objectifs d'Artiste Practique et Theorie des Objectifs et Teleobjectifs Anachromatique", pages 26 and 90.
Upon closer examination of blur widths I found the "strength" of the blur fall-off varied by lens implementation. While I'm not certain that was the case in old brass lenses, it is with the lenses I'm attempting to yoke to the Expressionist/Pictorialist Task.
Using extreme examples, here's an ISCO Iscotar 50mm f/2.8 Cooke Triplet with the first element fully extended (as the softness controler) and focused using an adapter helicoid.
Here's a Steinheil Auto-Cassarit 50mm f/2.8, _also_ a Cooke Triplet, and _also_ with the first element fully extended (as softness controler) and focused using an adapter helcoid.
Examine the left most garlic bulb and notice how the ISCO blur strength is greater than the Steinheil's. Which led to the following understanding.
Comments ~
While it is possible to measure and graph the fall-off rate in a blur radius, the effect is readily apparent and I feel little would be gained by knowing _exactly_ the shape of the fall-off curve. Instead, the examples should clearly illustrate and sufficiently describe the effect.
As a first quick pass, calculating blur radius of any lens might not be sufficient to fully describe just "softness" as an optical phenomenon and feel that adding the blur radius fall-off shape will help firm up my understanding of optical softness expression.
Blur radius and blur fall-off are, of course, not the only things that contribute to "softness"in a photograph. To get to the meat of the matter I've ignored the roles of spherical aberration, coma, field distortion, and astigmatism, while only briefly glancing at the role of field of view. I will cover these in a future post.
----------- Essential Reading --------------
Dr. Russell Young's thesis paper on soft focus photography
Biblioteque de la Revue de Photographie - L. de Pulligny - C. Puyo "Objectifs d'Artiste Practique et Theorie des Objectifs et Teleobjectifs Anachromatique" - Biblioteque national de France (BnF) (in French)
----------- On Soft Filters --------------
Soft Filters in Cinematography
M. David Mullen, ASC on cinema and photography
----------- Additional Resources ------------
An illustrated guide to early soft focus lenses
Frederick H. Evans was actually a soft focus photographer
History of Dallmeyer optics (on the Dallmeyer 3b in use)
Neo-Pictorialism by Guy Tal
Objectifs de flou artistique (in French)
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