peter brooks
10-Oct-2021, 11:36
Definition by Merriam-Webster -
Actinic focus: the focus at which the chemically most effective rays as distinguished from the visually most effective are brought together (as by a lens).
As I understand it meniscus lenses and semi-achromats (and possibly others) are affected by this lack of coincidence of the chemical and visual rays, and this can cause incorrect focus in the captured image if no correction is applied.
Traditionally the amount of correction required was said to be 1/40th of the focal length at infinity, but more at closer distances.
I just came across an ingenious device in the reprint of the 'Journal of the Photographic Society, Volume 1, 1854', described in a letter to the editor. The correspondent states that he finds 1/30 to be the most satisfactory adjustment and says that he 'constructed a very simple and inexpensive instrument to determine at once this difference, at whatever length the focal distance of the visual rays might be found'. The device is 'based on the principle of a proportional compass'.
He goes on to describe it - he obtained a 30" flat rule about an inch wide, which he split into to two half-inch wide pieces. He then fastened them together with a thumbscrew at the 15" mark (creating an 'X' shape), and cut 1" off both pieces of one half. The longer side he marked V and the shorter C.
He focuses, then sets the V side of the compass to be the distance between ground glass and lens. Reversing the compass to the 'C' side he then racks in the lens to the shorter distance.
This got me wondering, apart from meniscus lenses, which other types (and models, if you can be bothered) of lens would require actinic focusing? Are any of the 'modern', soft focus lenses affected? (Is this the reason for the 'focus on the tip of the nose' advice given for the 12" Kodak Portrait?)
And how does this apply to different media? Isochromatic films yes, what about ortho, x-ray, other modern films and paper negs nowadays?
Lastly - the use of a yellow filter is said to be an alternative correction for the coincidence of the rays, does anyone use a yellow filter specifically for this purpose, and how effective is it? (Yes, the obvious answer is 'try it and see' but I'm interested to hear the experiences of others).
(I'll have to stop reading those Journals and the like, I think I'm going all Victorian.. :) )
Actinic focus: the focus at which the chemically most effective rays as distinguished from the visually most effective are brought together (as by a lens).
As I understand it meniscus lenses and semi-achromats (and possibly others) are affected by this lack of coincidence of the chemical and visual rays, and this can cause incorrect focus in the captured image if no correction is applied.
Traditionally the amount of correction required was said to be 1/40th of the focal length at infinity, but more at closer distances.
I just came across an ingenious device in the reprint of the 'Journal of the Photographic Society, Volume 1, 1854', described in a letter to the editor. The correspondent states that he finds 1/30 to be the most satisfactory adjustment and says that he 'constructed a very simple and inexpensive instrument to determine at once this difference, at whatever length the focal distance of the visual rays might be found'. The device is 'based on the principle of a proportional compass'.
He goes on to describe it - he obtained a 30" flat rule about an inch wide, which he split into to two half-inch wide pieces. He then fastened them together with a thumbscrew at the 15" mark (creating an 'X' shape), and cut 1" off both pieces of one half. The longer side he marked V and the shorter C.
He focuses, then sets the V side of the compass to be the distance between ground glass and lens. Reversing the compass to the 'C' side he then racks in the lens to the shorter distance.
This got me wondering, apart from meniscus lenses, which other types (and models, if you can be bothered) of lens would require actinic focusing? Are any of the 'modern', soft focus lenses affected? (Is this the reason for the 'focus on the tip of the nose' advice given for the 12" Kodak Portrait?)
And how does this apply to different media? Isochromatic films yes, what about ortho, x-ray, other modern films and paper negs nowadays?
Lastly - the use of a yellow filter is said to be an alternative correction for the coincidence of the rays, does anyone use a yellow filter specifically for this purpose, and how effective is it? (Yes, the obvious answer is 'try it and see' but I'm interested to hear the experiences of others).
(I'll have to stop reading those Journals and the like, I think I'm going all Victorian.. :) )