Soft Focus Dagor?

[Bill_1856]

Does shooting wide open with one element of a Dagor produce true "soft-focus" effect?

[Scott Davis]

I'd think you'd have better luck with just slightly unscrewing the front or rear element group, kinda like the way the old Dallmeyer portrait petzvals or the Vitax did it.

[Mark Sawyer]

Not really. A wide open Dagor has a very small amount of spherical aberration, (the agent of softness in most true soft lenses), but it's not nearly enough to qualify it as "soft focus".

[ridax]

Changing the distance between a lens halves does not change the picture the same way as changing the space between certain elements of a Petzval or a Tessar or Heliar or Celor. To achieve a similar effect, one should change the position of the elements that in a Dagor are actually cemented together. Not too practical.

A half of a double anastigmat used alone usually makes the image somewhat softer then that of a complete lens, but certainly not the same as a meniscus or an Imagon image.... Though perhaps nobody was ever authorized to judge which of those was a "true" soft focus image and which were "not true enough".

[goamules]

I'd think you'd have better luck with just slightly unscrewing the front or rear element group, kinda like the way the old Dallmeyer portrait petzvals or the Vitax did it.

This won't do anything, it's a symmetrical lens.

[ridax]

This won't do anything, it's a symmetrical lens.

Again: No changing the distance between the halves would not do it. But NOT because of the lens being symmetrical. A Celor is also symmetrical, but changing the space between the 1st and the 2nd element or between the 3rd and the 4th element does this OK - just like changing the space between the 1st and the 2nd element in a Tessar or a Heliar. And changing the distance between a tessar's, a Heliar's or a Petzval's halves does not do it either.

[goamules]

OK, to be more clear; Scott's question about slightly unscrewing the front "group" made me think of using just one group in a Tessar, I wasn't paying attention to the spacing idea yet when I answered him.

And of course his premise was mistaken, you unscrew an element and change spacing within the group. With a Dagor it won't work because it's symmetrical with only 2 groups and because each group is totally cemented.

[Dan Fromm]

Um, er, ah, changing cell spacing changes focal length and, up, upsets the corrections.

[E. von Hoegh]

Dagors don't like to have the spacing messed with once it's correct.
The OP asked about using one cell, I've tried this and wasn't impressed in any positive way with the results. Too sharp to be soft focus, not well enough corrected to be a real convertible lens. A single cell would be OK for contacts, though. Maybe.

[desertrat]

From what I've read from Kingslake on the subject, changing the cell spacing on a Dagor has little effect on the central area of the field but upsets the corrections considerably near the edge of the field.