Focus Shift in Classic Lenses

[jeff_buckels]

I just looked at an auction for a 14" G. Dagor on Ebay. Not

interested in the lens, but the seller noted that the focus shifts

as you stop the lens down so "all critical focusing must be done at

f16 at least" etc. and that this is the way it is "with all classic

lenses." I don't believe this is true. Granted I'm not sure what

the guy means by "classic lenses", but this one is hardly a dinasaur

(coated lens in a Copal No. Something). Anyway, I don't think it is

true that all old lenses have focus shift. To my knowledge and

according to my experience, my real old uncoated 300mm Imagon has

focus shift (when you change from one disk to another), but I don't

believe any of my other lenses do and the newest of these is a

Wollensak (single-coated) Ia Convertible. I've been testing an

uncoated Zeiss Dagor 12" (vintage comparable to that old Imagon --

probably a little older). I focused wide open and shot closed all

the way down to f45. The negatives are sharp. Comments? -jb

[Troll]

Dagors and Protars have a reputation for significant focus shift, but I have 3 Dagors (120 and 150mm Berlin, and 8 1/4" American) none of which seem to show the problem. I wonder if it's related to incorrect spacing of this particular Dagor when it was remounted in the Copal #3? I also have a Staeble casket set (105, 135, 165, 195mm) every lens of which has +1/16" shift when stopped down, but if the rear element is used alone at 220mm there is a -1/16" shift. Turner-Reich elements were also well know for focus shift (see AA's description of making "Moonrise").

[jason_greenberg_motamedi]

Due to its design, the Imagon will have a focus shift, since it relies upon

spherical abberation for its soft effect.

 

Like the response above, I have never noticed a focus shift in my Dagors. I think that

the problem with focus shift in older lenses may, in addition to poor spacing, also

have to do with imperfect centering of the elements.

[nick_morris]

Dagors are convertible lenses. I may not be totally correct about this, but I believe the focus shift occurs when the front element is removeded, and only the rear element is used, converting the lens to a different focal length. If you focus with the apeture wide open, and then stop down, some focus shift will occur, and require re-focusing at the selected apeture. It may be if you stop down enough, the shift is not noticable. I use the rear element of my Wollensak 1a occasionally with a no. 15 filter, and will try to refocus, if there is enough light. But I usually stop down to 45 or 64 anyway. If you do a search on "focus shift" you will probably find some discussion on the subject.

[ernest_purdum]

Rudolf Kingslake in " A History of the Photographic Lens" says in regard the Dagor "it did suffer from a serious amount of zonal spherical aberration, which arose at the strong dispersive interface, with the result that the position of sharpest image tended to move along the axis when the lens was stopped down, a phenomenon known as focus shift. Nevertheless, the lens has always been populaar with photographers, both professional and amateur."

[dhananjay_n]

As pointed out above, focus shift is the result of uncorrected zonal spherical aberrations. Spherical aberration results from the fact that rays of light from the periphery of the lens are brought to a focus at a different point than rays of light from closer to the optical axis. As you stop down, you eliminate the rays from the periphery of the lens and this zone of focus shrinks. The actual point of best focus doesn't really change, it is just overlaid with out of focus rays from the periphery of the lens and this leads to a larger zone of focus and an apparently better focus - it's partly an optical illusion. If you judge the point of best focus by contrast, you are likley to experience focus shift.

 

Its not a surprise that your Imagon shows focus shift - the soft focus relies on sperical aberration for its soft effect - a sharp image overlaid by a softer image from the out-of-focus (at that point) rays from the periphery of the lens (the disks control the softness by controlling the amount of light from the periphery). Dagors do have a large amount of uncorrected zonal spherical (nad when used converted, the single cells also suffer from coma - the coma is automatically corrected by the symmetry when using both elements). The single cells of Protars are corrected for coma which explains their better performance when used converted. Single cells of Dagors need to be stopped down quite a bit for good performance.

 

Focus shift has more to do with the design of the lens than being classic or modern. Artars, which are the same vintage as Dagors (Emil von Hoegh designed the Dagor in the 1890s and the dialyte, which is the basis for the Artar some time later), show no zonal spherical. More modern designs such as the Imagon, which rely on spherical, will also show focus shift.

 

Cheers, DJ