View Full Version : focus technique

Neil Purling
29-May-2008, 14:20
If I am using my Dagor you should:
a. Focus at taking aperture.
b. Focus at around f16 then stop down a little more if intending to use f22, 32 or smaller.
The reason being that any spherical aberration is about gone by then.

If I am using any of my collection of pre-anastigmat lenses should I focus these the same way as for the Dagor or should I use a slightly different technique?
There's a few which are all aplanats (rapid, normal and wide-angle) and a lone Petzval type.

A bad worker might blame his tools for poor results so I want to sharpen my technique if necessary & get the best out of these antiques.

Mr Moderator: Feel free to shift this thread if you feel this is not the right place for it.

Ole Tjugen
29-May-2008, 14:42
IF using any lens which is reasonably well corrected for spherical aberration, I would focus wide open and trust increased DoF at taking aperture to take care of any focus shift.

Even with "bad" lenses I find that it is fairly easy to tell the difference between sharp-and-will-stay-sharp and the seem-sharper-but-will-turn-to-mush "fake focus".

Aplanats are pretty good in general; I'm not so sure about landscape lenses, Periskops and Rapid Rectilinears!

The sharpest negative I have ever produced was with a 1905 Aplanat, focussed wide open and exposed at f:32...

Maris Rusis
29-May-2008, 16:19
Spherical aberration is an interesting image artifact that can yield attractive soft focus photographs but it imposes some constraints. These are the rules I follow on a 8x10 camera:

For shooting at a fixed aperture just tweak the focussing controls until the ground glass image is right and make the exposure.

If the lens is going to be stopped down after focussing then focus OUT for the fine DETAIL, stop down, and note there is no focus shift. Or focus IN for the best CONTRAST, stop down, and note the focus shift. One method is going to be ok depending on the final result desired.

I reckon those folks using soft focus lenses a hundred years ago knew stuff about focussing that takes a power of concentration to re-learn.

Mark Sawyer
29-May-2008, 16:28
I have three Dagors, and checked them all for focus shift at near and far distances. I found the object I focused on stayed in focus as I shut down the aperture, even down to f/45. It may be that the depth of field is gained more in front or behind, but I can't detect it, so I've stopped worrying about it with those lenses.

I have heard other people talk of focus shift on Dagor lenses, so I'd suggest at first you focus wide open, then double-check after stopping down. If after a few tries, you don't find a focus shift, you're good to go focussing wide open. BTW, always use a good loupe!

Neil Purling
30-May-2008, 00:58
If I am to use my wide-angle aplanats at f32 or even smaller then focusing, even with a dark-cloth is going to be nigh on impossible on a dull day.
I wondered what techniques the old books advocated when these lenses were the Latest Thing?

Are Dagors a lens where its reputation is greater than what results it delivers?
Maybe the Goerz design traded some sharpness to maintain it over the whole of the intended plate size. Who knows?

If I can improve my work by changing such a simple thing as the way I focus, then it is worth burning through a few sheets of film to fine-tune for using lenses of ancient design.

Ole Tjugen
30-May-2008, 01:43
Neil, all early anastigmats traded some sharpness in order to maintain acceptable sharpness over a wider field.

The literature of the time (ca. 1910) is pretty unanimous in stating that the best Anastigmats "are now approaching the resolution of a good Aplanat". :)

30-May-2008, 01:44
Are Dagors a lens where its reputation is greater than what results it delivers?
Maybe the Goerz design traded some sharpness to maintain it over the whole of the intended plate size. Who knows?

Dagor lenses didn't get their reputation for no reason, they were a well computed design in the days when the mathematics of lens design was long winded and complex.

A fairer question is how good are they compared to modern optics. I always assumed probably not until I acquired a 12" Am Opt Dagor with my first 10x8 camera. I'm lucky the original owner of my lens (&camera) was a lecturer at the Clarence White School of Photography and always wanted the best, so he returned the late 30's Dagor to Goerz for factory coating shortly after WWII.

The lens is a superb performer, very sharp across the whole field, contrasty, in practice the results are indistinguishable from those made with my modern German optics. If it was tested on an optical bench then probably you'd see a difference.