Soft-Focus Lens Examples

[pjd]

[QUOTE=arri;1625060]

Single lens elements are not colour corrected which means that I have to reduce the found sharpness of the groundglass by a formular like this:
(extension²)/50xf = (350mm²)/50x250mm = 122500/12500 = 9.8mm
When I use this correction I get a very good focus to a point I prefer, otherwise it is impossible to use this lenses.
You can use this formular as well when a lens has two lens elements made of identic glass, like a Rodenstock Bistigmat or single lens element sets, some of it are named "Meteor" or the Busch Vademecum.
In the same way you can build your own lens with two single close up lenses, no other way to get a cheaper soft focus lens.

Interesting to read, thanks. I got hold of a 13x18 Bistigmat (last week, still untried) and the barrel extension setting has me thinking some experimentation is needed. Maybe that extension was fixed using this formula?

I couldn't get that link provided later to open on four browsers, incidentally.

[arri]

Interesting to read, thanks. I got hold of a 13x18 Bistigmat (last week, still untried) and the barrel extension setting has me thinking some experimentation is needed. Maybe that extension was fixed using this formula?

I couldn't get that link provided later to open on four browsers, incidentally.[/QUOTE]

Rodenstock sold the Bistigmat in two version, the more simple has a fixed barrel and the second has an outside tube with a shifting one inside.
You have to focus the lens with the larger extension and after this you have to push the lens backward and turn it into the groove.
So you are right, Rodenstock solved the problem with this lenses in a mechanical way.
Here it is usable from infinity to a ratio of ~ 1:10
The 13x18 Bistigmat has a focus length of 210mm I guess and so the focus shift compensation should be 4.2mm

When you want to go more in the direction to close up it will nescessary to correct more than this 4.2mm because of the longer lens extension in close up work.

[pjd]

Interesting to read, thanks. I got hold of a 13x18 Bistigmat (last week, still untried) and the barrel extension setting has me thinking some experimentation is needed. Maybe that extension was fixed using this formula?

I couldn't get that link provided later to open on four browsers, incidentally.

Rodenstock sold the Bistigmat in two version, the more simple has a fixed barrel and the second has an outside tube with a shifting one inside.
You have to focus the lens with the larger extension and after this you have to push the lens backward and turn it into the groove.
So you are right, Rodenstock solved the problem with this lenses in a mechanical way.
Here it is usable from infinity to a ratio of ~ 1:10
The 13x18 Bistigmat has a focus length of 210mm I guess and so the focus shift compensation should be 4.2mm

When you want to go more in the direction to close up it will nescessary to correct more than this 4.2mm because of the longer lens extension in close up work.[/QUOTE]

That's great, thanks. The lens I've got is about 21cm focal length and has the adjustable barrel. The barrel has about 4mm of travel, as you calculated using that formula. Would that formula work with a single cell, or isn't that necessary?

[arri]

That's great, thanks. The lens I've got is about 21cm focal length and has the adjustable barrel. The barrel has about 4mm of travel, as you calculated using that formula. Would that formula work with a single cell, or isn't that necessary?

You need this correction formular when you use a single meniscus element or with a two elements lens when it were made of two identic glass types, like the Bistigmat.
This optical systems are not colour corrected and this is the reason for the focus shift.

An achromatic system, like the Rodenstock Imagon has enough colour correction to avoid a focus shift.

The single cells of the set lenses don´t need correction because this once are colour corrected as well, like the cells of a Dagor, Protar, Eurynar, etc.
Here are two glass types in use, crown and flint, and with it a colour correction were possible.

What you can get with this lens cells is a focus shift by the closed aperture but you see it when you check the sharpness again when the lens is stoped down.
This focus shift has another reason and has nothing to do with a bad colour correction.

[Bob Salomon]

You need this correction formular when you use a single meniscus element or with a two elements lens when it were made of two identic glass types, like the Bistigmat.
This optical systems are not colour corrected and this is the reason for the focus shift.

An achromatic system, like the Rodenstock Imagon has enough colour correction to avoid a focus shift.

The single cells of the set lenses don´t need correction because this once are colour corrected as well, like the cells of a Dagor, Protar, Eurynar, etc.
Here are two glass types in use, crown and flint, and with it a colour correction were possible.

What you can get with this lens cells is a focus shift by the closed aperture but you see it when you check the sharpness again when the lens is stoped down.
This focus shift has another reason and has nothing to do with a bad colour correction.

The Imagon shifts focus with every change of disks and openings.

[arri]

Yes, you are right but this not the colour focus shift, it is the consequence of the closed aperture.
The Imagon is a two element achromatic system, made of two different glass types, it is not 100% colour corrected but good enough to avoid this focus shift.
That´s the information I have about this lens and I never have had a very large shift. I made a few das ago some table top shots with a 300mm Imagon and I don´t corrected anything and the pictures are correct focused.

[Bob Salomon]

Yes, you are right but this not the colour focus shift, it is the consequence of the closed aperture.
The Imagon is a two element achromatic system, made of two different glass types, it is not 100% colour corrected but good enough to avoid this focus shift.
That´s the information I have about this lens and I never have had a very large shift. I made a few das ago some table top shots with a 300mm Imagon and I don´t corrected anything and the pictures are correct focused.

The instruction book supplied with the Imagon specifically warns about the focus shift and states that focusingshould only be done at taking aperture.

[arri]

The instruction book supplied with the Imagon specifically warns about the focus shift and states that focusingshould only be done at taking aperture.

I see, we are talking about the same.

A colour focus shift will be reduce by stopping down, when I used my 9x12cm Bistigmat at oo and stoped down to f/22 it makes no different to compensate the shift or not, everything is in the DOF.

The other focus shift, the Imagon has, getting worse while stopping down the lens.

The reason for this phenomenon is an over or under corrected spherical abberation.

An achromat, like the Imagon, can´t be spherical corrected and a lot of lenses have this problem, the most of the Dagor type lenses f.ex. and its single cell even more.

The first anastigmat lenses were very good corrected, coma, colour, anastigmatism, nearly perfect but in the consequence of a worst spherical aberation.

The later plasmat style lenses don´t have this problem not anymore because the designer has had more ways to correct every optical foulds.

Also I never saw this problem when I use a four element dialyt system, this lenses are crisp sharp with all stops and with no focus shift.

The problem with this lenses were the 8 air/glass surfaces which makes much more problems with internal reflexions.

Like my father always said: all good things are never together

I hope I explained it understandable, English is not my native language.

[Jim Noel]

Try again. Google Chrome works best for me. Yesterday there were some problems with the entire internet.

[arri]

The Swiss made E.Suter Stellar 5/160mm, @f/5.0, 4x5", Fomapan 100 in Adox Fx-39 II
It is nearly impossible to find any infomation about this this. Some say it is an anastigmat but I can´t believe it.
It has a really unique kind of rendering when I use the full opened aperture.