CZJ Tessar - "critical f-stop"

[lbenac]

Hello,
I read in this forum a reference to Zeiss insiders by Peter K Peter K in a post from 2010.

>>>>>Reckmeyer, as I know a Zeiss staff member, coined the notion "critical f-stop", the f-stop a lens performes at it's optimum. The off-axis-abberations like spherical abberations are minimized and diffraction doesn't reduces the resolution in the center of the image circle. For a Tessar f/4.5 180mm he has measured the "critical f-stop" is f/12. In any case the resolution decreases in the center if a smaller f-stop is used but the resolution in the outer areas increases a little bit, the resolution in the whole image looks "smoother".<<<<

I could not find more details anywhere else.
I am curious to find similar theoretical information regarding other focal length.

Cheers,

Luc

[Ken Lee]

I can't speak to optimal resolution, but in the Tessars I've used, coma disappears at f/11.

[lbenac]

Thanks Ken.
I did find that under f11 the CZJ 250, 180 or 135 where a little bit soft anyway.

On a slightly different topic I have been thinking about your comments on aperture blades today...
I just acquired lens cells for a Kodak Agnastigmat 203/7.7 with a thread very close to Copal 1.
I mounted the cells on a Sinar DB board and I have been shooting wide open. I do really like the lens wide open but sometime, it needs a little bit of depth of field.
I shoot it yesterday at f11 using a lens hood. The bloody Sinar DB board gives horrible pentagons all over what would be otherwise nice round highlights - darn...

So I did order a couple of uncoated brass barrel lenses to see if I get the same look.
1) Kodak Agnastigmat 152mm/7.7 from Jim's Camera (dialyte)
2) B&L Zeiss Tessar 2b 5x8 (210mm)/6.3 from eBay (probably paid more than I should have but the flange size is 75mm just like my CZJ 250/4.5

Cheers,

Luc

[Ken Lee]

DB mounts have unusual apertures, but when I owned a Sinar shutter I noticed no difference between the blur rendition of lenses with circular apertures and those in DB mounts - except when the subject contained specular highlights.

For subjects with specular highlights it's very nice to have a lens with a circular aperture.

[lbenac]

except when the subject contained specular highlights. For subjects with specular highlights it's very nice to have a lens with a circular aperture.

Yes that is what I forgot to mention, this was for shots with specular highlights - flowers/bushes wet with dew in early morning raising light . I found that uncoated lenses seem to have a "nice" rendition for this IMHO.

http://www.lucbenacphoto.com/img/s12/v178/p1107172989.jpg

For general use, my very limited experience has been the same than yours, so I do not carry the heavy CZJ Tessar + Norma auto shutter around, only my small Kodak Ektars in Supermatic or Fujinons in Copal.

Cheers,

Luc

[Bill_1856]

The description "critical f-stop" sounds important, as though it defines the aperture which would give the "best" overall picture, but it's just another buzz-word, coined to justify the author's choice of parameters. As always, the aperture should always be chosen based on the appearance of the image on the ground glass.

[IanG]

The description "critical f-stop" sounds important, as though it defines the aperture which would give the "best" overall picture, but it's just another buzz-word, coined to justify the author's choice of parameters. As always, the aperture should always be chosen based on the appearance of the image on the ground glass.

It does depend on one's definition of "Critical", for most of my landscape images it's f22 with LF Tessar and other similar design lenses, wider than that and corner sharpness suffers. That's based on experience and also happens to be CZJ's recommendation for best overall sharpness.

Of course there are other reasons for using Tessars where sharpness is less critical and the falling of is benificial, portraits etc.

Ian

[ic-racer]

For 3-d objects, in many cases the best aperture falls at an aperture smaller than that ( see this: http://www.largeformatphotography.info/fstop.html ) .

The concept of critical f-stop is most valuable in enlarging. One can look at the MTF of the enlarging lens in question or use the popular rule of stopping down 2 or 3 stops.

I wouldn't call the critical aperture "theoretical" it is based on experimental data.

Looking at the ground glass to determine best aperture or critical aperture is difficult due to the very dim image at the edges when the lens is stopped down. Thus the need for MTF curves when doing 'flat' work or using focus spread when shooting 3-d objects.

[Liquid Artist]

Corner sharpness is again subjective.

I had someone drum scan some of my landscape negatives, and he said I messed up by having all the corners just a little soft. You can't tell unless your looking at a fair sized enlargment.
I didn't bother telling him that it was my intention, to help draw ones eyes into the photo and towards a subject.

If he was the one taking the photo it would have been a mistake, but for me it wasn't.

[Drew Wiley]

An optical engineer has his job to do, along with his own potential vocabulary, which is sometimes helpful to understand. But we also have our job to do, which
sometimes involves ignoring the technical parameters and just homing in on what looks good to us.

[IanG]

Corner sharpness is again subjective.

I had someone drum scan some of my landscape negatives, and he said I messed up by having all the corners just a little soft. You can't tell unless your looking at a fair sized enlargment.
I didn't bother telling him that it was my intention, to help draw ones eyes into the photo and towards a subject.

If he was the one taking the photo it would have been a mistake, but for me it wasn't.

Yes corner sharpness is subjective, but then so are many other aspects of an negative/image. What's far more important is knowing how your lenses performs and what you can expect from them.

I use quite a few Tessars (& type) from 50mm to 300mm and find them very capable lenses, sharp to th edges when you want them to be (by choice).

Ian