Sharpness

[bwaysteve]

What aperture would be most likely to deliver optimum sharpness with a 50mm or 35mm Summicron(Canadian).I want to shoot some portraits in even, available light(open shade) on Delta 100 to make digital negs for platinum prints.

 

[rogerchristian]

The rule of thumb is that 2-3 stops down from maximum aperture is about where sharpness peaks. At smaller apertures, diffraction from the diaphragm blades begins to degrade sharpness, wider may bring up other issues, depending on the lens.

The only direct evidence of this rule I have is in enlarging, seems to hold true most of the time, @ 2 stops or so from max aperture, and with a grain focuser, it can be easily observed.

 

[hans voralberg]

My Cron DR peaks at f5.6, most lens does that around f4 to f8

 

[payasam]

With nearly all lenses, f/8 and f/5.6 give the best results. Those are probably what you will use in open shade with an ISO 100 film.

 

[mfogiel]

I'd suggest to try apertures like 2.8-4.0. The modern lenses from Zeiss for example, tend to be at the sharpest in the center already 1 stop down from full open, and while the corners tend to peak at 3 stops from fuull open, the center actually resolves less already.
However, I'd say that beyond the f stop the two important elements to watch are: camera shake and precise focus, and this actually makes a bigger difference than the lens itself. Therefore, if you want to use a slow film in open shade, I'd say try f2.8 and a solid tripod, plus do multiple shots, bracketing the focus - keep it in mind you should not de-center the "sharp" eye after focusing, as this will cause a focus shift - the only option would be to use a rail and move the camera sideways after focusing, as per Ferider's advice.
Finally, try FX39 or Rodinal for optimal acutance of your negative.

 

[waileong]

Check Erwin Puts book.