How to induce spherical aberration with purpose ?

[Paul Schilliger]

This is not specific to large format, but it's a fun project and maybe someone will come with an idea. I like the effect I get when I attach a +1 diopter to my Nikon 2,8 80-200. Between 180 and 200mm at 2,8 to 3,5, I get a progressive diffusion effect and exceptional bokeh, even better effect to my eyes than what the Imagon produces with it's starred bokeh. I can obtain something approaching with the 300/4,0 as well. But I wish I could keep this effect with more distance to the subject than what the +1 diopter lens allows for on digital, which is no larger than 5" target, or obtain this with a shorter lens to have more angle, but it seems that only long range zooms produce this aberration. Of course I wished I had found a 0,25 diopter lens for my zoom to try first, but they have disappeared from the market in such sizes. The problem with SF lenses is that it's not possible to focus shorter than infinity, so I would probably need something else than the close up lens. I could perhaps buy a cheap midrange zoom and "reorganize" the elements if I knew what to do, but I have no idea how to get there. I remember reading from someone who had polished the front cell to change it's curve, but I am not equipped to do that. Would someone see another practical solution? But this is not the effect that can be obtained with diffusing or softening filters which I have also.

[Steve Hamley]

Paul,

The Universal Heliar produces diffusion by introducing spherical abberations. It does so by moving the center element which is part of the front cell and normallly very close to the iris, further away from the iris. I'd suggest trying something like a tessar design and find a way to move the center element away from the intended position.

Steve

[Ernest Purdum]

Do you have a bellows unit? These provide opportunity for lots of experimentation. You can adapt long focus lenses, particularly useful for portraiture. Some lenses are quite sensitive to a small inrease in cell spacing.

[Paul Schilliger]

Thanks for the suggestions. I guess there is place for experimentation there! BTW, I didn't know the Heliars were such lenses.

[Arne Croell]

Paul, only the Universal Heliars have this soft focus feature, not the regular Heliars - at the cost of being only available in barrel as the threads moving the center element are not compatible with a shutter.

[Paul Schilliger]

Just curious, are the Universal Heliars available in short focals, around 100mm ?

[Arne Croell]

No,

the shortest was 300mm.

[Paul Schilliger]

Way too long for a CCD! I remember now that the Super Angulon 5,6 used with a single element was producing something interesting. Not exactly what I am looking for though. But I doubt I could mount it on a digital SLR.

[Donald Qualls]

For a DSLR, you might look at lenses like Hastings triplets (sold as hand lenses for magnifying tiny flowers and the like), or try unscrewing the front element or group from a triplet or Tessar type lens and then adapting a mount to bring it back to focus (much longer, if only one element, or much shorter if the whole group removed). I've seen mounts made up from body caps and various pieces of tubing, even a bottle of headache tablets. Alternately, you could mount a front-element focused triplet or Tessar type of a "too long" focal length, remove the stop, and screw the lens *way* out; this will greatly shorten the focal length, but introduce considerable spheric aberration.

Watch out for focus shifts when you stop down a lens with a lot of spheric, though...