From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 2020-01-27 10:15, Steve wrote:   
   > This may sound like a strange question, but is it possible to add an external   
   > aperture stop to a fixed lens to increase it's depth of field?  If so, how   
   would   
   > you do it?   
   >   
   > The reason for this question is I have a very old Imacon Precision II film   
   > scanner.  I want to scan mounted slides with it, but when they made this   
   > scanner, they used an 8W high CRI fluorescent tube and had to get a fast lens   
   > because the light source was limited.  This resulted in a shallow depth of   
   > field, and you can't get good edge-edge sharpness with a mounted slide   
   (because   
   > of the curvature of the film).   
   >   
   > I can make a new light source using high CRI LEDs, and stop down the lens,   
   but   
   > I'm not sure how to do it so the lens sharpness isn't reduced.   
   >   
   > Thanks in advance.   
   >   
   > Steve   
   >   
      
   Once the aperture gets small enough that diffraction is important,   
   there's a tradeoff between DOF and resolution.  On the plus side, you   
   win DOF quadratically as you relax the resolution.   
      
   1970s-era Kodak slide projectors cheat by matching the field curvature   
   of the projection lens to the average shape of a free-mounted slide.   
   (Fairly brilliant actually.)  European projectors such as Leitz and   
   Rollei expected you to use glass-mounted slides, and so had a flat field.   
      
   So if you have only a few slides to scan, you could remount the slides.   
   There's some danger of spoiling them, of course, and you have to worry   
   about Newton's rings from the multiple reflections.   
      
   Aperture stops need to go at the pupil of the lens.  Some lenses have   
   pupils outside the outer glass surfaces, but most don't, so this is   
   probably pretty hard.   
      
   On the other hand, it might be worth experimenting with an iris   
   diaphragm right against side of the lens with the longer conjugate and   
   see if there's a useful tradeoff of DOF vs. vignetting.  A bit of black   
   Sharpie on the outside surface is a reasonable poor-man's version--it   
   comes off easily with a bit of methanol and won't leave a residue.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
   Optics, Electro-optics, Photonics, Analog Electronics   
   Briarcliff Manor NY 10510   
      
   http://electrooptical.net   
   http://hobbs-eo.com   
      
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