f-numbers of lenses in Epson V700 and V750 scanners

Discussion in 'Digital Darkroom' started by alan_rockwood, Jul 5, 2015.

  1. Can anyone point me to where I can find information on the f-numbers of the lenses in the Epson V700 and V750 scanners?
    I understand that there are two lenses in each scanner, and that the two are different. I am taking a wild guess that they might be somewhere in the neighborhood of f/8 to f/11, but that is only a guess. It seems unlikely to me that the f-number would be much higher than f/16 because diffraction would start to soften the image, and much less than f/5.6 would start to require more complex and expensive lenses.
     
  2. The two lenses are for the two modes of the scanner. One is for flatbed functions (focus set on surface) and the other is for transparencies (focus set above the surface). What ever mode the scanner goes into selects the appropriate lens.
    Why do you need to know the speed of the lens? I would think diffraction would be the least of any problems related to scanning film; getting a good focal point usually is.
     
  3. Peter,
    The reason has to do with an improvised fluid scanning method I am thinking about. The idea would be to fluid mount a block of acrylic to the glass of the scanner bed, and then fluid mount the film on top of the acrylic block.
    The acrylic block would be to raise the film above the glass by the appropriate amount. The appropriate height would be determined by experimentation. However, if there is air between the glass and the film then the appropriate height is somewhere in the neighborhood of 3mm. (The exact amount needs to be determined by experimentation, and probably varies between one example of the V750 scanner and the next. That's why they supply the scanner with height adjuster feet for the film holders.)
    If we assume for sake of discussion that the correct height is 3mm in air, then the correct height if the film is spaced using a transparent acrylic block is 3mm multiplied by the refractive index of the block, which in the case of an acrylic block would put it very close to 4.5mm thick, assuming the refractive index of the acrylic to be about 1.5. (It's actually closer to 1.493, but 1.5 is "close enough for government work.")
    The issue of f-number of the lens comes into play because the thick block of plexiglass will add some spherical aberration. I can calculate the amount of spherical aberration using WinLens, but the amount depends on the acceptance angle of the lens, which basically is determined by the f-number of the lens. (There's a bit more to it than this, being dependent on conjugate ratios, but the discussion is already becoming a bit complex, so for simplicity let's not go there.) I calculated (if I remember correctly) that if the f-number of the lens is above f/4 then the amount of spherical aberration caused by the thick block of plexiglass is well under the diffraction limit and can more or less be ignored. However, if the scanner lens has an f-number much less than f/4 then diffraction may begin to soften the image noticeably, assuming that the lens design is good enough to be diffraction limited.
    So, the short answer is that if I can know the f-number for the transparency scanner lens in the scanner I can do a feasibility calculation to see if this wet-mounting scheme has a reasonable chance of working. If so, then it could simplify life greatly by doing away with complicated and expensive fluid mount film holders.
    I should add that the transparent block will also add some chromatic aberration to the system, but that is a topic for another time.
     
  4. Alan: When set to 8x10 or "Film with area guide" the V7xx scanners will focus on the glass surface, not above it. No need for the acrylic block.
     
  5. Les, a quick question, does it use the high resolution lens when set to 8x10?
     
  6. Alan, I almost exclusively wet-mount on my V700. I mount the film between some acetate and glass. When the acetate side is bottom down, I get clear sharp scans. When the glass side is down, I have problems getting a sharp scan. Focus was adjusted correctly for both tests.
    All I can see happening from what you are trying to do, is to scan through a fog and a big reduction in sharpness.
    Les: I did not know that. I would make it easier to wet mount some 4x5 right on the scanner bed.
     
  7. The second lens that is supposed to focus at 3 mm off the glass is the one Epson claims is the higher resolution lens. It has a narrower field of view though and can't cover 8x10". People use my mounting station for 5x7" regularly so you aren't limited to 4x5" with this lens. Just like the second lens, the first lens that is supposed to focus at the glass may or may not focus right at the glass. Every scanner is different so you just have to test your unit which is why developed the variable height feature that Epson later implemented.

    Doug
     

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