More lens transmissions

Discussion in 'General Technical Discussion' started by Chris101, Dec 2, 2005.

  1. Chris101

    Chris101

    Feb 2, 2005
    Arizona
    This time I looked at two fast Nikkor primes, and two older lenses.

    The primes are my 35mm f/2 and 85mm f/1.8. Not surprisingly, these two lenses are very similar in their spectrum, but the 85mm is a few percent dimmer than the 35mm. I put the lens right up to the detector, so the divergence should not have affected the spectrum. Maybe the 85, being a non-D has older technology coatings, I cannot really explain the difference. Note the drop off in the IR as it approaches 1000nm. Again, I don't know if this is the type of glass or the presence of a modern coating on the glass.

    As expected, the older lenses are better UV transmitters, because they are uncoated and have less glass elements. The Xenatar is the big winner, with 50%T at 350 nm! And it's the most transmissive in the IR as well, with a nearly flat (but not as flat as the modern Componon, from yesterday) transmission through the visible. The Raptar is better in the UV than either Nikkor, but not quite as good as the Xenatar. However, the Raptar will let me focus to infinity on the Mamiya bellows, but the Xenatar (which came off an old Rolliecord) is only good for close-up photos.

    Here are the graphs:
    [​IMG]
     
  2. So what do the wavelengths mean?

    That is, where does red begin, for example?

    I love cool instruments. Back when I slaved over a hot test tube in a polymer lab, I got to use (and program) some interesting machines. HPLC, etc.
     
  3. Chris101

    Chris101

    Feb 2, 2005
    Arizona
    300nm to 400nm is what Bjørn calls pictoral UV, that is where you see things like the dark 'landing strips' on flower petals, then 400 to about 430 is violet, blue is centered at about 460, cyan at about 480, green at 520, yellow at 580, orange 620, and red from about 640 to 710nm or so. The colors change gradualy, so it's hard to give exact cutoffs for each color. Infrared starts at 710 nm. Near IR is the portion of the IR spectrum that does not feel hot, say up to about 2000nm. The range of 700-950 is used photographically, because 1) image sensors only go that far, and 2) most modern glass is opaque above that wavelength. IR filters let that band through, but cut off the visible light.
     
  4. Chris, have you made any attempts to correlate these spectra with actual performance taking pictures?

    I'm very interested in your wine color studies too. Have you tested any very old wines? I've got some Bordeaux from the 1970's which look pretty brown but are still excellent where it matters most (!):smile: .
     
  5. nfoto

    nfoto Guest

    No wonder they recommended using UV filters on those older lenses :biggrin:

    Please do us the favour of putting your graphs on more readable scales, Chris. As they are presented the information is hard to assess.

    By the way, I did some transmission measures many moons ago which pretty much coincide with the curves you presented.

    [​IMG]

    So, if you want to get decent UV images, you should use a lens with decent UV transmission, and this tend to exclude most modern lenses :mad:
     
  6. Thanks for the info. Very interesting.

    At some point I'd love to find a way to accomplish UV imaging, for obvious floral reasons. Seems that might be difficult though.
     
  7. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  8. Chris101

    Chris101

    Feb 2, 2005
    Arizona
    Sure thing Bjørn. F-stops represent halving of the light, so I measured the maximum of this curve, called that 100% throughput, and took the 50% as down 1 f-stop, 25% as down 2 stops, 12.5% as down 3 stops, etc., and then read the wavelength from the graph:
    [​IMG]
    So what would be a good filter for this lens to isolate the UV? Does it come as a gel, or maybe a polymer square (as the Xenatar has no lens threads)?
     
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