Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 71,808   Posts: 1,581,516   Online: 1004
      
Page 2 of 2 FirstFirst 12
Results 11 to 16 of 16
  1. #11

    Join Date
    Sep 2006
    Location
    UK
    Shooter
    35mm RF
    Posts
    3,520
    Quote Originally Posted by keithwms View Post
    Is it because the index of refraction for glass is all roughly the same?
    If you can recall the experiment in school, the prism bends the light, but the red and blue are bent by different angles, hence you can see the spectra of white light.
    There are (were) two classes crown and flint which dispersed the light by different amounts, separated the colours by greater/lesser amounts(dispersion) then Leitz discovered anamalous dispersion glasses, as calcite was a pig to manufacture, calcite has anamolous dispersion...
    So there are lots of different glasses, some very high refractive, i.e. bending the light by large amount, and variable in dispersion, there are catalogues of different glasses to feed into the computer design lens software.

    Noel
    P.S. Some one must be an optics engineer...

  2. #12
    keithwms's Avatar
    Join Date
    Oct 2006
    Location
    Charlottesville, Virginia
    Shooter
    Multi Format
    Posts
    6,079
    Blog Entries
    20
    Images
    129
    Thanks, I know there are different glasses, my question was where the number 250 comes from, so that I can ascertain whether the expression is generally useful. My guess is that it assumes a specific change in the index of refraction as a funciton of wavelength.

    Anyway here is my answer...

    Last edited by keithwms; 10-22-2007 at 12:15 PM. Click to view previous post history.
    "Only dead fish follow the stream"

    [APUG Portfolio] [APUG Blog] [Website]

  3. #13

    Join Date
    Mar 2005
    Shooter
    35mm
    Posts
    373
    Quote Originally Posted by Mike Kennedy View Post
    My second time out with Kodak HIE will be this morning.I'll be using a Nikon FE with a Tamron SP 28-80mm lens.
    Do I focus on my subject ,then turn the lens to the IR mark on the lens barrel?

    Thanks
    If you're using a non-opaque filter, there is no need to adjust your focus point. Just use a fairly small aperture. If you are using an opaque filter and your lens has no IR markings, focus slightly in front of the subject matter. I routinely meter, focus and expose my Kodak HIE through a deep red #29 filter, and get consistentlyy good results.

    Kiron Kid
    Last edited by Kiron Kid; 10-24-2007 at 10:43 PM. Click to view previous post history. Reason: add snaps

  4. #14
    AgX
    AgX is offline

    Join Date
    Apr 2007
    Location
    Germany
    Shooter
    Multi Format
    Posts
    9,031
    keithwms,

    Sorry for not answering you. I can no longer search for my own postings (and the follow up) here.

    Yes, you are right. I based that number 250 on old literature where numbers between 200 and 300 were stated. Obviously it is a matter of refraction related to effective (filtering) wavelenght. Keep in mind that there are specially constructed lenses which do not need adjustments.

  5. #15
    keithwms's Avatar
    Join Date
    Oct 2006
    Location
    Charlottesville, Virginia
    Shooter
    Multi Format
    Posts
    6,079
    Blog Entries
    20
    Images
    129
    Quote Originally Posted by AgX View Post
    keithwms,

    Sorry for not answering you. I can no longer search for my own postings (and the follow up) here.

    Yes, you are right. I based that number 250 on old literature where numbers between 200 and 300 were stated. Obviously it is a matter of refraction related to effective (filtering) wavelenght. Keep in mind that there are specially constructed lenses which do not need adjustments.
    Thanks. Indeed, I recently started using APO process lenses for IR and have found no need for refocus.
    "Only dead fish follow the stream"

    [APUG Portfolio] [APUG Blog] [Website]

  6. #16
    Ed Sukach's Avatar
    Join Date
    Nov 2002
    Location
    Ipswich, Massachusetts, USA
    Shooter
    Medium Format
    Posts
    4,520
    Images
    26
    Trying for simplicity:

    Glasses, and other optical materials; Crystalline Quartz, and plastics, have widely different Indices of Refraction.

    The working Index of Refraction WILL change, slightly, with the wavelength of light concerned.

    Lens systems with many "first surface mirrors" (reflective) will be affected LESS - in general - than those depending on refraction. Less refraction, less concern.

    "Apochromatic" lenses are - usually - more "highly corrected" in regard to chromatic abberation. Or so it says. sometimes I wonder. They are certainly NOT "miracle" lenses, completely immune to the laws and effects of optical theory.

    Whether or not try to compensate for "focusing shift" with IR film ... It depends a great deal on the character of light involved and the sensitivity of the film receiving that light. With the films whose sensitivity "peak" is closer to that of the visible spectrum (Konica IR, SFX ...) and where focusing is done in a SLR complete with a "mild" IR filter (Wratten #25, or like that ..), I'd forget trying to "compensate". Through experience, I've found that has caused far more out-of-focus-ness than it has corrected.

    Kodak HIE, first focusing without a filter, and adding filter attenuating most visible light, should be another ball game entirely. There, the IR Index Marks seem to make sense.
    Carpe erratum!!

    Ed Sukach, FFP.

Page 2 of 2 FirstFirst 12


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin