Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,708   Posts: 1,548,579   Online: 1165
      
Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 24
  1. #11
    Helen B's Avatar
    Join Date
    Jul 2004
    Location
    Hell's Kitchen, New York, USA
    Shooter
    Multi Format
    Posts
    1,557
    Images
    27
    Quote Originally Posted by Ed Sukach View Post
    ***

    BTW -- It really does NOT have anything to do with lens design. Snell's Law and the effects of wavelength apply to any and all configurations.
    Ed,

    The relative amount of focus shift is dependent upon lens design.

    Best,
    Helen

  2. #12
    Ed Sukach's Avatar
    Join Date
    Nov 2002
    Location
    Ipswich, Massachusetts, USA
    Shooter
    Medium Format
    Posts
    4,520
    Images
    26
    Quote Originally Posted by Helen B View Post
    Ed,
    The relative amount of focus shift is dependent upon lens design.
    Best,
    Helen
    Why?

    This gets complicated, but the relative amount of refraction due to differing wavelengths remains constant in the only characteristic that would affect the ray trace - the refraction index of the glass itself.
    Hopefully, this makes sense. Reminds me of trying to describe "pinhole diffraction" in three words or less.

    Uh ... quietly, with hat in hand ... been there - done that.
    Carpe erratum!!

    Ed Sukach, FFP.

  3. #13
    gr82bart's Avatar
    Join Date
    Mar 2003
    Location
    Culver City, CA
    Shooter
    Multi Format
    Posts
    5,224
    Images
    37
    Umm .... I just 'nudge' the focus ring over a little bit. Seems to work fine for every image I've taken in IR so far.

    Regards, Art.
    Visit my website at www.ArtLiem.com
    or my online portfolios at APUG and ModelMayhem

  4. #14
    Helen B's Avatar
    Join Date
    Jul 2004
    Location
    Hell's Kitchen, New York, USA
    Shooter
    Multi Format
    Posts
    1,557
    Images
    27
    Quote Originally Posted by Ed Sukach View Post
    Why?

    This gets complicated, but the relative amount of refraction due to differing wavelengths remains constant in the only characteristic that would affect the ray trace - the refraction index of the glass itself.
    Hopefully, this makes sense. Reminds me of trying to describe "pinhole diffraction" in three words or less.

    Uh ... quietly, with hat in hand ... been there - done that.
    Ed,

    Why? Because of chromatic correction. That messes up the simple relationship between wavelength and focal length that exists for an uncorrected lens. How much detail would you like me to go into?

    Best,
    Helen

  5. #15
    Ed Sukach's Avatar
    Join Date
    Nov 2002
    Location
    Ipswich, Massachusetts, USA
    Shooter
    Medium Format
    Posts
    4,520
    Images
    26
    Quote Originally Posted by Helen B View Post
    Ed,
    Why? Because of chromatic correction. That messes up the simple relationship between wavelength and focal length that exists for an uncorrected lens. How much detail would you like me to go into?
    Best,
    Helen
    When you say "chromatic correction", are you talking about correction for "Chromatic Abberation"?
    Carpe erratum!!

    Ed Sukach, FFP.

  6. #16
    Helen B's Avatar
    Join Date
    Jul 2004
    Location
    Hell's Kitchen, New York, USA
    Shooter
    Multi Format
    Posts
    1,557
    Images
    27
    Quote Originally Posted by Ed Sukach View Post
    When you say "chromatic correction", are you talking about correction for "Chromatic Abberation"?
    Yes. Chromatic correction is a general term that does not define the degree of correction. Terms such as achromatism can be taken to infer a certain degree of correction of chromatic aberration. At least that is how Alexander Conrady used the terms in Applied Optics and Optical Design.

    Best,
    Helen
    Last edited by Helen B; 01-04-2007 at 04:21 PM. Click to view previous post history.

  7. #17
    Ed Sukach's Avatar
    Join Date
    Nov 2002
    Location
    Ipswich, Massachusetts, USA
    Shooter
    Medium Format
    Posts
    4,520
    Images
    26
    Why - oh WHY do these involved discussions always appear only when I am flooded with darkroom work?

    If you understand the nature and design techniques for Chromatic Abberation correction - briefly put: incorporating glasses of differing indices of refraction and various lens contours and configurations, they are done with one object in mind - to cause a broad spectrum of light to converge on the same plane; i.e. ALL be in focus at the same place. If we were to remove all light of one particular wavelegnth (I know, I know ... it will not be "perfect"), the focal plane location will NOT change - at least not as long as that spectrum is within that specifed in the design criteria. Not much, anyway... and that "not much" comes from a former Metrology Lab Technician ... "not much" = "damn little".

    Most of the IR film we have today is really "visible red to near infrared" and it is doubtful that there will be anything like a great deal of focus change. Possibly with the "old" Kodak HIE and fillters that attenuated all - or most - light below 900 nm or so there would be a noticeable difference - IF original focus was determined without the filter ... but, even then a concrete "all inclusive correction value" would be nearly impossible to determine - the lens would be used outside of its design criteria, and to me that translates to "all bets are off". It would be possible that the theoretical IR correction could go either way ... closer than indicated, or further away.

    Again, as I've said - focus the Hasselblad through the filter, if possible - and given that it is possible, leave the focusing where it is, ignoring the IR Index mark. If the filter is opaque to ALL visible light, the only thing left would be trail and error.
    Carpe erratum!!

    Ed Sukach, FFP.

  8. #18
    DrPablo's Avatar
    Join Date
    Aug 2006
    Location
    North Carolina
    Shooter
    Multi Format
    Posts
    796
    Images
    63
    I hate to make a trite suggestion, but what's wrong with just stopping down? I shoot some IR film on 4x5, and even with near subjects at f/22 I have front to back sharpness. I'd think that the increase in depth of field will atone for a subtle focus error.
    Paul

  9. #19

    Join Date
    Mar 2005
    Shooter
    Multi Format
    Posts
    3,427
    Quote Originally Posted by Ed Sukach View Post
    Why - oh WHY do these involved discussions always appear only when I am flooded with darkroom work?

    If you understand the nature and design techniques for Chromatic Abberation correction - briefly put: incorporating glasses of differing indices of refraction and various lens contours and configurations, they are done with one object in mind - to cause a broad spectrum of light to converge on the same plane; i.e. ALL be in focus at the same place. If we were to remove all light of one particular wavelegnth (I know, I know ... it will not be "perfect"), the focal plane location will NOT change - at least not as long as that spectrum is within that specifed in the design criteria. Not much, anyway... and that "not much" comes from a former Metrology Lab Technician ... "not much" = "damn little".

    Most of the IR film we have today is really "visible red to near infrared" and it is doubtful that there will be anything like a great deal of focus change. Possibly with the "old" Kodak HIE and fillters that attenuated all - or most - light below 900 nm or so there would be a noticeable difference - IF original focus was determined without the filter ... but, even then a concrete "all inclusive correction value" would be nearly impossible to determine - the lens would be used outside of its design criteria, and to me that translates to "all bets are off". It would be possible that the theoretical IR correction could go either way ... closer than indicated, or further away.

    Again, as I've said - focus the Hasselblad through the filter, if possible - and given that it is possible, leave the focusing where it is, ignoring the IR Index mark. If the filter is opaque to ALL visible light, the only thing left would be trail and error.
    Ed, first off very few lenses have absolutely no chromatic aberration in the visible part of the spectrum. Secondly, very few lenses are corrected for the invisible non-photographically useful parts of the spectrum. Some enlarging lenses are, according to their makers' propaganda, well corrected for 350 nm - 700 nm, few taking lenses are well-corrected for as wide a range.

    There are old taking lenses which worked well enough on orthochromatic emulsions (not very sensitive at the short end of the visible spectrum) but that worked poorly on panchromatic emulsions (better sensitivity at the short end) because of poor corrections for blue. That's why so many semi-nice old folders have built-in yellow filters and that's why the take sharper b/w pictures with the filter in place.

    Why don't you admit that you've overstated your position or made exagerated claims for how wonderful modern lenses are?

  10. #20
    Helen B's Avatar
    Join Date
    Jul 2004
    Location
    Hell's Kitchen, New York, USA
    Shooter
    Multi Format
    Posts
    1,557
    Images
    27
    Quote Originally Posted by Ed Sukach View Post
    Why - oh WHY do these involved discussions always appear only when I am flooded with darkroom work?

    If you understand the nature and design techniques for Chromatic Abberation correction - briefly put: incorporating glasses of differing indices of refraction and various lens contours and configurations, they are done with one object in mind - to cause a broad spectrum of light to converge on the same plane; i.e. ALL be in focus at the same place. If we were to remove all light of one particular wavelegnth (I know, I know ... it will not be "perfect"), the focal plane location will NOT change - at least not as long as that spectrum is within that specifed in the design criteria. Not much, anyway... and that "not much" comes from a former Metrology Lab Technician ... "not much" = "damn little".

    Most of the IR film we have today is really "visible red to near infrared" and it is doubtful that there will be anything like a great deal of focus change. Possibly with the "old" Kodak HIE and fillters that attenuated all - or most - light below 900 nm or so there would be a noticeable difference - IF original focus was determined without the filter ... but, even then a concrete "all inclusive correction value" would be nearly impossible to determine - the lens would be used outside of its design criteria, and to me that translates to "all bets are off". It would be possible that the theoretical IR correction could go either way ... closer than indicated, or further away.

    Again, as I've said - focus the Hasselblad through the filter, if possible - and given that it is possible, leave the focusing where it is, ignoring the IR Index mark. If the filter is opaque to ALL visible light, the only thing left would be trail and error.


    Ed,

    You now appear to be agreeing that the amount of IR focus shift is dependent upon lens design, and that there is no single relative focus shift value that applies to all lenses, even if one IR wavelength is chosen. Is my interpretation of your current opinion correct?


    Thanks,
    Helen

Page 2 of 3 FirstFirst 123 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



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