Archived posting to the Leica Users Group, 2006/01/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Robert, What controls the focus point is the wavelength of the light and the optical design of the lens. Some lenses control the focus point of wavelengths beyond 450nm much better than others. Obviously, a true apochromatic lens would be much better than your usual high aperture 50mm. Contrawise, a lens designed for focusing UV light would be absolutely horrible at IR focusing, if it even passes IR light. Take a typical glass prism, it will difract light in the usual ROYGBIV. If made of anomalous dispersion glass, the typical pattern will be much different. No matter what filter you put in front of the prism, the diffraction pattern will be the same for whatever wavelengths of light the filter allows to pass. Don don.dory@gmail.com On 1/17/06, Robert Meier <robertmeier@usjet.net> wrote: > > The filter controls what wavelengths get through, of course. The filter > therefore can be ground to compensate the focus point for those > wavelengths. > What the film records should not be a factor, since it can only record > what > gets through to it. > > Bob > > > >> > >> > >>Compensation varies according to the film, as well as the lens. The > >>focusing > >>distance should be adjusted by 1/200 to 1/400 of the focal length of the > >>lens, at infinity. Which fraction to use depends upon trial and error. > > > > Compensation should depend on the spectral response of the film. Those > > such as HIE that are sensitive further into the IR region need more > > compensation than films such as Konica 750 or Maco 820 that don't have > as > > wide a spectral response. The further the incoming 'light' is into the > IR > > region, the further behind the film plane the main bundle will be > > focussed. > > > > Compensation also depends on the design of the lens; many modern lenses > > have a much flatter spectral response and don't need as much > compensation > > or none at all into the near infrared, whereas the 1/200 compensation is > > needed for older (60's or older) lenses using HIE. In any case, stopping > > down to middle or small apertures is needed with HIE in any case as > hardly > > any lenses display severe aberrations at 850-900nm that can be brought > > into line by stopping down. > > > > -- > > * Henning J. Wulff > > /|\ Wulff Photography & Design > > /###\ mailto:henningw@archiphoto.com > > |[ ]| http://www.archiphoto.com > > > > _______________________________________________ > > Leica Users Group. > > See http://leica-users.org/mailman/listinfo/lug for more information > > > > > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >