Filter auf der Rückseite des Objektives

  • Hallo Fotografen ,
    Ich möchte gern einen Graufilter auf der Rückseite des Objektives anbringen.
    Ich lese da was von Focusverschiebung ....
    Was muß man beachten?
    Objektiv Super Angulon 165 mm verwendet für 8 x 10 , als Weitwinkel
    Filter Graufilter - 6 Blenden etwa 2 mm dick ?
    Danke für Eure Infos im vorraus

  • Hallo von Frankreich !

    Zusammenfassung auf deutsch : kein Problem mit filter auf der Rückseite des Objektives
    Focusverschiebung ca. 0,66 mm für 2mm Glas n=1,5 ==> Focusverschiebung = d(1-1/n) d = Dicke n = Brechungsindex

    (meine detaillierte Antwort, auf Englisch)

    I apologize not no be able to answer in German for technical questions. I hope that English is OK!
    There is no problem to use a filter at the rear of a lens.
    And yes, you get some focus shift.
    With a weak filter at the rear of a large format camera, you just need to focus with the filter in place. With a bayonet-mounted lens on a SLR body, it means that you wont' be able to focus at infinity with the filter in place at the rear.
    However, with a strong absorbent grey or neutral density filter (6 f-stops = 64X = ND 1,8) in place at the rear of the lens, the problem is that you'll not be able to see the image!

    Hence you have to guess what is approximately the value of the image shift. Actually the image can be located anywhere, not only the focal plane: if you are working in macro, for example at 1:1 ratio in the 2f-2f configuration, the image shift will be the same alhough the image is located one additional focal length beyond the focal point.
    For a glass filter of refractive index n=1,5 the image moves backward by about 1/3 of the filter thickness. The general formula is d(1-1/n) where "d" is the filter thickness and "n" the refractive index of the material (glass, plastic, gelatin ..) used in the filter.
    For example, if your filter is 2 mm thick, glass with n=1,5 in principle you have to move your rear standard backward by 2/3 of a mm = 0,66 mm.
    Another way to solve the problem would be to get a blank filter, or a weakly absorbing filter, of the same thickness, focus through it, and substitue the dark filter just before taking the picture.
    Gelatin filter being thinner than glass filters will induce a much smaller image shift and thus you can probably neglect the image shift with a gelatin filter installed at the rear.

    Some orders of magnitude to remind:
    if you work in, the 4x5" format and take a conventional criterion of sharpness with a circle of confusion of about 90 microns, 0,09 mm, the depth of focus for far distant objects will be plus or minus Nx90 microns where N is the f-number. Assume that your image is recorded with N=22, f/22, hence the depth of ofcus will be plus or minus 22x0,09 = plus or minus 2 mm !

    So my recommendation would be to make test shots with the glass filter in position, and first do not change the focus setting as determined without filter. Then take a 2-nd picture by shifting the rear standard back by about 1 mm; formally you would need 0,66 mm with a 2mm filter of refractive index 1,5.
    This is quite a small displacement of the standard, smaller than the estimated depth of focus@f/22 in 4x5"; and such a small adjustement could not be easy to achieve on a conventional 4x5" camera. For example classical monorails move the standard by about 2 cm per one full knob turn (this is the value for Arca Swiss monorail cameras)
    1mm means 1/20-th of a turn, i.e. a rotation of 18° on the knob. If the camera works fine in principle you have enough tactile sensitiity do set this angle precisely, but it really depends on the camera you are using and how well it has been maintained.

    The last remark will concern the use of a filter in front of the lens : for far-distant objects, whichever the filter thickness could be, you have no focus shift, because infinity plus 0,66 mm = infinity; and the sharp image will be located at the same place with filter or without filter i.e. in the focal plane ;)

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