Metrogon II Yellow & III Red (3') center spot filter history

Discussion in 'Aerial and Drone' started by murrayatuptown, May 7, 2022.

  1. Does anyone remember what the II Yellow and III Red designations meant on aerial photography filters?

    I have some for 6" Metrogon Lenses with these Roman numeral markings.

    Red 25A requires 3 stops of exposure correction, which may be coincidental. I have seen 25A red filters without the vapor-deposited center spot filter on Aero-Ektar lenses. The red appears to my eye to be the same color.

    The yellow also looks like the same color as other non-spot filters, but 2 stops of correction seems a bit much for that yellow color.

    I'll take a guess that the II and III refer to the filters not having equal light transmission across the diameter...or that II and III refer to stops of exposure correction for the spectral response of aerographic black & white film under the conditions seen from an airplane (different from sea level).

    One of the red filters also has a marking 3' (read like '3 feet' or '3-prime').

    Thanks

    Murray
     

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