On Mer, 4 feb 1998 14:24, Gene Mayeda <mailto:gmayeda@xxxxxxxxxxxxxxxxx>
wrote:
>How does one install the #87 filter behind the lens and in front of the
>shutter and can you still see through the lens or not;that is since the
>#87 is opaque to visible light is this a possible way to use this filter
>without having to remove it in order to use the viewfinder? I do not
>understand why this results in correct infrared focus unless you are
>just using the depth of field that you get with the aperture setting
>that you are using.If this is the case then it would not matter where
>the filter is placed as far as focus is concerned.It seems to me that
>you are claiming that the #87 can refract infrared wavelengths to the
>correct focus.I was not aware of this focus correcting ability of the
>#87 filter.
>-Gene Mayeda
>
Every lens focuses different wavelenghts light at different distances (that
is called axial chromatic aberration). In common lenses, exp. long lenses,
there is a considerable gap between blue and green from one side and red to
the other. So most lenses have a red dot on the barrel in order to correct
focus when using infrared or b&w red filters. It is a reference for the red
focusing plane. 87 filter cannot correct focus, the fact is this: that red
focuses on a different plane that other colours. Some lenses have not red
dots because are corrected for all the three colours, that is are APO
lenses.
btw: all my prime lenses have a red dot, while no zoom lens has...
Marco
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