Marco wrote:
snip...
> 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
Another reason for the missing "R" red-dot is cost-saving; one less color
(and process) during manufacturing. As for the zooms, the IR focus may be
different for various focal lengths, so the easy way out is to leave it
out.
Regards,
Richard
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