Re: Focusing Scope with BLUE filter?

RE;Richard Knoppow wrote:

Its supposed to prevent mis-focusing due to chromatic
aberration in the enlarging lens. ...
In the early days of photography, when lenses were
not very good and plates sensitive nearly exclusively to
blue and UV light, focusing by the visible image would often
result in blured pictures.
Focusing using blue light is supposed to eliminate the
error caused by the chromatic aberration of the lens.


I can't see anything there to argue with. I would state the
case a little differently. The blue filter is there to assist in
focusing the image on blue only sensitive materials. A
Graded paper is the usual blue only sensitive target.
If the lens is corrected for only the blue, as I believe is the
case for some process lenses, then use of the blue filter
is almost manditory. Dan

.

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