Chris Barker wrote:
<< Surely the Neutral filter is like the ND filter - to reduce the light
level reaching the film without changing the colour. >>
No, I don't think so. You see, there are just 3 positions in all. There are
orange and yellow filters for black and white photography presumably, then
only ONE other position, and that was L39(UV), but it was later changed to
NEUTRAL.
I asked the original question because was curious as to why there wasn't a NO
FILTER position. I go with the other guys' explanations saying that there
must be some glass in this 3rd position, to make it consistent with the other
filters, ie the lens is designed to have a certain optical thickness of glass
there, and so it _must have_ something there all the time.
Originally it had an L39(UV) either because as the guys said films didn't
used to have a UV absorbing layer, or because as I originally thought it
might be needed for exposure meter accuracy because there is often a lot of
sky in the fisheye picture! [This second suggestion might also apply to the
8mm fisheye, which often is used for sky photography.]
Maybe as UV absorbing layers were introduced into film, the filter position
was changed to NEUTRAL - just a blank piece of glass, to keep the lens
optically the same as when you have the orange or yellow filters in place.
I can understand why this is so: if the light is converging or diverging at
all as it goes throught the internal filter, the glass will refract and
retard the light a bit and change its path with respect to if there is no
filter at all. It will then reach the rear elemts and the film plane in a
different position and focus. That would affect the focusing of the final
image, so you have to have a piece of glass there. [Imagine if you look
through a flat pane of glass at an angle - your view is shifted a bit from
what it would be if you don't look through the glass.]
If you think about it, there would be no point in having a neutral density
(ND) filter in place, because with only 3 selectable positions you would have
to have it in all the time (unless you're using the yellow or orange), so for
colour photos you would be stuck with the ND filter and you might as well
have an f/8 lens instead of an f/3.5 or whatever.
Kind regards,
Dave Bellamy.
http://members.aol.com/synthchap/
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