On Wed, 23 Oct 2002, Robert G. Earnest wrote:
> So, would adding a filter to the front of the lens after focussing
> require adjustment as well?
I don't think so.
> And, in the case of a Lee Gel, which is paper thin, would stopping down
> an extra stop take care of the difference?
Maybe. Depth of field is not the same as depth of focus, but with some
gels it would be minimal (test, test). Most older field cameras are pretty
misaligned anyway. With a thicker filter, a la Lee or Cokin resin type,
the problem becomes larger.
I am interested in how one makes this rear-mount perfectly plano-parallel
with the lens.
Maybe Ansel was blowing smoke rings up our behinds ? After all, he *was*
explicitly talking about the paper-thin gel type of filters.
> Everyone I know of focuses with the aperture wide open anyway
Of course.
> I would say that using it inside offers benefits
> that far outweigh using it on the outside.
Less flare. But... how much less ? I wonder what the difference would be,
specially using a pro-caliber shade ? (One should be doing that
anyway, or one is giving away the equivalent of one paper grade in
contrast on the average) What about losses from planar misalignment
of the filter ?
> We have always focused with the filters in place (unless we are talking
> about a polarizer which are difficult to use when inside the camera).
That's the way to do it, but it is easier said than done under cloudy
skies with a slow lens and a 25, 29, dark green, Orange, heavy ND or IR
filter in place, front-or-rear mounted.
> I am referring to cc filters (mostly kodak) which we would leave on the
> camera as long as we are using a specific film emulsion.
cc filters usually do not exact heavy light loss, making it easier
to focus through them. If you do that, then there is no problem. But
Greg's question would involve a heavy ND filter, meaning
it'd be inserted after focusing. I wonder if anyone with an optical bench
and collimator has actually tested the front-vs-rear filter thing
recently and published results ? Just wondering.
In Greg's case, why not just shuck the gel in place and try it ? Burn
one sheet, see what happens. If the results are acceptable, plow on.
A gel runs $20 or so...a mere trifle by photographic standards.
--- Luis
