[LargeFormat] Intro; filter placement

[Michael Briggs]

On 01-Aug-2003 David Galway wrote:

> And here's my question: I have already received the lenses that came
> with the kit. I am going to send the 14" Red Dot down to Grimes to have
> the Gold Dot mounted in the 14's oh-so-sweet Copal 3. I also want the
> Grimes guys to set me up for filters ... I understand one can mount
> filters in front of or behind the lens. (I used to hang my 35mm filters
> off the Dagor in the 5x7, behind if the camera was pointing down, in
> front if I had it pointing up ... !) Is there any web literature on this
> subject? Any advice?
> 

Glass filters and other thick filters should be mounted in front of the lens.
If you place a thick filter behind the lens, the filter will change the
effective distance from the lens to the film by about 1/3 of its thickness.  
This means that the focus is changed.  If you must use a thick filter behind a
lens, then you should focus with the filter in place.  This can be difficult
with a darker filter.  Another approach would be to focus with a filter that is
identical except for color, then swap the filters.   A thick filter in a
converging beam also introduces some spherical abberation -- I'm not sure, but
probably not that important at typical taking apertures for LF photography.

When the filter is on the front and the lens focused on a distant object, the
beam of light is parallel.  The filter doesn't introduce spherical abberation. 
Because the object is distant, the shift of the light by the thickness of the
filter is completely neglible.   In short, no optical problems with a thick
front in front while photographing distant objects.

If you want to use filters behind the lens, it is probably best to use thin
filters.   The classic type is gelatin filters from Kodak.  These are still
available (via a different distributer), but are rather fragile.  Another type
is thin polyestar filters from Lee (also sold as Calumet house brand).

In macro photography, both the front and rear beams of light are non-parallel
and the lens to object/image distance is small.   On either side of the lens,
the filter will change the effective distance and thus the focus.  So for macro
photography it is always best to focus with the filter in place.

--Michael