Re: Behind the lens filters

[iris & mario cater <caterl@netzero.net>]

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okay, so here is the deal. 
when you are filtering behind the lens the image quality you get depends
very much on your tidyness 
of course, but also on a variety of other factors. 
there is the depth of focus of your lens. it follows the same principles
as depth of field but it lies behind the lens. 
depth of focus is determined by the focal length and the f-stop. 
the shorter the focal length the shallower the depth of focus 
the wider open the iris the shallower the depth of focus 
  

 

the bigger the circle of confusion the more apparent any dirt on the
filter will become in the image. 
that doesn't mean you can't use gels on a long lens or at wide open, you
just have to be even more meticulously clean. 
the other major factor determining the circle of confusion is obviously
the distance between the filter and the film plane. and having an acl we
got lucky. 
gels might not be en vogue nowadays, but latest generation arri and pana
35mmm cameras all provide for  behind the lens filtering. why nobody
really uses it is because the filterholder sits right in front of the
film plane, rendering the tiniest piece of dust nicely and sharply on
the negative. 
in an acl the filter sits about half an inch away from the image plane.
in terms of depth of focus this is a huge distance, making gel filtering
far less risky. as an added bonus the filter sits in front of the
mirror, so what you see is what you get. 

another effect you should be aware of is that with a gel you are
effectively introducing a plan-parallel optical element into the
lightpath. this increases the back focus of a lens. 
 

gels are very thin,so there is no need to be overly worried about this. 
however, in critical situations focus through the viewfinder and avoid
zooming. 
also make sure the filter sits absolutely flat in the holder. 
and, yes definitely don't "stack" gels unless you like newton rings as
an effect. 

gelatine filters are more delicate to work with than glass filters, but
i think they often can be a cheap, practical and lightweight option.
sometimes maybe the only option. extremely wide angle lenses e.g. are
almost impossible to filter from the front. 

by the way this whole acl revival thingy is seriously cool 

mario 
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