Latitude and filter-behind-the-lens

["David N. Horn" <dh@xxxxxxxxxxxxx>]

Latitude means tolerance to errors in exposure.
A Latitude of three stops means that one can vary exposure within a
three stop range and still get a usable negative.
Latitude should be expressed in plus X stops, minus Y stops.
Most negative film is more tolerant to overexposure than underexposure.
For example, you could say such and such film has a -1/+2 stop latitude.
In my limited experience, HIE has little latitude.
Most consumer color negative films, and Ilford XP1, have a lot of
latitude. Three stops is a lot.

> 
> Just in the process of getting my head round some of the info in Laurie
> White's 'Handbook'.
> 
> It says that HIE has a latitude of 3 stops - is that to say that if you
> expose HIE on the straight line portion of its curve, a change in
> exposure of 1/3 stop will be equivalent to a change of 1 zone in density
> on the negative. That is are the ten zones spread over the 3 stops of
> latitude ?
> 
> Or does it work differently ?
===============================================================

Placing a filter of significant thickness behind the lens WILL alter 
the focus.
If you put a filter, other than a wratten type, behind the lens, you
will have to increase the film/lens distance to compensate.
Some polaroid film backs that cannot bring the polaroid film to the
original film plane, use a glass or plastic plate to move the plane
of focus back to where the polaroid back places the film.

> > As owner of same equipment I'm very interested in the subject you
> forward to the List. IMHO a good quality filter placed in front of the
> lens doesn't alter the focus. When placed behind the lens there will be
> no alteration either. Also in your case where the filter is placed in
> the filmback. So you still have to correct your focus for IR as usual. I
> don't have that special 70 mm back, but I think that this also applies
> for your back. What do others think about this?
> 
Hope this helps!

Dave Horn
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