[LargeFormat] Bellow extension and film refraction

[lnphoto]

on 1/7/02 11:11 AM, Robert Mayrand at dnaryam@videotron.ca wrote:

> I found a few formula for exposure correction relative to the bellow
> extension. But since I never really exposed any film for a very long time, I
> never had to consider film refraction curve. Is there some clear explication
> on the net about this and large format.
> 
> I will probably use mainly kodak film (t-max, tri-X) and ilford hp
> 
> Thank you.
> 
> Robert


Dear Robert,

the easiest bellows formula I know of is to divide the distance between the
lensboard and the ground glass by the focal lenght of the lens. This will be
the number of stops you need to open up.  If you square this number you will
get the exposure factor. Multiply this number by the time read by the meter
to get your new effective exposure.

for example  It's a standard rule that at 2twice the focal lenght of the
lens you get a life size 1:1 image and the there is a two stop light loss.


The lens is a 6" focal length, the bellows are 12" long.  12/6=2 stops. Yep
that checks.  2 squared is 4, a 4x factor on the exposure.  Yes that checks
out too.  (The metric system works too but the numbers are too big for my
head)

Now let's say your new exposure is around 10 seconds.  Reciprocity is the
film's ability to give the same exposure when exposed at f8 @1/60 as does
when exosed at f11@ 1/30.

For the most part film works.  during low light, long exposure situations
reciprocity laws fail. You need to give even more light, usually by extening
the time, than your calculated exposure.

Tri-x had horrific reciprocity.  Once into the 10-30 second range, Plus-X
was faster.

Tmax is much much better at not having reciprocity failure.  At around 10
seconds there's little compensation if I remember (take Kodak's word over
mine)  In the 30 second range there's probably a stop added.

les