[LargeFormat] Bellow extension and film refraction

[Karl Wolz]

Easier way to do this calculation, especially once you're used to doing it
this way.

Convert your lens length into inches (150mm = 6")

Measure your lensboard to film plane distance (say, for argument's sake, 8")

Convert those measurements into f: stops (6" = about f:5.6, and 8" =, well,
f:8).

Count the stops between, and you have your compensation, which in this case
is one stop, plus a smidge.

Trust me; it works.

Karl Wolz

----- Original Message -----
From: "Robert Mayrand" <dnaryam@videotron.ca>
To: <largeformat@f32.net>
Sent: Monday, January 07, 2002 1:48 PM
Subject: Re: [LargeFormat] Bellow extension and film refraction


> On 1/7/02 3:11 PM, "lnphoto" <lnphoto@twmi.rr.com> wrote:
>
> > 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.
>
> Does this mean that you even got to do some adjustment at infinity?
> 6 inch lens / 6 inch of bellows extension = 1 = I stop increase and time
> exposure stay the same??? It seem there is something wrong here.
>
> I also found the web site with the quickdisk (small disc you put in the
> picture and measure on your groundglass) for calculation, but I would
prefer
> the measure between GG & lens board, maybe I'll do a ruler with the info
for
> each lens.......unless such a device is already available somewhere on the
> the net?
>
> Sorry about the refraction and reprocity mixed up, thanks for giving me a
> cue about that, I'll go check on the film manufacturer web site to get the
> data....i thought it was a bit more complicated then that! Thanks again!
>
> Robert
> Mtl, Quebec
>
>
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