[Alt-photo] f stop and log values

Bob Cornelis bob at colorfolio.com
Fri May 26 20:48:35 UTC 2017


Christina, Loris, et al

Here is the small chart available on the stouffer website for the 21 step
wedge:

To Change Exposure 1 Step 2 Steps 3 Steps 4 Steps
To Increase Exposure 1.41x 2.00x 2.82x 4.00x
To Decrease Exposure .71 .50 .35 .25

With the info you gave me (and a little help from my son, who has a
Master's Degree in math) I was able to construct the same chart for the 31
step wedge:

To Change Exposure 1 Step 2 Steps 3 Steps 4 Steps
To Increase Exposure 1.246x 1.59x 2.00x 2.52x
To Decrease Exposure .80 ..63 .50 .40

I think that's right and a handy reference table to have.

Thanks for the help!
Bob

On Fri, May 26, 2017 at 12:48 PM, Christina Z. Anderson via
Alt-photo-process-list <alt-photo-process-list at lists.altphotolist.org>
wrote:

> Even easier.
>
> Numerical factors for fractional stop exposure time sequences
> Increment       Description     Value   Inverse
> Half stops      square root of 2        1.414   0.707   Multiplying 2x
> gives    2.0000
> One third stops cube root of 2  1.260   0.794   Multiplying 3x gives
> 2.0000
> One fourth stops        fourth root of 2        1.189   0.841
>  Multiplying 4x gives    2.0000
> One-sixth stops sixth root of 2 1.122   0.891   Multiplying 6x gives
> 2.0000
> One-eighth stops        eighth root of 2        1.091   0.917
>  Multiplying 8x gives    2.0000
> One-tenth stops tenth root of 2 1.072   0.933   Multiplying 10x gives
>  2.0000
> > On May 26, 2017, at 1:12 PM, Loris Medici via Alt-photo-process-list <
> alt-photo-process-list at lists.altphotolist.org> wrote:
> >
> > Let me do it for you Keith;
> >
> > 1) Increasing the exposure means multiplication with the factor,
> > 2) Decreasing the exposure means division by the factor,
> > 3) For 1 stop the factor is 2^1 (^ being the exponentiation operator)
> > 4) For 1/2 stop the factor is 2^(1/2)
> > 5) For 1/3 stop the factor is 2^(1/3)
> > 6) For n stop the factor is 2^n
> >
> > Regards,
> > Loris.
> >
> >
> > On May 26, 2017 9:44 PM, "Keith Schreiber via Alt-photo-process-list" <
> > alt-photo-process-list at lists.altphotolist.org> wrote:
> >
> > Bob -
> >
> > To reduce exposure by 1/3 stop, multiply by 0.79.
> >
> > If you are interested in how this this figure is derived I'll explain it
> > late. Gotta run now.
> >
> > Cheers,
> > Keith
> >
> > Keith Schreiber
> > jkschreiber.com
> >
> >
> >
> >
> >> On May 26, 2017, at 12:05 PM, Bob Cornelis via Alt-photo-process-list <
> > alt-photo-process-list at lists.altphotolist.org> wrote:
> >>
> >> I have a 31 step stouffer step wedge and need to know what fraction to
> >> increase/decrease exposure time when steps need to merge or separate.
> >>
> >> I know each step is 1/3 of a stop but wasn't able to find the
> > corresponding
> >> fraction to adjust exposure. The Stouffer website has this info in the
> FAQ
> >> section for the 21 step wedge only.
> >>
> >> My math is not good enough to wade through all the equations on
> Wikipedia
> >> to get the answer!
> >>
> >> Thanks
> >> Bob
> >>
> >> --
> >>
> >> http://www.bobcornelis.com
> >> _______________________________________________
> >> Alt-photo-process-list | altphotolist.org
> >
> > _______________________________________________
> > Alt-photo-process-list | altphotolist.org
> > _______________________________________________
> > Alt-photo-process-list | altphotolist.org
>
> _______________________________________________
> Alt-photo-process-list | altphotolist.org
>



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