[argyllcms] Re: Camera matrix profile, adding ti3 perfect white data set

  • From: Elle Stone <l.elle.stone@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 4 Feb 2010 17:48:21 -0500

On 2/2/10, Ben Goren <ben@xxxxxxxxxxxxxxxx> wrote:

> Something else that hasn't been mentioned yet is peripheral light falloff,
> aka vignetting. Shoot at f/8 and keep the target in the center half (or
> less) of the frame (don't fill the frame) and you should be fine, unless the
> lens is truly miserable.

Hi Ben,

I think you've pin-pointed the reason that ALL my target shots over
the last year, indoors and out, despite stringent efforts to get even
lighting, show a gamma of around 1.05. Every single target shot I've
done has been essentially full-frame.

Yesterday I made as uniformly-lit flat exposure as I could think of,
using a DIY "light-dispersing filter" made from a square of flat thin
dense styrofoam, held over my (older, manual focus, 55mm) lens with a
filter holder, aimed at a uniformly cloudy sky, f5.6 (what I probably
used with the target shot), focused on infinity. Eye-droppering
around, I found that the pattern of brightness increased toward the
center, compared to the values at the corners. The exact same pattern
that I find with all my targets. The amount of light fall-off, center
to sides, is not nearly as noticeable as the visible vignetting one
tends to find with wider angle lenses at low f-stops. But it is
measurable and consistent.

Oh, well. Live and learn. Next time I'll only fill the center half of
the frame. And use f8.

Elle

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