Re: some observations (and a question) regarding using the sun as a light source

From: Tom Ferguson ^lt;[email protected]>
Date: 08/06/04-08:39:38 AM Z
Message-id: <>

I don't know of a chart. Why not start with simply metering the light
hitting your print frame with a camera/gray card or incident meter.
You should meter through an 80A filter (medium blue) to get a more alt
(UV) centric reading. It would definitely require some testing, but
your results would look something like this:

If I get F/16 at IOS 125 and 1/30 second, then expose for 45 seconds
If I get F/11 at IOS 125 and 1/30 second, then expose for 1.5 minutes
If I get F/8 at IOS 125 and 1/30 second, then expose for 3 minutes

I've done very limited sunlit printing. One "fact" often reported is
that contrast changes between direct sunlight hitting the paper and
indirect (skylight) hitting the paper. That may be a simple time
difference (skylight exposures will be much longer) or a color temp (UV
content) difference.

On Saturday, August 7, 2004, at 07:05 AM, Keith Gerling wrote:

> A little while back I mentioned that my old Violux light source had
> bit the
> dust. For two weeks I've been using the sun as a light source for gum
> printing. Strange, but even though I often coat and dry gum prints
> out-of-doors, I've never before considered actually using the sun.
> This has
> been a challenging task! Surrounded as I am by trees, it is often
> hard to
> find openings that are large enough for long enough to expose a print.
> I was astonished to learn just how bright the sun actually is! Six
> minutes
> under my fluorescent bulbs equals three minutes under my Violux equals
> about
> 45 seconds under the noonday sun. But the time of day causes very
> extreme
> variances in the amount of light. A minute at noon might be 4 minutes
> at
> 3:00 PM.
> A question: Is there some kind of chart I can refer to that will
> provide a
> sort of adjustment factor for the amount of sunlight falling at a given
> time? Something that will take into account latitude and the date?
Tom Ferguson
Received on Mon Aug 9 12:02:46 2004

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