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

  • From: "Iliah Borg" <ib@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 02 Feb 2010 09:48:25 -0500

Dear Edmund,

> Personally, I think that making a target shot without
> very detailed
> instructions is not within the abilities of an average
> photographer,

Here is something I came up with:

This is the most important part of the profiling. Without a
quality shot of the target profiles are poor - GIGO.
Quality of the shot is more important than having an
expensive profiling target. Classic CC24, ColorChecker
Passport, or even a mini CC is all what is needed to start.

If you have a studio you can use Solux 4700K lamps with
black-painted backs or HMI lights (Mole-Richardson are a
good source). Broad-spectrum fluorescent lamps (Normlicht
or JTI branded) are a solid choice too. If you are on a
budget any halogen lamps with stabilized power supply and
gelled to reach about 5000K are OK.

For a decent setup, check
http://www.imatest.com/docs/lab.html#lighting /

You can also use daylight if a studio is out of reach. It
is interesting to make different profiles for different
types of the daylight, and with different positions of the
Sun.

1. To start, use a lens as simple as possible, a 50mm prime
is ideal. You will need a separate profile for your
polarizing filter. You will need separate profiles for
CC40m and CC30m filters. You will discover that some lenses
need a different profile, especially those old manual focus
primes. A good deep hood on the lens is a must. Any
reflections or flare will make for a poor profile.

Clean the sensor, the lens, the hood; and of course the
filters if you are going to use them.

2. Affix the target on a grey card, 2-3" larger than the
target in each dimension.

If the target is not perfectly flat resulting profile will
be compromised with shadows and colour casts. This is very
important for smaller targets like mini ColorChecker.

3. Position the card so that it will be easy to compensate
for distortions in Photoshop; do not fill the frame with
the card - the card should occupy about 1/3 of the frame in
each dimension.

4. Cover the target with another grey card and set white
balance from it.

Take the covering grey card off and blow off any dust from
the profiling target

5. Shoot at f/5.6 or about that, very slightly out of focus

6. Take a shot and ensure that all 4 readings from the grey
close to the corners differ no more than +-4 RGB from each
other.

Cover the viewfinder so that it will not allow any stray
light.

7. Expose so that the whitest patch is 248..252; 250 is
ideal. Take several shots bracketing with shutter speed to
ensure good results. Adding a black trap and BabelColor
white target
http://babelcolor.com/main_level/White_Target.htm helps
producing very consistent profiles.


--
Best regards,
Iliah Borg

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