[argyllcms] Re: White Point

  • From: Iliah Borg <iliah.i.borg@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 7 Nov 2015 19:07:36 -0500

Dear Hening,

FTP refuses the connection, I will try later.

Please allow me to start with a banal thing: the accuracy needs to be placed
where it belongs, and the flatness of the spectral response of first two grey
patches is proven to be enough for practical purposes. You can use the teflon,
but you will need two shots for the following reason.

To use something for white balance, it better be exposed not too close to
clipping (2 reasons, raw converter clipping, that often happens to stay on the
safe side and try to avoid pink clouds; and individual camera mis-calibrations
within 1/6 of a stop, usually), and If you are going to use several layers of
teflon tape as the white, you can take another exposure, 1 stop lower, and take
white balance readings from that piece; next, apply those settings to the final

In my previous e-mail, "decrease the exposure" also means "choosing a lower
exposed frame".

On Nov 7, 2015, at 5:27 PM, Hening Bettermann wrote:

On 07 Nov 2015, at 21:27h, Iliah Borg <iliah.i.borg@xxxxxxxxx> wrote:


In floating point, the Teflon tape reads 1.000-1.000-1.000. (As does the
whitest patch of the left/upper part of the ColorCheckerPassport; but
RawDigger shows higher values for the Teflon than for the Passport patch).

You probably have clipping in the raw conversion, since different raw values
are mapped to the same value in the raw converter.

First, it would be good to see the raw file in question (if anything, to get
an idea of the exposure and flare. It may help if there is a shot of the
whole setup, even taken with a phone, to see the locations of the camera,
the lights, the target. Second, if a raw converter clips the highlights, the
right thing to do is to decrease the exposure. If it is a substantial
clipping, you may want to present the case to the developer of the raw
converter. Third, what is that you are using for white balance? Second grey
from CCP?

Best regards,
Iliah Borg
LibRaw, LLC

Hi Iliah,

thank you for your very fast response.
For white balance, I used the Teflon tape, since I read that it is almost
100% neutral.
The raw is here for FTP download:
file name: _DSC2375.ARW
server: landshape.net
login name: ad_hoc@xxxxxxxxxxxxx
password: come_in
and an iPhone shot of a quick reconstruction of the setup at the same place,

I can not show the camera, because it's in for repair (that's why I find time
for the darkroom ;-) )

By 'decrease the exposure', you mean in the raw converter, or choosing a
lower exposed frame?
Lowering the exposure in Iridient requires 0.62 EVs to make the last red
pixel disappear (at 100%). I might as well choose a frame exposed 1/3 EV
lower and then do the rest in Iridient?

And there is another thing I don't understand how interferes with all this.
Iridient has for the a7r a default black level of 78 and white level of 246.
How do these come in?

Best regards -
Hening Bettermann, www.landshape.net

Best regards,
Iliah Borg
LibRaw, LLC

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