[argyllcms] Re: Camera profiling with bracketing

  • From: Iliah Borg <ib@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 24 Feb 2011 21:07:33 -0500

Dear Graeme,

>> Such shots are done using tripod and remote, so if user (or scanin) samples 
>> one chart
>> all other locations are the same. I prefer manual sampling (like it is done 
>> in RPP)
>> because that allows to avoid any imperfections right from the start. It 
>> seems that
>> profiles with manual sampling go easy to 6dE max, while with scanin they are 
>> usually
>> about 12 to 20 dE.
> I'd be interested to have a sample that shows such a difference, as scanin
> should work better than that.

I will soon be profiling some new cameras for RPP and hope to send you the 

>> I would start with a merge utility that re-calculates spectral reference for 
>> each
>> individual .ti3 file based on the exposure of grey patches (around neutral 
>> grey)
> Right, this sounds much like HDR image merging. It is also a bit outside the
> scope of the next release :-)

Yes, it does sound like HDR. I don't know of your timeframe for the next 
release, but I do not think coding such a simple utility needs your personal 

>> filters veiling glare can reach 2/3 EV easily when shooting a chart. Adding 
>> a simple
>> black trap (hole) to a chart allows for a good estimation of veiling glare.
> That would only work for a specialised chart though.

Yes, but it is a very common modification to add the black hole.

> I wonder if there is
> a more general way of estimating flare.

3% is typical high grade film camera, 5% is for a high grade dSLR.

> Is it reasonable to assume that flare decreases in line with everything
> else if the exposure is reduced ?

Veiling glare is in percents, and the best way to decrease it s effect is to 
use targets with a sufficiently narrow range between Dmin and Dmax (2.6 stops 
max) and to put the targets onto a deep black non-reflective background.
Common formula is VG=Lblack/Lwhite - that is for a linear data, small black 
hole shot on a large white background, L is illuminance at the sensor. However 
this is not very useful as the flare changes across the scene. Analyzing a 
series of shots with progressive change of the exposure allows to estimate 
flare map. However the problem is, changing the aperture one changes the flare 
too. So a light source with a sufficient regulation is a better choice. An LCD 
that reaches 1:600 contrast ratio is something usable for at-home veiling glare 
Iliah Borg

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