RE: Primer on photographic exposure, etc.
RE: Primer on photographic exposure, etc.
- Subject: RE: Primer on photographic exposure, etc.
- From: Wayne Bretl <email@hidden>
- Date: Wed, 15 May 2013 22:26:30 -0500
It's not obvious if this went through with a 2MB attachment, so I'm
resending it with the text only. If the previous post with attachment did
not go through, could someone tell me if it's possible to post the
attachment and how to do it?
-----Original Message-----
From: Wayne Bretl [mailto:email@hidden]
Sent: Wednesday, May 15, 2013 8:42 PM
To: 'Lars Borg'; 'Iliah Borg'
Cc: 'email@hidden'
Subject: RE: Primer on photographic exposure, etc.
When you get the best match to reflective test patches, you normally will
get some spectral colors outside the spectral locus and some inside. On the
other hand, fitting a selection of spectral colors to the spectral locus
does not necessarily give the best possible match to reflective patches,
which have broader spectra.
This comes about due to the mismatch of the camera spectral responses to
eyeball responses. Most camera responses are narrower than eyeball cone
responses, so as scene spectra become narrower (colors become more
saturated) the camera response saturates (one or two channels goes to zero
or an abnormally low value) before a spectral color of complete purity is
reached. At some point before a pure single wavelength is reached, the
camera stops indicating an increase in saturation. At this point, no further
adjustment by lookup table or any other means can help. If the reflective
patches with high (but not 100%) saturation are to be reproduced accurately,
pure spectral colors must be sacrificed, because the camera cannot see the
difference between high saturation and 100% saturation.
This is explained in the paper I gave at the 2011 SMPTE conference, which
was published in the May-June 2012 issue of the Journal. The abstract is
here:
http://journal.smpte.org/content/121/4/69.abstract
I have attached the slide presentation, which illustrates the effects of
real camera responses of various types, and particularly has some
illustrations of the effects of a series of spectra with progressively
narrower width, so you can see the response of these cameras to highly
saturated colors and spectral colors when they have been matrixed for the
best fit to the 24 patch chart. Of course, using more patches will give
somewhat different results, but the saturation compression will still be
present. Once the compression becomes severe, it is impractical to spread
these colors apart by application of a profile of any kind.
This effect is especially severe with typical film responses, less so with
the wider, more overlapping responses typical of DSLRs.
I'd also note that both surface colors and reproduced colors are typically
limited in saturation in the cyan area, so attempting to match spectral
colors in this range is mostly futile as well as not needed. My presentation
says a few things about this too.
-Wayne Bretl
-----Original Message-----
From: colorsync-users-bounces+waynebretl=email@hidden
[mailto:colorsync-users-bounces+waynebretl=email@hidden] On
Behalf Of Lars Borg
Sent: Wednesday, May 15, 2013 5:20 PM
To: Iliah Borg
Cc: email@hidden
Subject: Re: Primer on photographic exposure, etc.
Still, with such a target you're calibrating only for reflective colors.
Shouldn't you include more colors near the spectrum locus?
Don't you get a lot of on-spectrum-locus colors ending up outside the
spectrum locus?
(A common problem with camera calibration)
On a separate issue:
Sure there are target tolerances for the same SKU.
(And camera tolerances for any given camera model) Do you have actual dE
numbers?
Are these tolerances bigger than the color inaccuracy of the camera system
itself (which is maybe 10 dE over a large set of colors after calibration)?
I'm curious, as AMPAS has similar issues with camera calibration.
BTW, any experience with the Image Engineering camSPECS?
Lars B
At 11:05 AM -0700 5/15/13, Iliah Borg wrote:
>Dear Ben,
>
>I'm just saying that C1 prefers LUT profiles, and there are 2 ways of
>doing it - make a matrix and convert to LUT; or make LUT directly. One
>can do both and keep both, they both have their own advantages.
>
>For a minimal profile 24 patches is too little with current profiling
>engines, matrix or LUT, does not matter. ColorCheckerSG with its 140
>patches is a far better choice. Making a combined target from 3 shots
>of SG bracketing the exposure by 2 stops is even better. Target needs
>to be individually and accurately measured, canned measurements do not
>fit well. Scaling for a combined 420-patch (140*3) target is easier if
>measurements are spectral.
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