Re: Primer on photographic exposure, etc.
Re: Primer on photographic exposure, etc.
- Subject: Re: Primer on photographic exposure, etc.
- From: Ben Goren <email@hidden>
- Date: Thu, 16 May 2013 05:41:31 -0700
Wayne sent me the file under separate cover. It is now available here:
http://trumpetpower.com/files/trumpetpower.com/Wayne_Bretl_Theoretical_and_Practical_Limits_to_Wide_Color_Gamut.ppsx
Cheers,
b&
On May 15, 2013, at 8:26 PM, Wayne Bretl <email@hidden> wrote:
> 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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