Re: PS CMYK Conversions
Re: PS CMYK Conversions
- Subject: Re: PS CMYK Conversions
- From: Marco Ugolini <email@hidden>
- Date: Thu, 14 Aug 2008 10:46:57 -0700
- Thread-topic: PS CMYK Conversions
In a message dated 8/14/08 7:57 AM, Derek Cooper wrote:
> On 14-Aug-08, at 10:13 AM, Roger Breton wrote:
>
>> As opposed to getting images off a drum scanner?
>
> No, as opposed to getting accurate colours in CMYK. They always shift
> when going from RGB to CMYK, and you have to spend quite a bit of
> time manually tweaking.
Derek,
You certainly know about the differences in gamut between CMYK color
spaces/profiles and RGB color spaces/profiles. Some CMYK spaces are larger
than others, and so are RGB spaces (from smaller ones like sRGB to very
large ones like ProPhoto RGB). In general, CMYK spaces do not match RGB
spaces exactly (they often fall short, at least in part), though some RGB
spaces are large enough to encompass just about all standard CMYK output
spaces.
Depending on how wide the gamut of the source RGB profile is and how many of
the colors actually present in the image exceed the ability of the target
output (CMYK) color space to reproduce them, you *must* map any out-of-gamut
source colors to the best possible colors that are within the target's
capabilities. There is simply no choice but to compress, which is what the
rendering intent (plus BPC - Black Point Compensation) will do at conversion
time.
Photoshop allows you to soft-proof the image to the output profile while
still in RGB, and tweak the source colors (using adjustment layers, masks
and other standard tools) until they look best to you in the target space as
previewed on your calibrated and profiled monitor.
Bottom line: any colors in the source RGB image that exceed your output's
gamut will *have* to be subjected to re-mapping (compression), while
striving to maintain visible and believable detail in areas of the image
that are out of the target's gamut.
Honestly, I don't know of any applications or procedures that allows you to
soft-proof (and eventually convert) better than Photoshop does. So, I would
answer your original question ("Does anyone have a great explanation for why
the conversion to CMYK in Photoshop really isn't the best approach for
preparing images for press?") by saying that if anyone has a better way to
do this, I'd like to know it.
One last thing: try using the Relative Colorimetric intent (*with BPC*)
instead of Perceptual. Sometimes that works, specially when you want to keep
certain colors looking as close to what you see in the source image as you
can. The Perceptual intent privileges detail over color fidelity, and will
often move more or less ALL the colors in the image in order to fit in
colors that are out of gamut into the output space. On the other hand,
RelCol will privilege color (and saturation) fidelity over detail. Which
also means that with RelCol you must be careful to avoid clipping detail in
saturated areas.
Marco
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