Re: CMYK separations: a correction

Hi Katherine,

First let me say that I am sure there are folks who make great gum prints
both from RGB files and CMYK files. I think different people develop their own
custom workflows and end up with different routes that get them roughly to
the same destination. Someday when I have the time to try full color gum I
think I would go the RGB route, because it is simpler and more direct and I've
seen Sam Wang's full color gums using this method ;) That doesn't mean I might
not throw a 4th layer of black or near black in to get deeper shadows.

Another reason that I might choose RGB is that the CMYK conversions are
designed for laying down inks, not making negatives, so there are compensations
built in to them for that reason which have little to do with how gum would print—
especially given that one might choose a variety of colors for the CMY.

However, since I haven't done it, I'll leave it to the experts to hack each
other to pieces over the RGB/CMYK controversy, and wisely shuffle off into the
shadows.

On the other hand, regarding my earlier comments:

1. I was merely remarking that I didn't see how the file size/resolution
would affect color conversion. I looked at your website and see the two
examples, but I don't see any information that shows what the variables were in
terms of file size/resolution that caused the two different results. That you
got different results is evident from your two jpg's—what caused the two
different results to me is logically something other than file size/resolution. If
you can repeat these results and give me the values to try, I'd be happy to do
so. It wouldn't be the first time I've had to eat crow or some other
distasteful substance.

2. Regarding the best printing resolution. I am not sure what printer you
are using and whatever works best for you then by all means do so. I have
tested a number of Epson printers with special resolution targets and have not
been able to achieve better definition than when using 360 ppi files to feed
the beasts. I've tested everything from 240 ppi to 1440 ppi. Epson suggests
using 360 ppi—now that doesn't mean that I believe them. They do seem to be
rather mysterious about the inner workings of their printer drivers.

There are many people who feel (and this is based upon some Epson
documentation that I have downloaded from the Epson site and read) that the current
desktop printers have an optimum file resolution of 720 ppi (the file you are
sending to the printer—the printer is still printing at it's "native resolution"
of 2880 DPI) and that the large format printers have an optimum file
resolution of 360 ppi. Some swear that they have tried both and agree with this.
All I can say is that when I tested it with sharpness/detail targets, 360 ppi
won hands down. Part of what seems to "blur the issue" so to speak, is the
dithering algorithm.

Also, I might add that if you send a file to an Epson printer that is above
360 ppi, at some point (and I don't have time in this lifetime to determine
that point), the Epson driver will interpolate the file down so that it can print
a reasonable facsimile of the image. Since DPI and PPI are not a one to one
relationship with inkjet printers—it takes more printers dots to represent a
pixel. The exception to this rule is the Lightjet type printers that print
one dot for one pixel. Usually when you try to increase the resolution, you
do so at the loss of the number of tones in the file—eg, imagesetter
negatives, when you violate the "rule of 16".

Anyway, thanks for your work and efforts on the RGB/CMYK controversy. I'll
now fade back into oblivion.

Mark
Purchase the eBook & System for Your Own Custom Workflow@
Precision Digital Negatives
PDN's Own 31-Step Tablet Now Available—produced by Stouffer Industries
Credit Card & Paypal now accepted
www.MarkINelsonPhoto.com
Workshop info on Homepage

In a message dated 12/17/04 10:23:44 PM, kthayer@pacifier.com writes:

> Hi Mark,
> I must have explained that very badly, because I don't think we're
> talking about the same thing. Did you look at the separations? The
> problem isn't in the image; the colors in the image don't change. The
> problem is in the separations. The problem seems to be that if the file
> isn't big enough or within a certain range of resolution/size values,
> the separation engine doesn't create accurate separations, but makes
> separations that are basically strange and don't have a lot to do with
> the actual image. The yellow and magenta separations (I've seen this now
> in three images) are much too thin and look almost solarized; they look
> very strange, flat and low contrast, with outlines around the edges of
> some shapes.  And if you print them on gum, you get a really otherwordly
> color balance.   But if I start with a bigger or less complicated image,
> the separations are a more accurate reflection of the CMYK profile.
>
> I disagree about 360 being optimal; the higher the image resolution the
> better negatives I get, especially with regard to small test negatives.
> Some of my best test negatives were made from 35mm negatives scanned at
> actual size at the highest resolution of the film scanner and printed
> directly from a file that size and resolution, at 1440 dpi which is the
> highest resolution my old printer will print at. These negatives print
> gum really great even that small, whereas the same size negative from a
> 360 ppi file wouldn't work very well at all. 
> Katharine
>
Received on Sat Dec 18 00:41:51 2004