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| The State of Color and Converting |
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Good Conversions Lead to Good Output
Converting RGB to CMYK for final output is, unfortunately, essential. It’s a critical step that makes or breaks the quality of the final
printed piece. An RGB file contains far more colors than that of the CMYK color space. As a result, when one of these files is converted
colors are stripped and in many cases appear muted once in CMYK. As more photographers begin to scan their film or shoot with digital
cameras, their clients are increasingly demanding Separated CMYK files. This is a benefit and a curse. The curse is the process that
does the conversion. The quality of the output rests squarely on photographers’ shoulders. They now have full control over the color
from inception to final ink on paper. In addition, the wise photographers will find this critical step a new profit center in increasing
their fees. More services are offered to their customers that previously someone else down the line did. In addition, in the old days
film was scanned directly into CMYK for final output usually by a high end prepress house or service bureau. Now, in order for quality
to be ensured, photographers need to provide this and do so with the same quality as those prepress people. Let’s examine some of the
options required to do this as well as look at some of the inherent problems in doing so.
The first problem with CMYK files (and to a lesser degree RGB) is that CMYK is highly device dependent. What this means is you can
have a file you converted from RGB to CMYK, send it to a dozen different CMYK printers and get a dozen different looking prints.
CMYK needs to be optimized for a specific output need. This of course is true for RGB output as well. As we will see, that’s sometimes
not possible. But we can get close. The most common application for RGB to CMYK conversions is Adobe Photoshop. One could simply open
a RGB file and go under the (Image -> Mode-CMYK) menus and end up with a separated file. But will the conversion be sound? Most likely
not. We have to inform Photoshop how we wish to handle this. Assuming that Photoshop’s default settings are going to produce satisfactory
results is like assuming that setting the F-Stop to f8 on a lens will always produce an acceptable exposure. Remember those service
bureaus and prepress houses? They did the conversions. High end scanners containing proprietary algorithms were operated by expertly
trained personnel who knew the unique characteristics of their output devices. Each scan was targeted for a specific printing condition.
In today’s desktop workflow, we want to scan once, convert the RGB file to CMYK, and output the file on a variety of devices.
Photoshop has at least two ways in which to produce our conversions. But first we need to know the conditions in which our data will
be output. This information is critical. Therefore we must to open a conversation with our clients or the printer. In the past, Adobe
and others would simply recommend, "ask your printer". This doesn’t provide much in the way of instruction. How could anyone know
what to ask if everything was an unknown?
Here are a few simple rules about getting the right information. Write the following information down as it’s given to you.
1. Know the linescreen at which the job will print. These are known as LPI. Common numbers are 133 or 150 with other
variations available. Remember those dots per inch? The dots stand at attention in a line. If we say the linescreen is 133 that means
there are 133 lines of dots per inch of printed material. Take a magnifying glass and you’ll immediately see the lines of dots.
2. Know the conditions of the press.
3. What system is used to proof? Often the information in numbers two and three are readily available. Many print shops
simply say, "We print to SWOP (Standard Web Offset Press) standards." SWOP is fairly generic information. Telling someone that a
press is printing SWOP is much like describing all E-6 transparency films are using the same processing and chemistry. In other words,
SWOP only tells us just a little bit about the supposed method of printing. In reality, not as many print shops are really dialed into
SWOP as you’d think. This condition is a pretty broad based. SWOP specifies a line screen of 133. If you print 150 lpi, that\'s not SWOP.
Yet it's done and it can be made just as consistent and predictable as 133 lpi It should be noted that there is a true standard emerging
for SWOP known as TR001. SWOP as a specification was deemed to not be that specific. So ANSI had a CGATS committee perform a SWOP
certified press test conforming to SWOP. On this press test they printed the IT8.7/3 press test target (a standard target of 938 some
odd patches of different combinations of CMYK values). They measured the target with colorimeters and spectrophotometers and averaged
it all out and then called it TR001 and said "this is SWOP".
Many users know that prior to printing, they will get what has incorrectly been referred to as a "MatchPrint". We prefer to call this
a "Contract Proof" because MatchPrint is a specific contract proof manufactured by Imation/3M. There are many other such proofs that
your printer may use such including Fuji ColorArt, Kodak Approval, and Kodak Contract. While your client may be expecting a MatchPrint,
in reality, depending on the shop making the film and proofs, you may not end up with a Matchprint at all! It’s important to have a
conversation with the people providing your film and Contract Proof. Ideally you will optimize your conversion from RGB to CMYK for
this proof! It’s called a Contract proof because it’s a contractual agreement between you or your client and the people printing the
job. With print jobs that involve making separated film ( a B&W neg for all four colors), those negs will be used to actually burn a
plate the press will use to lay down the inks. A contract proof will also use the actual film to also make a proof and thus both client
and printer will accept this as something they agree on will represent what comes off the press. We should point out that there are
filmless proofing and printing systems such as Kodak Approval used for direct to press jobs. The concepts of the Contract Proof remain
the same. When the client is satisfied with the contract proof, they will "sign off" the proof and the film (or digital file) and
Contact proof will go to the printer. The pressman will control the press to the best of his ability to produce a printed page that
matches the contract proof. Usually the client will come to a "Press Check" and compare the prints coming off the press to the Contract
Proof under controlled lighting. At this point, the client should be able to approve the output from the press and the entire job is
run. So you see that the Contract Proof is a critical element we hope to optimize our conversions to.
Again, many shops will simply tell you to convert based on something like SWOP and hope the Contract Proof is also going to represent
this output. But often, a print shop will specifically inform you of the contract proofing systems they use. This can allow you to
tailor your conversions to this kind of proofing system like 3M MatchPrint or Kodak Contract. Bottom line; ask the provider of the
proof and film what proofing system they will use. Try to dial in your conversions to that process.
Once you have some idea of the press conditions, or better the contract proof,
it’s time to optimize your conversions for this process. Photoshop offers a few ways to do this. Depending on your budget
and determination to produce the best possible output, you have several choices. Naturally making a custom conversion for
your output needs is ideal and we will talk about a few ways of doing this. Sometimes this just isn’t possible so providing
a CMYK conversion that’s as close to the true conditions will be the way to go. Photoshop has 2 main methods of setting a
conversion; Custom CMYK accessed in the CMYK Pop-up menu in Color Settings or buy loading an ICC profile in this CMYK Pop-up
menu. Both methods use ICC profiles for conversions but the Custom CMYK method allows the user to create or alter specific
settings for the actual conversion. Also, when the Custom CMYK setting is configured, this interface allows one to save out
ICC profiles reflecting the current settings when the user clicks on the "Save CMYK…" menu option. The "Built In" mode is
what most users are familiar with.
Notice in
Figure 1
that the top of the dialog allows us to select the ink options or what we prefer to call the Ink model. This is supposed
to describe the color of inks you are using. It’s currently set to SWOP Coated but as mentioned, there are many different
results one can achieve even when using different conversions. We also have control over dot gain and other ways of
controlling the black generation of the conversion (the black channel which will create the black plate). At this point,
it’s really appropriate to truly know the correct settings for these options and that means a long conversation with the
printer. |
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Figure 1
(enlarge view) |
Also, the various ink models built into Photoshop are really intended for press conditions. There are no models that mimic a contract
proof although if the press and contract proof match each other, you could certainly pick one of these models. Picking one of these
ink models and perhaps tweaking the settings for dot gain and other settings like total ink limit and so on are going to get you in
the ballpark assuming the information provided by the printer is valid. But be aware that these settings are absolutely critical to
the outcome of your printed piece. Just being off a bit with something like dot gain can make or break a job.
Here the real power of conversions can come into play. Photoshop 6.0 allows users to create or use existing ICC profiles for making
mode changes. You have at least two options here; make your own or try and find existing profiles that correctly describe the
conditions of your printer, press or contract proof. There are some very good profiles floating around the web that were created for
contract proofing devices. The Radius/Miro line of displays ships with a group of excellent profiles for such proofing devices as
3M MatchPrint, Fuji ColorArt and several flavors of SWOP. In addition, printers and service bureaus may have ICC profiles they’ve
made or have been provided to them, which they should be able to provide to you for making your conversions.
The ICC CMYK profiles supplied by Photoshop 6 are also excellent. Lastly you can make your own profiles, which provides the best
possible conversions. This of course means you need a Spectrophotometer and associated software (although you can use the method
described above to measure the 61 patches and end up with an ICC Profile). If you have the capability to make your own, you’ll
likely want to profile the contract-proofing device. Profiling the press is a viable option as shown above. This means sending the
shop a file with a few hundred color patches which are then read into a package that generates profiles. Once that is done the
profile is loaded into the computer, Software accesses this profile for conversions to this specified device. No matter which method
is used to create custom conversions, we recommend sending a test off to the same shop with a mix of different images just to insure
the conversions are sound. Your job is to inspect them carefully. After all, it’s your career that’s on the line. Most software
profiling packages will allow the conversion to be tweaked as necessary. Dot gain, where the dot swells once the output material
has dried. Normally, this is taken into account if everyone has done their job correctly. In a pinch, where no information is provided,
keeping the total ink limit somewhere between 300% and 340% along with a light to medium black generation will usually work well for
printing on coated stock. Realize that when profiling and converting for a contract proof, the goal is to produce film or files (if
direct to press) that provide sufficient latitude for a pressman to make adjustments to match the proof. Presses can be profiled but
most are constantly changing the parameters that include the inks, papers and so on. Short of profiling the press the goal is to match
the contract proof. This the agreed upon result.
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