Should the press "match" the proof? Or should the proof "match" the press? There is a great deal of confusion among print suppliers, buyers, consultants, and even some vendors on this important question.
The two main strategies
There are two main print production strategies the printer can adopt: targeting an industry defined specification for presswork and/or targeting a shop-specific specification. Either strategy will impact everyone in the print production process so it is critical that the decision is based on a complete understanding of the consequences. Once the business decision is made, presswork and proofing can be brought into alignment so that color expectations can be effectively set and communicated.
Strategy One:
Targeting an Industry Defined Specification - the press aligns to the proof
Ideally, printers who do not control 100% of the digital files entering their pressroom would target an industry-defined specification as a way of bringing some degree of order to the various inputs that they may receive. In this case the proof is the target and the presswork is brought into alignment with it. This strategy is most appropriate in a distributed printing environment where prepress and scans from a variety of sources must conform to a common print characteristic. Examples are magazine and newspaper advertising or brochures that will be printed at a variety of locations.
This strategy enables prepress tradeshops and production graphic designers to prepare image scans and digital artwork appropriately even when they do not know which printer will be doing the final presswork.
Aligning the press to the proof is embodied in the traditional, analog film-based, laminate proofing systems with the most popular implementation for offset printing being SWOP. In this production workflow print purchasers produce one set of digital files and proofs, and send the set to the various print locations. The proofs, using current digital halftone as well as inkjet proofers, are made in a standardized manner according to the registered SWOP application data sheets (ADS) to ensure conformance to the SWOP specification. It is then the printer's responsibility to align their presswork, by whatever means are appropriate, to the supplied SWOP proof.
Strategy Two:
Proofing to a Shop-Specific Presswork Target - the proof aligns to the press
With this strategy, the presswork color is the target and the proof is aligned to it. This strategy would most commonly be used in a non-distributed printing environment where prepress and scans are completely controlled by the printer or where the printer has a close relationship with the print specifier. Examples include brochures, collateral materials, annual reports, corporate brochures, art reproduction, and so on. This strategy is often used by printers wishing to differentiate their presswork from their competition. In this case print purchasers don't care about aligned color across presswork at multiple locations but instead are concerned that their particular project looks as good as possible in print. The presswork is unique — either to the individual print shop or even to a specific press in the shop. The printer may print with higher ink densities, higher contrast, finer halftone screens, or perhaps, custom ink sets, depending on their customer's specific needs. This strategy may also mean that prepress and separations are customized in order to take the unique press condition into account.
A Third Strategy Option:
Multiple Targets
Just as one size does not fit all - one print characteristic may not suit all. In today's highly competitive print environment, it can make competitive sense for a print shop to leverage the flexibility of digital proofing and plating systems to offer multiple print characteristics to their print buying customers.
Effectively they would select/offer the color flow that is most appropriate for the needs of their customers - presswork aligned to an industry-defined specification proof for certain projects. Proofing aligned to a unique shop-specific print characteristic for others. This involves greater complexity in workflow, however, once set up can provide the printer with tremendous flexibility in differentiating themselves and better meeting their customer's expectations and needs in print.
Achieving gray balance in presswork is the major mantra in today's print production world. It is the paramount metric - now defined in unambiguous CIE L*a*b* values - for the G7™ calibration method to align press and proof color as well as for achieving presswork that conforms to GRACoL® 7. That being said, one must keep a "balanced" view on gray balance as a metric in presswork. While it certainly has value - interpreting that value requires some understanding of the peculiarities of the press room as well as a bit of history.
- As far as I can determine there has never been a formal study of gray balance targets measured in press sheet color bars and how they relate to the live image content of the press sheet.
- Press operators do not "make color" on press. They concentrate on what a press is designed to do - lay down a consistent film of ink approximately one micron thick on the substrate.
- Press operators use densitometers - not to measure color, but to indirectly measure ink film thickness.
- The deepest study that has been done on gray balance was by System Brunner using densitometers rather than spectrophotometers to measure gray balance.
- The ISO specification (e.g. ISO 12647-2, ISO 2846-1) defines gray balance as "neutral" - a very vague term.
- A press lays ink down in a series of zones which run from the lead edge to the tail of the sheet. If a specific process color (C, M, Y or K) is not require in that zone because of image content, that ink zone is switched off and gray balance is no longer achieved in the color bar.
- Image content that requires a substantial increase in solid ink density in some colors (e.g. a sunset scene) will result in gray balance no longer being achieved in the color bar.
- The correlation of gray balance distortion (i.e. color bias) vs perceived color shift in quarter, mid and shadow areas has not been studied. It is probable that it is very non-linear. That is to say that the shifts we see in the gray balance target in the color bar may shift quite a it, however, we will not necessarily see an equal shift in highlight or shadow color.
- Gray balance targets are likely too sensitive to normal color fluctuations on press to be of practical use. Just like a car's speedometer would be useless information if provided 3 decimal place readouts of speed (e.g. 50.392/kmh, 50.471/kmh, 50.148/kmh).
- It is dangerous to apply gray balance principles derived from scanning and proofing to the pressroom since the mechanics of how color is achieved is radically different.
- The job of the press operator is to align the "live" image area of their presswork with the proof by achieving the appropriate solid ink densities since printers sell the live image area of the press sheet - the color bars go into the recycling bin.
Gray balance targets in presswork color bars certainly have some value - but interpretation of the information they contain must always be considered in the context of the mechanics and chemistry of the press itself.
It comes as a surprise to many professionals in the graphic arts that quality very often depends on optical illusions. One optical illusion in particular, "Mach bands", is critical in the graphic arts and fundamental to quality image reproduction.
Mach bands are named in honor of Ernst Mach the Austrian physicist and philosopher who is also the namesake for the "Mach number" (also known as Mach speed). The Mach band optical illusion is the basis for how most image sharpening, such as the "UnSharp Mask" filter in Adobe Photoshop, is done in photo editing software.
Mach bands reveal a bit about how our eye/brain compensates for a lack of actual detail resolving ability. Look closely at the below image which has six patches of gray set between a black patch and a white.
Do the gray patches looked scalloped? Lighter on the left than on the right? In fact, the gray patches are of a uniform tone as can be seen by simply moving one patch out of its context.
To make up for our eye/brain's detail resolving deficiency, it processes incoming light in such a way as to exaggerate the contrast wherever two different tones meet. Increasing the contrast at those edges gives us the perception of more detail.
Here's how the sharpening filter in Adobe Photoshop leverages this optical illusion to make images look more detailed. First I'll apply it to the original gray patches. The top half is the original and the bottom half with the sharpening filter applied:
Now we'll see the same sharpening applied to a real image.
On the left is the original globe image straight out of the camera with middle top image is a section of the original enlarged. On the right is the globe after sharpening is applied with the bottom center enlargement showing the effect of sharpening.
Technically, the sharpened image actually has less real detail because image pixels had to be converted to black and white lines bordering the actual image detail. However, despite having less detail our eye/brain perceives a sharper, more detailed, image all thanks to software leveraging the power of an optical illusion.
