Re: Graphics Resolution Confusion Once Again

[David Neeley <dbneeley -at- gmail -dot- com>]

The best resolutions for master files that are destined for offset 
printing depends upon the resolution of the offset job.

A digital file is ripped by a digital imagesetter, giving an output 
image of the target lpi (offset resolution is in "lines per inch" dating 
from the old days of silk screens being applied).

Generally, the rip engines work best if the master images are at a 
resolution that is an even multiple of the output resolution desired. 
That way, it is a straight mathematical transform and nothing needs to 
be interpolated in the software.

Thus, if your offset job will be at 150 lpi, you can get reasonably good 
results from even multiples (300, 450, 600 for example).

It is usually best to use at least double the output resolution.

Occasionally, you will find jobs run at other resolutions entirely--at 
one point, 133 lpi was a fairly popular choice. Thus, any images for 
that output res would again be even mulitples of that. (When I say "even 
multiples," I am also referring to numbers like 1.5, 2.5, etc.)

Some offset work is higher resolution than the 150 I mentioned 
first--although that would likely be fairly rare in documentation 
printing. Images printed at higher resolutions can be fairly tricky to 
run at times, especially if the press being used requires multiple 
passes to lay down all the colors for the job. For example, you may have 
a four-color press printing a four color job--but printing both sides at 
once. Thus, the first pass would print two colors on each side. The 
moisture from the ink can cause the paper to swell slightly, making the 
second pass through the press be off registration slightly. This is 
especially true with uncoated stock--again, typical for manual 
printing--and most especially when printing at high resolution since the 
dots are so much smaller.

Nothing substitutes for having a serious chat with the offset company to 
get the best advice as to how to get the cleanest master with your 
combination of output desires.

These days, it is common for at least some clients to supply a file in 
.pdf format. That is fine if you know what you're doing--but I always 
preferred to work with a printer who could deal with the original file 
where possible, so that any necessary adjustments could be made in the 
pre-press phase. Of course, today many tools exist which can make 
manipulating the acrobat files themselves relatively trivial in terms of 
issues such as signature creep and the like. However, it remains very 
good practice if you can produce output files with colors already 
separated and crop and registration marks in the file--although this may 
be well beyond the comfort zone or software capability in many cases.

Obviously, if you are not offset printing in color, things are much 
simpler.

David


>
> From:
> Gene Kim-Eng <techwr -at- genek -dot- com>
> Date:
> Tue, 8 Feb 2011 19:27:00 -0800
>
>
> If you're taking your work to offset printing, anything less than
> 300dpi is going to look dodgy.  I prefer at least 600dpi for my offset
> projects.
>
> Gene Kim-Eng
>    

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