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When you want to license an image for some print use, most people ask,
"Can you deliver the image at 300dpi?" But this doesn't make sense—"DPI"
has nothing to do with image resolution, which is really what you need
to ask for. Figuring that out is the point of this article.
What most people get hung up on is the fallasy that they need images at
300DPI—or "dots per inch." They don't really understand what that means
because that value doesn't tell you how large the file is. I can give you
an image that's 500 pixels wide, and another that's 5000 pixels wide, and
both can be set at 300DPI. But obviously, one can print onto a large
piece of paper, and the other cannot.
To determine the best size image for your printing needs, first,
determine the size of the area you want the image to cover, and then
determine what the best "dpi" for the printer itself and the materials
you are going to print on. Not all materials "require" 300dpi.
The following table includes commonly used sizes and papers.
20-50dpi
Areas larger than 10 feet, such as billboards, wrapped materials for trucks, wall paper |
100-150dpi
Areas larger than 20", such as posters, signs, banners, calendars, product packaging, retail signs |
150-200dpi
Plain paper, newsletters, books; 70# Matte Paper |
250dpi
Basic magazines, textbooks, most everything; 80# Matte Paper |
300dpi
High-density catalogs or covers; 100# Matte or Gloss Paper |
The term "300dpi" is a historical expression that refers to to a technical
the maximum resolution density that a printer could accept before it
throws away the excess pixels. But this "maximum" dpi isn't the same as
a printer's optimal dpi. In almost all cases, most modern, high-end
printing devices will throw away even those excess pixels as well in order
to achieve the "optimal" dpi for the paper being used. People don't notice
this because the system will produce exactly the same visual result from
images at 250dpi or 300dpi. Most mid-range printing systems will produce
the same results from images scanned at 200dpi. (That is, the results are
slightly worse, but that's because of the printer, not the quality of image.)
Don't be deceived by "home printers" that say they produce ink
droplets at 1440dpi. A single droplet of ink does not correlate directly
to a pixel in an image. Ink droplets are small for the sole purpose
of mixing proper quantities of different colors to yield more accurate
colors represented by the pixels in the image. For example, if the pixel
in the image requires 3 parts blue ink, 10 parts red ink, and 20 parts
yellow ink, and 3 parts black ink, the printer is going to be able to
reproduce that because it can mix lots of ink droplets into a single
point on the page. This has nothing to do with the dpi of the image.
Most (if not all) home printers will downscale all images to about
200-250 dpi during the printing process. You can try to print images
that are at 300dpi and higher, but you won't get any different results.
So, putting all this together, say you're laying out a catalog page where
an image will be used. First, know the physical dimensions of how much space
the image will take. Not things like "¼-page" or "½-page."
Actually measure the amount of space in inches. (All page-layout software
programs will tell you this information.)
If you don't know the "optimum dpi" of a device, you can almost assuredly
assume that 200dpi will work just fine. That value can drop further if you
cover more area, such as posters, etc.
Film Scans
Though most people get images from digital cameras these days, there's
still an awful lot of old film images being sold and delivered. However,
people have a misunderstanding about film, thinking that it's just a
continuous tone of of color that can be scanned at higher and higher
resolutions to get finer detail in the photo. But film isn't a continuous
tone of colors. Rather, it's more like digital cameras, in that it
has its own pixels; here, called film grain. Therefore, all film
has a "maximum" resolution: the a point at which you don't get more
detail in higher-resolution scans, you just get a lot of "grain." In fact,
scanning film beyond it's maximum resolution will yield worse digital
renditions, not better. Also, because different films have different
grains, each has its own "maximum" density as well.
Most professional 35mm color films has a maximum scan yield of about 5500
pixels in the long dimension. (This has the equivalent of a 21 megapixel
digital camera.) Yes, there are scanners that will go up to 8000 pixels,
but the digital image you get as a result is actually poorer, because it
will only get more of the grainy surface of the film, degrading the colors.
Additionally, there are scanners that only yield 4000 pixels in the long
dimension, but they produce better digital results due to having a better
imaging sensor (the CCD). So, the quality of a film scan is not measured
simply by its resolution.
This then begs the questions of what do you do if you have a 35mm slide
and want to print a 60" image. At 300dpi, 5500 pixels is only 18", a far
cry from 60". If you try to print this digital file at that size, you'd
get an effective dpi of only 91! Can you get a reasonable result from an
image printed at 91dpi?
Yes. It's almost universally true that most good, high quality printing
systems will render perfectly acceptable results from a 91dpi image,
provided that 1) the scanner that was used had a high quality CCD sensor,
2) the operator used the right color-management workflow to assure the
proper translation between source and destination media types (film to
paper, for example), and 3) the final material used was of sufficient
quality.
By contrast, one can get a very poor result from that same image if it
were scanned at 300dpi at 60" because the digital file would have consisted
of mostly film grain, not color. This also makes proper color management
nearly impossible since the original source colors would be too noisy
from the grain.
Testing the Theory
Look at the two photos below. The one on the left was shot on film
and scanned at a resolution that yielded 4000 pixels horizontally.
The 35mm frame of film has tiny dots, and the truck is covered
with a huge sheet of paper that has big dots (that are also spaced
apart). Where "DPI" comes is when you're talking about how many of those
dots represent an inch.
The output device that prints the paper that wraps around the truck will
tells how many "dots" make up an inch. The whole reason for specifying
a DPI value is to adjust your image to match the output specifications to
that of the output/printing device. It turns out that, for the paper that
goes on this truck, 20dpi was sufficient.
People who know this stuff don't just ask for a high-res image or a
300dpi image, they ask for an image either at a specific resolution, or
indicate the final output size along with the desired DPI. For example,
a company that makes postcards might request images at 4x6" at 330dpi
because they know that most postcard printers print image optimally
at that resolution. If a European company ask for an image at size A4
at 200dpi, you can determine that size by converting the metric system
to inches. (Although Americans have to first figure out what the heck
"A4" means.) This translation is easily done in Photoshop's "New Image"
dialog.
Summary
Most people never actually need images at 300dpi because it's an overkill.
It rarely hurts to have an image larger than what is needed because it can
be reduced. But that's a technicality. In today's image-licensing world,
higher-resolution images cost more, which needs to be taken into account.
You don't want an image sized at 300dpi if it costs twice as much as the
"proper" image size. Therefore, it's smart to know exactly what you need,
and not just expect 300dpi.
Though most people will still hand-wave this technicality aside, they
will butt up to the problem noted above: what do you do if you think you
need an image that simply can't be delivered? Digital cameras can only
produce images up to a certain size, and film's highest-resolution scan
usually tops out at 5500 pixels. If you do the math, these won't print
on posters or other larger sizes at 300dpi. Well, by the simple fact
that we've seen these posters forever, and they were printed just fine
with lower-dpi images, it disproves the theory that you "need" 300dpi.
You can feel perfectly comfortable printing images using lower-resolution
film scans.
Of course, business is business, and services often deliver clients what
they ask for, especially if it translates to more money, or to appease
an "insistent" client. Invariably what happens is that such a client asks
(and pays for) a higher resolution image than what is technically possible,
so the provider simply opens that same image in photoshop, and "resamples"
the image to be larger. In essence, it simply doubles the pixels (or some
magnification factor) to yield what the client asks for. This is actually
a lower-quality image, but by the time the printing system gets it, it's
going to simply throw away those new pixels anyway, effectively printing
the same thing it would have had the image not been unnecessarily altered.
In summary, most people don't need to worry about this stuff. Do the
simple math to figure out the total image size you need and ignore the
"dpi" value entirely. For example, if you're printing in a typical magazine,
newspaper, or poster material, assume you can get a good print at dpi values
of anywhere from 150-200. Therefore, if you want to print up to 30", a 200dpi
image would be 6000 pixels wide. If the photographer wants to give
you an image that's 5000 pixels, that would be 166dpi (for a 30" area),
which is perfectly fine.
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