Preparing Photos for Print Projects
This whitepaper is one in a series of helpful how-to's from
PrintingForLess.com, America's Print Shop. In this guide, you'll learn
what resolution is, how much you need when printing, and how
to change it without lowering image quality. You'll also learn how
to edit digital photo files so that they look crisp and sharp in your
printed projects.
Understanding Resolution
Image resolution can be a tough concept to wrap
your brain around. When referring to an image,
resolution is the number of pixels displayed per
unit of printed length. It's a measurement used
in printing and it's stated in dots per inch (dpi).
This makes sense because printers print dots,
and that's what a printed image is composed
of. When referencing an image onscreen—on a
computer monitor, TV, or projector—resolution
is stated in pixels per inch (ppi). This too makes
sense because digital images are displayed in
tiny individual blocks of color called pixels.
How Resolution and Pixels Work Together
The resolution measurement dictates how closely an
image's pixels are packed together. Increasing an image's
resolution means the pixels will be packed together more
tightly, resulting in a smaller physical size, but generating
a smoother, higher quality print. Lowering an image's
resolution means loosening the pixels, resulting in a
larger physical image size, but generating a blocky, lower
quality print.
Think of the resolution measurement as density. For
example, the tighter a substance is packed, the denser it is
and the less surface area it takes up (like brown sugar). The
more loosely a substance is packed, the more surface area
it consumes and it becomes less dense.
The confusing part is that when it comes to imagery,
printers are the only devices that can do anything with the
resolution measurement. Because our eyes can process
only so much information, a 72ppi image onscreen
looks identical to a 600 ppi image onscreen. However, a
printer isn't hampered by the human eyeball and can take
advantage of resolutions much higher than 72.
How Much Do You Need?
The resolution necessary for a good print depends on the
printing device itself. For instance, consumer inkjets do
a nice job at 225 to 250 dpi. For a black and white laser
printer, you need between 150 to 200 dpi. For business
cards, brochures, and other PrintingForLess.com products,
you'll do best with 300 dpi or higher.
Let's put theory into action on a photo from a high-end
digital camera. I opened Figure 1 in Adobe Photoshop,
then chose Image > Image Size (Command+Option+I/
Alt+Option+I).
This photo (courtesy of iStockphoto.com/Lisa Gagne) is
measured at 72 ppi. But that doesn't mean it's a low-quality
image. It's 14.1 megabyte, and its pixel dimensions are
2716 x 1810. That means you have a lot of pixels to work
with. In fact, if you tried to print this image at its current
resolution, you'd need a piece of paper more than 37
inches wide by 25 inches tall.
At the bottom of Photoshop's Image Size dialog,
uncheck the Resample Image option. This locks the
pixel data, thereby locking image quality. Watch what
happens in Figure 2 (next page) when you enter 300
into the resolution box. The physical size of the image
changed! It decreased to roughly 9 x 6 inches—much more
manageable!
However, look at the pixel dimensions toward the top
of the dialog. They didn't change, and that is the power of
the Resample Image option. I just successfully changed the
resolution without altering image quality. The number
of pixels is exactly the same as it was before, but now
the pixels will be packed more tightly together when the
image reaches a printer.
Upsample with Caution
If you leave Photoshop's Resample Image box checked
and increase the resolution of an image, you'll be
adding information (pixel data) to the image that wasn't
originally there. That's called upsampling. It's usually an
extremely bad idea, but there may come a time when you
have no choice; for example, maybe the only available
image is a small, low-resolution one from the Web, and
you have to print it.
Here's how to add enough pixels for printing: Open the
Image Size dialog in Photoshop and leave the Resample
Image box checked.
Choose Bicubic Smoother from the pop-up menu to
its right and change the document dimension pop-up
menus to Percent. Enter 110% in the width box and press
OK (Figure 3).
Repeat this process as many times as necessary to
enlarge the image. Resist the urge to increase the size more
than 10% at a time. Adding data 10% at a time doesn't
cause a huge amount of quality loss.
If you must upsample low-resolution images frequently,
you may want to buy a plug-in, such as Genuine Fractals by
onOne Software, Blow Up by Alien Skin, or PhotoZoom Pro
by BenVista. They manage to increase pixel data without
totally destroying the image.
Pre-Sharpening
Sometimes you want to print an image with
enough resolution, but the photo is a little blurry.
You can't take something that's completely out
of focus and make it a masterpiece of clarity, but
you can significantly improve an average photo.
Optimize Image Contrast
When you see a sharp edge in an image, it's because the
difference between dark and light—that is, contrast—is
higher along that edge than in other areas. When you
apply a sharpening filter in a program such as Photoshop,
you boost contrast only along edges, creating the illusion
of heightened sharpness. But it isn't just edge contrast
that affects whether you perceive an image to be sharp.
Every adjustment that affects overall contrast contributes
to what looks like sharpness. You can apply contrast to the
entire image, to specific tonal ranges within the image, and
to specific areas.
Shape the Tonal Range to Enhance Details
Sharpening will be more effective if you first enhance
the definition of image details by adjusting the photo's
tonal range.
Set a proper black point and white point. Setting the
darkest and lightest tones in the image is the first stage
in affecting how much an image pops (Figure 1). In
Photoshop CS3 and CS4 (Figure 2), you do this by dragging
the black and white triangle sliders in the Levels or Curves
dialog box or panel (in Photoshop CS2 or earlier, the sliders
are in Levels only). In Adobe Lightroom and Adobe Camera
Raw, the black point is controlled by the Blacks slider and
the white point is controlled by the Exposure slider. In all
cases, you can Option-drag (Mac) or Alt-drag (Windows) a
slider to preview which tones are clipped.
Shape the tone curve for details. The next level of
contrast adjustment involves shaping a tone curve, which
lets you boost or cut contrast in specific ranges of tones. If
you want to make certain image details more distinct and
you can isolate that part of the image by tone, steepening
that part of the tone curve will help differentiate image
details in those areas (Figure 3).
In Photoshop CS4, create a new Curves adjustment
layer by clicking the Curves button in the Adjustments
panel. In Photoshop CS3 and earlier, click the Create New
Adjustment Layer button at the bottom of the Layers panel
and choose Curves. To increase contrast within
a specific range of tones, click two points on the curve
where that tonal range begins and ends, and drag the
points to steepen the curve there.
In Camera Raw, click the Tone Curve tab to view the
Parametric curve. In Lightroom, use the Tone Curve panel
in the Develop module. In both programs, drag horizontal
sliders that control four segments of the tonal range
(Figure 4). In Camera Raw, you can edit the curve as in
Photoshop by clicking the Point tab.
Enhance Local Contrast
Sharpness is ultimately about contrast. More specifically,
sharpness is ultimately about local contrast.
Unlike a tone curve, which lets you adjust contrast
among different ranges of tones within an image, a
local contrast adjustment affects areas of tones within
an image. When you boost local contrast, you boost the
difference between all adjacent dark and light areas in
the image (Figure 5).
Because local contrast is a fundamental component
of sharpening, the way you control local contrast in
Photoshop is by applying the Unsharp Mask or Smart
Sharpen filters.
The basic technique is simple: Set a high Radius value
and a low Amount value compared to those you would
use for sharpening edges.
These values provide a subtle contrast boost in areas
extending some distance from the edges of image
content, instead of at edges as in normal sharpening. (In
normal sharpening, the Amount is closer to 100% and the
Radius is closer to one pixel.) The Radius value controls
how far from an edge the contrast boost is applied.
Local contrast changes are most effective in
the midtones. You may want to keep local contrast
adjustments from affecting highlight detail. If you're
using Smart Sharpen in Photoshop, click the Advanced
button, click the Highlights tab, and increase the Fade
amount. If you use Unsharp Mask, after applying the filter,
double-click the layer thumbnail in the Layers panel, and
in the Layer Style dialog box, drag the white Underlying
Layer slider to the left to hide the effect of the layer from
the highlights (Figure 6). Option-drag (Mac) or Alt-drag
(Windows) one half of the white slider to split it, feathering
the transition.
In Camera Raw and Lightroom, controlling local
contrast is easy—just increase the Clarity value.
Applying Clarity is like applying the high-Radius/low-Amount
technique above but includes automatic protection of
image quality so that there's much less need to manually
protect highlights.
By now, the details in your image should appear
much clearer, even before you've applied any traditional
sharpening settings.
And Finally, Ready for Advanced Sharpening
If you've performed the previous steps correctly, you should
have images that respond easily to the sharpening features
in Photoshop, Camera Raw, Lightroom, and other image
editors. In Figure 7 is the final image after
sharpening was applied, compared to the same amount
of sharpening applied to the image in the beginning
where no contrast optimization was applied. You can see
that manipulating contrast contributed as much to better
definition of image details as sharpening did.
Advanced Sharpening Workflow
If you're ready for some heavy lifting in Photoshop, read
Out of Gamut: Thoughts on a Sharpening Workflow by
Bruce Fraser on CreativePro.com.
This whitepaper is based on materials written by Lesa Snider and Conrad Chavez for CreativePro.com.
All trademarks are the sole property of their respective owner.
