Digital Photography Hacks [Electronic resources] نسخه متنی

Hack 62 Match Resolution to Output

Image resolution remains one of the great
mysteries to hobbyist photographers; there's one
setting for computer viewing and another for print output.
Here's how it works.

I can't resist starting
this chapter with an anecdote. Usually, I wouldn't,
but I feel that this story sets the tone for the entire chapter.

About a year ago, after teaching a two-hour digital photography
seminar, I was fielding questions from attendees while packing up my
equipment. One gentleman approached me and said, "I
have just one question for you. How do I change the resolution of my
pictures without changing their dimensions? I know
there's one little thing I need to do, but for the
life of me, I can't remember what it
is."

This guy had patiently sat through an entire seminar, probably about
stuff he already knew, just so he could ask this question afterward.

I reopened my laptop, launched Photoshop, and showed him the magic
box that he needed to uncheck. His eyes lit up, he grabbed my right
hand, and he shook it vigorously. "Thank you, thank
you!" he exclaimed and ran out the door. Most
likely, he headed directly home and went to work.

For me, that experience sums up Photoshop for digital photographers.
You know there's some way to accomplish everything
you want to do, but for the life of Moses, you can't
figure out half of them. Or, as in the case of my student, you
can't remember what you already know. How to adjust
picture resolution for printing falls squarely into that category for
many photographers. This hack will solve that problem for you.

6.2.1 Your Camera's Resolution

Resolution
is expressed in dots per inch (dpi) or
pixels per inch (ppi). The higher the ppi, the
more densely packed the pixels are, and the higher the resolution.
Because of the inherent differences in various media, printing
requires higher resolution than computer viewing.

Your digital camera is basically a miniature computer that contains a
built-in scanner. It scans its images at 72 ppi. That happens to be
the same resolution at which most computers display their images. So,
going from digital camera to computer is like going from apples to
apples. If you were always going to view your pictures solely on a
computer, that would be the end of this conversation. But chances
are, you want to make prints too. Prints don't look
so good at 72 ppi. The ideal printing resolution for photo quality
starts at 200 ppi. So how do you get there from here?

6.2.2 Working with Image Size

Open your picture in Photoshop and find the
Image Size command. In Photoshop
Elements, the path is ImageResizeImage Size. In
Photoshop CS, the path is ImageImage Size.

You'll notice in the dialog box that
Pixel Dimensions is the
top section and
Document Size is the middle area.
Pixel Dimensions shows you the current size of the image at the
current resolution. Chances are, the resolution is set to 72 ppi.

The Document Size box shows how big a print you can make at the
current resolution. As shown in Figure 6-1, I can
make a 31" 23" print from this four-megapixel picture. Though
that's technically true, it
wouldn't look very good printed at 72 ppi.
That's just not enough resolution.

Figure 6-1. The Image Size dialog box in Photoshop Elements

I want a photo-quality print, so I'm going to change
the resolution. But first, here's the secret: before
you do anything else, uncheck the box labeled Resample Image, as
shown in Figure 6-2. Now, enter 200 in the
Resolution box. You'll notice that your previously
impressive 31" 23" print has shrunk to 8" 11". But it will look darn
good.

Figure 6-2. Changing the image's resolution

If you resample images when adjusting them for
printing, Photoshop will add more pixels. You don't
want Photoshop to do that, because it doesn't add
pixels at the same quality as your camera records them.
That's why you turn off resampling. Notice that the
file size (listed next to the Pixel Dimensions label) stays the same
when Resample Image is turned off. That's your best
clue that you're doing this right.

I recommend you use the Save As command at this point and leave the
original image alone. You can add print to the
new filename to remind you that this version of the picture is for
print output.

6.2.3 A Final Photo Tip

You saw how drastically the print size shrunk when I changed the
resolution from 72 ppi to 200 ppi. You can fudge this process a bit
by using 150 ppi, and you'll still get good-looking
prints.

But the real point is to shoot at your camera's
highest resolution so that you always have the option of making a
reasonably sized print. The images in this hack were recorded with a
four-megapixel camera set to the highest-quality settings for
resolution and compression. I recommend you use those settings too,
regardless of how many megapixels your camera has.

You can always make copies of the image and size them down for other
uses [Hack #64] .