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Directions: Got Enough Pixel Power?
many megapixels do you need to get the quality you want? As you can
see here, it all depends on print size.
a digital camera for the first time can be a thrilling eventas
more and more of you are discovering these days. But trying to figure
out how to set all of the camera’s controls in order to maximize
both image quality and performance can be frustrating. For example,
a typical 4MP camera user must choose from as many as eight resolutions,
ranging from 640x480 pixels to 3200x2400 pixels, and at least four
different compression levels. But a typical camera manual doesn’t
tell you which setting to use to make a photo-quality 5x7- or 8x10-inch
print or which setting best captures images for your personal web
page or for e-mailing. Choose the highest resolution setting, and
you’ll barely get a single image on the puny memory card that
ships with the camera. Select a low-resolution setting, and you’ll
get dozens of images on the card that can be e-mailed in a flash,
but aren’t fit to print. How in blazes do you figure out which
setting works best for the application you have in mind?
a simple resolution chart that you can use to choose the resolution
and compression settings that you need. It starts with a column listing
the most common pixel resolutions for a variety of digital cameras
and their equivalent in terms of megapixel ratings.
To give you a
visual comparison between a variety of megapixel resolutions and what
you can expect from a properly exposed ISO 100 color print film, take
a close look at the detailed enlargements on this page. The detail
in the first five eyes comes from 11x16-inch prints made from their
respective megapixel file sizes on an Epson Stylus Pro 7600 printer.
The original images were accurately focused and exposed using a manually
focused digital SLR. (On the other hand, a 4MP point-and-shoot camera
with a so-so lens or focusing system might not capture as much detail
as a good 2- or 3MP camera.) Compare the digital prints to the detail
from a popular ISO 100 color-negative film optically enlarged to 11x16
inches by an automated minilab printer.
What most POPULAR
PHOTOGRAPHY & IMAGING staff members noticed was the lack of grain
and the pleasing skin tones on the 11x16-inch prints made with a 9MP
camera. (The shot on the previous page was taken with the Super CCD-based
Fujifilm S2 Pro digital SLR in 12MP mode.) However, as we reported
in our S2 Pro test in the November 2002 issue, the 12MP setting delivers
closer to 9MP resolution.) The higher detail reproduced in the film
print wasn’t as noticeable when both prints were viewed at
normal distances (arm’s length). In fact, when viewed more
closely, its sharpness and noticeable film grain actually worked against
it, which is why most pro photographers choose a higher speed, lower
contrast film such as Kodak Portra 160NC Pro or Fujicolor NPS 160
Pro for portrait shots.
Our 200 ppi resolution
rule of thumb was based on comparisons between digital prints produced
by the best inkjet and dye-sublimation printers available at the time
and prints from color-negative film made at a top minilab. All were
displayed at standard viewing distances and judged by both professional
and amateur photographers. For example, 4x6-inch prints were judged
from about one-foot away from the subject, while 8x10-inch prints
were viewed at arm’s length. The pros among the group, who
were used to viewing most enlargements up close or with a loupe, found
the inkjet prints made at settings of 250300 ppi more to their
liking, but agreed that prints made with files set at 200 ppi would
have acceptable photographic quality for consumers. That’s
why both of these resolution settings and their corresponding print
sizes are listed in the resolution chart.
confused about our use of pixels per inch to describe the resolution
of an image from a digital camera or film scan, that’s not
surprising. Most inkjet printers list their resolution in terms of
dots per inch (dpi), and not ppi. That’s as it should be, since
the dots produced by an inkjet printer are not the same as pixels.
Originally used to describe individual picture elements (that’s
where we get “pix-els” from) along a TV scan line, pixels
now refer to the photo-sensitive elements in a camera or scanner’s
CCD or CMOS sensor. The term is also used to describe the individual
elements that make up a digital picture file, as well as the output
resolutions of a computer monitor or a digital projector. In a digital
file, each pixel contains color information corresponding to a point
in the original scene. When these pixels are organized in a grid on
a computer monitor or LCD screen, you get a picture.
When you choose
to print an image file, printer driver software turns every pixel
into dots corresponding to a printer’s color ink set. Unless
the original pixel color is exactly the same as one of the inks found
in the printer, different color dots must be mixed together to produce
the desired color.
merging of all these microscopic dots of various colors (and in some
cases, sizes) that produces the illusion of a continuous tone print
with millions of colors (but without the tile pattern that you might
see if you zoom into an image on your computer monitor). Smaller droplet
sizes, in some cases as tiny as 3 picoliters (i.e., 3 trillionths
of a liter!) allow the printer to place more dots next to each other,
creating smoother tonal gradations, finer detail in highlight areas,
and wider color gamuts. (Note: Because of the way different inkjet
printers overlap and merge colors, a print from an HP Photosmart thermal
inkjet printer featuring 600 dpi resolution might rival the image
quality from an Epson micro-piezo printer featuring 1440 dpi. And
dye-sub printers with only 200 dpi resolutions can readily produce
a photo-quality print, but don’t do as well generating sharp
Until the majority
of photographers know the difference between ppi and dpi, or how to
resize an image file for printing without excessive interpolation
(see sidebar), I’d like to see a section of every camera and
printer manual dedicated to explaining the formula for converting
pixel counts to print sizes. Unfortunately, what that formula is and
how to apply it is an ongoing disagreement among the manufacturers
of camera and printing equipment, and all too often we hear about
a new 2MP camera that is just great for making photo-quality 8x10
to 11x14 inch prints. Or worse, we find a manufacturer promoting the
megapixel resolution of its inkjet printers. Is either possible? You
do the math.
Can you cheat reality by adding pixels?
imaging programs allow you to resize an image file quite easily by
adding pixels in a process known as interpolation. This helps smooth
out jagged lines and it reduces the effects of pixelization that would
otherwise occur if you printed a straight image file too large. It
works best if you just want to add 50 percent more pixels to the original
file. Increasing a file size beyond that may result in soft images
with color banding in fine gradations, and other artifacts.
To get the best
results, mix smart interpolation with careful sharpening. For example,
let’s say your image is from a 2MP (1200x1650 pixels) camera.
Following the 200 ppi rule, you should be able to make an 6x8 1/2
-inch print from this image without using interpolation. But in order
to make an 8x10 print, you’ll need to boost the pixel count
to 1600x2000 pixels via interpolation. (If your program offers you
a choice, use bicubic interpolation.) Once the resizing is complete,
use the unsharp mask filter to sharpen the image (that’s right,
the unsharp mask filter actually sharpens an image!). I recommend
experimenting with the unsharp masking controls a bit, starting at
100 for Amount, 0.3 for Radius, and 03 for Threshold. First,
increase the Amount setting, then increase the radius by 1/10 point
at time. If you’re using Photoshop, you might try switching
the color mode to Lab and apply the unsharp mask to the L level only.
This sharpens detail without blurring colors. Other sharpening plug-in
filters, such as the Sharpen Filter from Nik Effects, can be used
to improve the look of an interpolated file.
For images destined
for poster size, there are stand-alone programs and plug-in filters
such as Genuine Fractals and VF Zoom, that use fractal algorithms
to analyze an image and resize it 100200 percent without obvious
artifacts. But even these have their limits, and both work better
if you use images from 3MP-or-higher cameras. So don’t expect
to turn a 1.5MP image into a stellar 8x10-inch print.