When printing an image, the
amount of detail the print will show is directly related
to the resolution of the camera. Put simply, the higher the
resolution, the better and sharper the printed image will
be.
To illustrate this relationship between print sharpness and image resolution,
a simple demonstration has been set up. A Minolta F300, a five megapixel
camera, is used to capture 4 photos of the same subject and from exactly
the same position, each at one of the camera's 4 available resolutions
(2560, 2048, 1600 and 640). The photos are then printed on a Sony Digital
Photo Printer (DPP-EX5) as borderless 6 x 4 inch prints.
As with many digital
cameras, the F300 records an image with a 72 ppi (pixel per
inch) resolution*. The pixel per inch count of a digital
image has a relationship to the DPI used in printing, as
it relates to the 1:1 image size that can be produced from
a photo without any up, or down, sampling.
In our example, the printer used requires 403 pixels per inch, and images
that cannot produce a 6 x 4.5 inch print at 403 pixels per inch require
some degree of up-sampling to meet the needs of the printer. However, the
greater the up-sampling is, the coarser the image becomes.
*
The ppi value of 72 originates with the ppi of colour monitors.
Most were limited to 72 pixels per linear inch. Nowadays
most monitors have ppi values of 96, and many digital cameras
record images with a higher ppi than 72.
At this point, a look at what ppi represents, and how it relates
to dpi is probably in order. The pixels per inch (ppi) of a digital
image is really a bit of an arbitrary value as it does not assign precise
dimensions to the pixels. The dpi, on the other hand, is a precise measure
of the numbers of dots per inch used to print the image with a given
device.
With the ppi, one manufacturer may set it to 72 ppi — as we saw
above, a leftover from the resolution of older monitors — while
another will peg it at 180 ppi, and yet another at 300. To the user,
the only real function of the value used for the ppi is to provide an
idea of the print size that can be produced by the resolution of any
given image.
Let's take a look at an example, and see how variations in the ppi value
of a 3-megapixel image affects the image size if we treat ppi as
having a 1:1 relationship with dpi:
2048 x 1536 at 72 ppi = 28.44 x 21.33 inches or
72.25 x 54.19 cm
2048 x 1536 at 180 ppi = 11.37 x 8.53 inches or
28.9 x 21.67 cm
2048 x 1536 at 300 ppi = 6.82 x 5.12 inches or 17.34
x 13 cm
Since most printing devices — inkjet, laser and dye
sublimation — output images at dpi values ranging
from 150 to 400, the higher ppi numbers provide a better
idea of what will be realistically possible in terms of
print size. The values are however of limited importance,
in and of themselves.
With our test images, the chart below shows the differences between the
resolution of the image and the size it can produce at the resolution of
the printer. Remember, these numbers are in relation to our output device.
Resolution
Image Size
if the ppi is
set to 403 (printer res.)
PPI of
the image when the image size is forced to
6 x 4.5 inches
(15.24 x 11.43 cm)
640 x 480 (VGA)
1.588 x 1.191
in.
(4.03 x 3.03 cm)
106.667
1024 x 768 (0.8 MP)
2.541 x 1.906
in.
(6.45 x 4.84 cm)
170.667
1280 x 960 (1 MP)
3.176 x 2.382 in.
(8.07 x 6.05 cm)
213.333
1600 x 1200 (2MP)
3.97 x 2.978
in.
(10.08 x 7.56 cm)
266.667
2048 x 1536 (3 MP)
5.082 x 3.811
in.
( 12.91 x 9.68 cm)
341.333
2272 x 1704 (4MP)
5.653 x 4.228
(14.32 x 1074 cm)
378.667
2560 x 1920 (5MP)
6.352 x 4.764
(16.13 x 12.1 cm)
426.667
As can be seen by the PPI numbers shown above, in the case of our test
images, only the 5-megapixel image would be able to produce a 6 x 4 inch
print without any resampling. If instead of a dye sublimation printer
an inkjet type printer was used, a lower resolution would yield a larger
image. However, this printer has a fixed DPI, much like
commercial digital photo printers.
The images below are scans made of the test prints. The scans, measuring
400 x 300 pixels at 300 PPI, where done of the same section of the prints.
(It is worth noting that scanning produces a lower resolution image.) The
images were then saved in JPEG format with the quality set to 60% to avoid
creating further artefacts:
640 x 480:
To produce a 6 x 4 print at 403 PPI, the 640 x 480 image needs to be resampled
considerably. The result is an image that exhibits a lot of artefacts,
and only some of the words in bold are legible. Printing this image without
resampling is possible, but although it appears more crisp and has fewer
colour artefacts, it still lacks sufficient detail to be readable.
1600 x 1200:
Surprisingly, the 2-megapixel image is quite acceptable, even after the
resampling necessary to bring it up to the printer's resolution. Some colour
moires can be seen, but obviously, a 2-megapixel resolution is already
sufficient to produce an acceptable 6 x 4 image with this type of printer.
2048 x 1536:
If 2 megapixel is acceptable, then 3 should be even better. Indeed, the
3-megapixel image shows that the minimal resampling required permits a
good sharpness. Finally, we see even less colour moires than in the 2 megapixel
image.
2560 x 1920:
At the 5-megapixel resolution, the image contains more data than is required
for a 6 x 4 print at 403 DPI. The text is sharp and very legible. There
are no colour moires. To produce this image, the printer driver actually down samples
the data, a process that can have the effect of augmenting the perceived resolution
by eliminating any artefacts that might be present in the image.
To the naked eye the text in
the printed photos is small (approximately equivalent to
a 5 point face), but completely legible in all but the print
made from the 640 x 480 pixel image.
As explained at the beginning, the point of the exercise is to illustrate
the fact that the higher the resolution, the better the print quality.
The Sony DPP-EX5 has a resolution that is commonly used for commercial
digital printers, the machines that are used to produce prints in stores
and at photo kiosks. The results shown here clearly demonstrate that to
obtain a high quality 6 x 4 inch print from such a machine, a minimum resolution
of 2 megapixel is required. Moreover, it also shows quite clearly that
an even higher resolution is better.
For anyone purchasing a digital camera with the intention of replacing
their film camera, and for anyone who intends to either print, or have
the photos printed, resolution should be the foremost consideration
when making a selection.
