Slightly compressed spider image
Image Resolution, size, and
What does it really mean?
Over compressed spider image
Ok, so your
"5 mega-pixel" digital camera can capture at different
"resolutions" like 1024X768, 800X600, 640X480, or 320X240 and also with
varying levels of "compression". But
what does this mean? What is "compression" and how big a print can
Well, we hope to answer these questions
and more for you
as we demystify this subject.
| What does image resolution mean?
Resolution refers to the number of pixels in an image. Resolution is
sometimes identified by the width and height of the image as well as the
total number of pixels in the image. For example, an image that is 2048 pixels wide and
1536 pixels high (2048X1536) contains (multiply) 3,145,728 pixels (or 3.1
Megapixels). You could call it a 2048X1536 or a 3.1 Megapixel
image. As the megapixels in the pickup device in your camera increase so does the
possible maximum size image you can produce. This means that a 5
megapixel camera is capable of capturing a larger image than a 3
image resolution play out on my computer monitor?
The computer screen you
are looking at right now is set at a particular resolution as
well. The larger the screen, the larger you likely have your
screen resolution set. If you have a 17" monitor, likely you
have it set at 800X600 pixels. If you have a 19" screen it is
likely set at 1024X768. You can change the settings but these are
optimum for those screen sizes.
Now, if your monitor is set to
800X600 and you open up an image that
is 640X480, it will only fill up a part of your screen. If
you open up an image that is 2048X1536 (3.1 megapixels) then you will find yourself moving
the slider bar around to see all the different parts of the image.
It just won't fit. (To see the spider above at this resolution, (Warning!
536 Kb file!),
click here. Add to that the fact that the computer monitor
has a finite number of pixels per inch available (like 72) so if you are
going to display your image on a monitor only, you would want to drop
the quality down to 72 to save file space. If you are going to put
your image on a webpage or email it to a friend then you will want to
first make it a useful size. Not too big, not too small.
Maybe 200-300 pixels high would be a nice size. You can also
reduce the size of the file (not necessarily the size of the image) so
it loads faster. You reduce the file size by compressing the image
(see the next question).
|What does image
In addition to image size, the quality of the
image can also be manipulated. Here we use the word
"compression". An uncompressed image is saved in a file
format that doesn't compress the pixels in the image at all.
Formats such as BMP or TIF files do not compress the image. If you
want to reduce the "file size" (number of megabytes required
to save the image), you can choose to store your image as a JPG file and
choose the amount of compression you want before saving the image.
JPG compression analyzes images in blocks of 8X8 pixels in size and
selectively reduces the detail within each block. At higher
compression rations, the block pattern becomes more visible and there
may be noticeable loss of detail, especially when you attempt to make
prints larger than recommended. The subject and pattern in the
image is also a factor. For example, a picture of the blue sky can be
compressed quite a bit without any noticeable effects but a picture of a
colorful bird would "pixelate" quite quickly. Take
another look at the two images at the top. The first is somewhat
compressed, about 200 pixels high (size) and the file is only
14Kb. The image on the right is the same size but compressed quite
a bit more reducing the file size to 4Kb.
JPG compression, you can keep the physical size of the image the same
and reduce the amount of disk space required to store it but you will be
sacrificing the quality of the image.
|OK, so I have an
image that is 640X480. How big a print can I make?
Well, the true answer is you can make as
big a print as you want but very quickly you will start to see
"blocks" (pixelization) and the quality will drop off. To maximize the
capability of your printer, you should print a picture a size that the
printer can handle. Here we introduce a new term "dots per
inch" or "pixels per inch". Example, you have a
640X480 image and you want to print it at 200 dpi (dots or pixels per
inch). 640 divided by 200 equals 3.2 and 480 divided by 200 equals
2.4 so if you print this picture at 3.2"X2.4" you will get a
print with 200 dots per inch. We recommend 200 dpi as a minimum
for good quality prints.
Now, let's work one of these problems
backwards. Let's say we want to print an 8X10 picture at 300
dpi. What resolution must we have to do this? (stop here and
figure it out, or read on ;~) OK, 300 times 8 is 2400
and 300 times 10 is 3000. So we would need a 3000X2400 image to do
this. Let's see, 3000X2400 is 7.2 megapixels! That would be
one very nice digital camera and one very large file, especially if it wasn't compressed.
|Do you have any
rules of thumb for print size vs resolution?
Yes. First try to determine what your use
for the image will be. Will you want an 8X10 print or will you
only be emailing it to a friend? Choose the image size and
amount of compression to meet these needs and capture at the least
possible compression. The trade off is large file sizes and you
will fill up your media quicker but, later on you can dump the original
uncompressed image to a CDROM or hard drive, compress the original and resave it with a different file
name. You cannot expand a previously
compressed file so keep the uncompressed (or low compressed) file as a
master. To determine what resolution you will need for particular
print sizes, see the chart below. (from a printing company)
||Wallet size only
By doing the math, you can see that
this printing outfit wants a minimum of 150-200 pixels per inch.