One aspect of digital images which seems to cause a lot of confusion to
beginners is the matter of image size. There are three basic measures of image
Pixel count - e.g 3000x2000 pixels
Physical size - e.g. 4" x 6"
Resolution - e.g. 72 pixels per inch (ppi)
The confusion seems to arise because people aren't sure of how these are
related. They wonder if ppi affects the size of the image you see on a monitor
screen. We know that displays are measured in pixels (e.g. a 1024x1280 display),
and that screens are a given with (e.g. 15"), it seems logical that the
pixels/inch setting should affect the size of the displayed image. Logical maybe,
but wrong. Although monitors do have a measurable figure for ppi (pixels per
inch), the ppi information in an image is NOT used for monitor display in web
Digital image files are "tagged" with other information. For example the width
and height in pixels is in there, as is the resolution (ppi). Some image files
also contain all sorts of information about the image, such as exposure data,
focus data, flash data - and this is stored in what is referred to as the EXIF
header. EXIF stands for Exchangeable Image File Format, and is a standard for
storing interchange information in image files, especially those using JPEG
compression. So how is this information used, and what uses it?
When you display a digital image on a monitor using a web browser, the only
thing that determines the size of the image is the width and height in pixels.
All the other data contained in the image file including resolution (ppi data)
is ignored. If your image is a 480Kbyte file which is 800 pixels wide by
600 pixels wide, it will display as a full screen image if you are using an
800x600 display. It doesn't matter if your ppi is set to 1 or 1000 This is
100% true as far as web display goes and as far as any monitor display goes -
unless some software intervenes. For example the IE6 browser may take large
images and resize them so they fit on the screen. However ppi is still ignored. A
few advanced page layout programs and advanced image editors are capable of
taking ppi into account when displaying images.
So I'll say this once again. The way you control how large an
image appears on someone's monitor screen when viewing your images on the web is
by changing the height and width in pixels If your original
image is 1600x1200 pixels it will probably be too large to see all at once on 95%
of the video monitors out there. It will also be slow to load since it will be a
large file. If you want someone using an 800x600 display to be able to see your
image clearly, you need to change the size to, say, 600x400 pixels (remember the
browser window is smaller than the full monitor display). You change image size
in software. All image editing programs can do this. It's called "resampling". If
you make the image smaller it's often called "downsampling" or
"downsizing". if you make it larger it may be called
"upsampling" or "upsizing". See your image processing software
manual for details on what options your software offers.
For example here's a screen from PhotoShop (and PhotoShop Elements) which
allows you to change the pixel width and height of the image.
You can see here we are starting out with an image which is 1000
pixels wide and 640 pixels high. Let's assume we want people who still have
640x480 displays to be able to see all this image at one time. To do that we have
to reduce the image size. Let's say we want to make it half as big, 500 pixels
wide by 320 pixels high. What do we do? Well, we ignore what's in the document
size box, since that ONLY affects the way the file will be
printed. The data in the Pixel Dimensions box is all
that counts as far as web display is concerned. So what we do is
simply type "500" in the width field of the Pixel Dimensions box as shown below,
with both the "constrain proportions" and "resample image"
Note that since we had the "Constrain Proportions" box
checked, changing the width also changed the height in correct proportion.
Clicking on "OK" will resize the image to 500 x 320 pixels using a
"bicubic" resampling algorithm. The "bicubic" algorithm usually gives
the best results but takes most time. There are alternative resampling algorithms
called "bilinear" and "nearest neighbor" which are faster but which might not
give as good results. On a small file like this, speed doesn't matter, but if
you're working on a 180MB file, it might. Resampling is the name for the
process which figures out how best to subtract (or add) pixels when the number of
pixels in the image is changed.
Of course you can make images larger as well as
smaller. You can take an original image which is 500 x 320 pixels and
make it 2000 x 1280 pixels. The problem is that you have now created an image
with an extra 1.92 million pixels which have to be made up by the software. The
larger image will have no more real information in it than the original, so it
won't look as sharp. If you must do this, the best way to do it is using
bicubic resampling and to do it in small steps. By this I mean you may
get a slightly better result by enlarging in 5 or 6 smaller steps than
by enlarging in one big step. So for example you might change width (and height
in proportion) in steps from 500 - 750 - 1000 -1500 -2000, rather than in one
step from 500 - 2000. Sometimes you may see a small difference, sometimes you may
Pixels per Inch
You'll note that the width and height in the document size box
have changed along with the number of pixels in the image. This makes sense
because the resolution (pixels/inch) has not been changed. "Pixels per
inch" and is almost exclusively used for printing, not video display. If
you take an image that is 1000 pixels wide and 640 pixels high, and you print it
with a ppi setting of 180 pixels per inch, the print will be 5.556 inches wide by
3.556 inches high. If you resample the image down to 500 pixels wide by 320
pixels wide and you keep the ppi setting at 180 ppi, the print (document size)
will now be 2.778 inches wide by 1.778 inches high.
You can, of course, change the ppi setting. 180 is probably as
low as you want to go for quality images. For the very highest quality you might
want to go to 360 ppi as shown below.
Note that the resample image box has been unchecked
since we don't want to change the number of pixels in the image, just how those
pixels are printed. Instead of printing 180 pixels to every inch, we now want to
print 360 pixels to every inch. Obviously this will result in an image which is
half the size (but with twice the linear pixel density). You will see the new
print (document) size is now 1.389 inches wide by 0.889 inches high (pretty
What happens if you only have a 500 x 320 file, but you want a
10" x 6.4" print? Well, you either have to set the resolution to 50 ppi, or you
have to set the resolution to something like 200 ppi and upsample the image by a
factor of 4 (to 2000 x 1280 pixels). Either way image quality will suffer,
showing you can't make good large prints from small image files. There are some
programs (e.g. Genuine Fractals) which claim to be able to upsize with less
quality loss. Sometimes they give results which are a little better than using
bicubic upsampling in PhotoShop, sometimes they don't.
Dots Per Inch
There's another term, dpi, which is associated with printers. DPI stands for
"dots per inch" and is a property of a printer and printer driver software, not a
digital image. It's a measure of how finely spaced the droplets of ink can be in
a print. However the number is a bit misleading since it's not always measured in
the way you think it might be! Printer settings of 360dpi, 720dpi, 1440dpi and
2880dpi are often found. However the difference between then is subtle at best.
Most people probably couldn't tell the difference and 360dpi usually looks great.
Changing DPI does not change the size of the print. ppi controls that.
dpi controls print quality (though as I said, over 360dpi you typically don't see
Resizing images for use on Photo.net
Photo.net requests that images attached to forum posts be at most 511 pixels
wide, and that images uploaded to the image gallery be at most 800 pixels wide.
This is done for speed, bandwidth, and ease of display. So what you'd do here is
take the image from your digital camera, click on "Image -> Resize -> Image
Size" (in PhotoShop Elements II), or "Image -> Image Size" (in PhotoShop 6).
Then you check the "resample image" and "constrain
proportions" boxes and type in either "511" or "800" in the width box of the
Pixel Dimensions section and hit "OK". Your image will now be resized and you can
save it, preferably as a JPEG using the "File -< Save for Web" dialog. Ideally
images should be under 100Kbytes in size and you can adjust this with the JPEG
settings in the "Save for Web" dialog box
Right now photo.net will probably further resample your image, even if it's
small enough to be within the requested size parameters. This is due to the way
the existing software was originally designed. At some point in the future this
may be changes so that no further changes are made to submitted images which are
within the stated size requirements.
Note that there are some programs which create JPEG files which the photo.net
software for attached images doesn't understand. If you attach such an image to a
forum post it will display as a link, not as an image, even if it's under 511
pixels wide. One program known to do this is Micrografx Picture Publisher 8.0.
There may well be others. The problem seems to be that the photo.net program
incorrectly reads the file dimensions, so it doesn't know that it's 511 pixels
wide or less. In that case it attaches the file as a link rather than attempting
to display it.
What if you don't have PhotoShop?
Not everyone can afford PhotoShop - and indeed not everyone needs or wants
it.ust about every image editor ever made allows you to adjust the size of images
and just about every camera or scanner sold comes with one or other of them. They
all have different menus so it's not possible to give definative instructions for
every single program. If for some reason you don't have any image editing
software, take a look at IrfanView (
www.irfanview.com). It's easy to use and free for
non-commercial use.You can adjust image size via the "Image ->
is another alternative. It's a program originally developed for UNIX and ported
to windows. It's not as easy to install or to use as IrfanView. If you're a
novice, I'd recommend thinking twice about GIMP.