PIXEL DIMENSIONS
Technically your image size only depends on the pixel dimensions. If
you only intend to use your photos on the web, this is all that
matters.
DOCUMENT SIZE
The document size is the size of your image if you print it. Here
the pixel dimensions alone are not enough. It very much depends how
much resolution means PPI (pixels per inch) your printer will need.
What is the right value for PPI? This depends on the printer and
process you use. Some printing services have very strict
requirements for the PPI settings. For Epson printers like
2200/7600/9600 we have used successfully 180,240,300,360 DPI.
Note: Do not mix PPI and DPI. We are talking here about PPI while
you printer will be able to print in DPI (dots per inch). Advertised
DPIs of 1440 and 2880 can be confusing. Not a single dot in an
inkjet prints defines a colored pixel of an image. The color is
actually created by a dither pattern of different sized and colored
dots. This means that 360 DPI may often be enough for use with most
inkjet printers. What happens if you change the width, height or
resolution? This depends
on the following properties:
• Constrain Proportions: You most likely want to have it checked all
the time as otherwise the width might change but not the height.
Your image will look distorted.
• Resample Image: If checked you can upsample or downsample your
photo. However, there is a loss of quality involved.
• Checked: The pixel dimension of the images will change
• Unchecked: The pixel dimension will not change
Note: Never resample an image if it is not absolutely needed!
SCALING AND RESIZING
One of the most common operations on images is upsizing and
downsizing. Both are very critical operations and should be taken
seriously.
CHANGING PPI
If you just change the PPI of an image, you do not change any real
image data. This is a very easy and safe operation.