WONKO'S IMAGE SIZING TUTORIAL
DPI (SYNONYMOUS WITH DOTS PER INCH, PIXELS PER INCH, RESOLUTION)
If you measured an inch on your photo at the size you want it, this measurement is how many pixels you would count in that inch. Again, this is really a measure of the quality of the picture. This measurement is expressed ###dpi, or if the image was to be represented at 300 dots per inch, you would then say it is at 300dpi.
So why is this important to know?
The number of pixels wide and high, and the DPI are both necessary to get the size and quality of the image correct.
In my image program all I have to do is tell it what size in inches/centimeters and it saves it to that size. Why do I care about all this other confusing stuff?
I get that one a lot. Editing programs often, to make things more simple, set a default or average setting that they think will work for most things. For instance, my wife's image editing program that came with her camera sets all the pictures at 150dpi. This works OK for some things, but is really not acceptable for most things. Let me show you, using the windmill image again:
This image is 36x50 pixels. I knew that was the size I wanted because most computer monitors show pictures at 72dpi. I wanted a half inch wide image, so I took half of 72 and it came up 36. Restated, this image is a half inch wide at 72dpi.
Notice I stated the size two ways, "36x50 pixels" and "a half inch wide at 72dpi". The WxH measurement is generally preferred in most applications.
When professionals are discussing the size of a button on a page they are working on, they are thinking, "I would like this to end up about an inch wide by half an inch high." Again, they know most monitors show images at 72dpi. Rather than confusing the issue with all of the other information, they will ask for a "72 by 36" button. When they are making the button, it does not matter if they choose 1dpi, 72dpi, or 1,000,000dpi, he will end up with the button being the size he wants.
Most of the advertising banners and the like that you see on web pages are standardized sizes set, not by inches, but by pixels wide by pixels high. This makes sure that any image that goes in that space will be the intended size.
There are times when you will still use the "ruler" measurements. Usually these are applications where a set dpi has been specified or is expected. My printer specs out at 600dpi. What this means is that when my printer prints an image, it will print out with 600 different dots of color in an inch. This lets me make pictures that have enough detail that you have a hard time telling them from a photo. If I always set my photos at 600dpi, I will know that I am getting the finest quality image possible, with any size print, on my printer. There is no point in saving it at 1200dpi, because my printer will either print it out too large, or all of that extra information will just be wasted disk space.
The one thing to remember is that when specifying a "ruler" measurement, to be accurate, there must either be an agreed upon or specified dpi. If this is not the case you will most likely end up with something you did not want.
On the next page I will show you some comparisons of what happens when you change various settings.