"But the display thing is obviously untrue. with the exception of paint, all the programs I use on the PC open the images at a reasonable size, certainly not the default 72dpi that Canon has in its images!"
Do you actually think Canon picked 72 DPI? Let me tell you where it came from. It is also currently the resolution standard used for displaying images on web pages, for historical reasons.
Once upon a time when PCs were first invented, text used to produced as an 8x8 dot matrix, both on your screen and on your dot matrix printer. A single dot was used to separate adjacent characters. This meant that each character occupied 9 pixels, and if you wanted to display 8 characters per inch, then you required 72 DPI to reproduce 8 characters per inch.
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"Full size" images are displayed on your screen at a lower resolution than what is actually saved in many files. So, how do they go from X megapixels to your screen resolution? As noted, MS Paint displays images without scaling the pixels to fit the screen. Obviously, the images are rescaled in some fashion. Depending upon the desired result, the scaling can be done in different ways.
To make a long story short, groups of pixels in the image file are averaged to produce one pixel that you see on your display. At 300 DPI, that is approximately a 4x4 matrix [for a 72 DPI display] of pixels are processed to produce a single pixel of a given color. A larger matrix, 8x8, would be capable of yielding higher color resolution per pixel. A smaller matrix, 2x2, would have less color resolution.
So, the "playing field is leveled" by requiring everyone to produce images at a specific DPI, and canvas size, so that everyone's images are scaled for viewing equally.
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"The right mouse button is your friend."
I did and do understand you don't understand what dpi is. I tried anf failed to help you understand. For that I am sorry but people like those 'judges' don't help when they consistently ask for something that doesn't matter.
