by Tom Lee Published 01/11/2006
In a nutshell then, if your Nikon D100 has a sensor of 2,000 by 3,000 pixels your image size is 2,000x3,000 = 6,000 (ie a 6 mega pixel chip) and this gives 6,000x3 = 18,000 bytes as your file size and you divide by 1,024 to get the 17.6 megabyte file size that shows up in Photoshop.
It is not surprising that people get confused but we have to take the time to explain it fully otherwiseyou ponder why Photoshop always reports a smaller file size than you expect if you are used to, say, kilometres which are actually 1,000 metres rather than 1,024 metres! Now remove the wet towel from your head and carry on reading.
As we have already said, all the computer cares about from a quality standpoint is how many pixels of information it has to play with. If the image is 1,024 pixels wide x768 pixels high and you display that image on your monitor at a resolution of 1,024x768 (the so-called SXGA), it will just fit nicely on your screen, providing the software which is driving the screen does not start making adjustments. The screen runs at a nominal 72 pixels per inch or 96ppi. This again causes no end of confusion. An image of 1,024 pixels stretched across a 24-inch screen will be displayed at a different resolution to one that is displayed on a 17-inch screen.
You cannot even do the sums on that scenario easily, as the screen resolution is measured up and down the screen and the screen size often measured across the diagonal - once again we are being bamboozled by the advertising men who prefer the bigger number of the screen diagonal and measure under the bezel of the CRT screen! They have more recently been forced by advertising standards to tell us the truth about screen sizes and measure it across and accurately. Even so, they cannot even agree amongst themselves on which pixel number should equate to which 'GA' number, but one take on the subject is listed below.
Some of the older ones originally expected to be totally phased out, but the arrival of mobile phones has partly saved their bacon, as the tiny screen can only display the smaller resolutions. At the big end of the scale the acronyms get ever-more hysterical; I wish somebody would take their pencils off them!
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