Digital Photography vs Traditional Photography print vs. WEB use
	Back to the Numbers Question? So what is the difference between 1600x1200@75 DPI and 800x600@150 DPI and 400x300@ 300 DPI? Answer! Nothing! (Depending on a couple of things!) But how can that be? In the Digital World we need to look at the total number of pixels used to image a single scene, and how it is viewed in a particular environment. DPI (dots per inch) only has a meaning when the output method can be varied to use it. If we are using a monitor to view all of our images, then the total number of pixels is relevant, but not the DPI scale. Remember : all monitors work at a fixed DPI. Let's look at the next three images (Each is a screen capture of a computer monitor.), one image on the screen is 500x400 @ 75 dpi, one is 500x400 @ 150 DPI and the other is 500 x 400 @ 300 DPI.But notice each one is being viewed on the computer screen and each appears to be exactly the same size. How can this be? The answer comes from the total number of pixels in each image. Each is 500 x 400 pixels and no matter what resolution in DPI is used, they must display at 75 DPI on the computer monitor. So what changes? The SIZE of each image in inches or cm.
	500x400@ 75 DPI ( 6.67" x 5.3") 500x400@150 DPI (3.34"x2.65") 500x400@300 DPI (1.67"x1.33")	Notice the first image the Width is 500 and the Height is 400, and it has a resolution of 75 DPI. But the overall image size is 6.667 inches by 5.333 inches. If we do the math: 6.667" x 75 DPI = 500 total pixels 5.333" x 75 DPI = 400 total pixels In the second photo the image size has changed from a width of 6.667 inches to 3.333" and the height has changed from 5.333 inches to 2.667 inches. The second image is at 150 DPI. 3.333" x 150 DPI = 500 total pixels 2.667" x 150 DPI = 400 total pixels In the third photo the image size has changed from a width of 1.667 inches to 3.333" and the height has changed from 5.333 inches to 1.332 inches. The second image is at 150 DPI. 1.667" x 300 DPI = 500 total pixels 1.333" x 300 DPI = 400 total pixels Since all of the images contain 500 pixels in the width and 400 pixels in the height they are the same size as far as the monitor is concerned. It will display them all at 75 DPI, as that is all the monitor can do. Would there be any difference in their quality? NO! Each displays the same pixels and has the same color value per pixel. ---------------------------------------------------- If we were to send each of the images to the printer, the story changes. The printer can vary its output and can print at different resolutions. So he first image would print at 75 DPI, the second one at 150 DPI and the third at 300 DPI. Since each one has the same total number of pixels, the first would print at 6.667 inches by 5.333 inches, and the second one would print at one half that size (3.334 inches by 2.6555 inches), and finally the third image would be one half the size of the second or one quarter the size of the first (1.667 inches by 1.332 inches) Would there be any difference in their quality? YES! The first image would be big and fuzzy looking, the second one sharper but smaller, and the third would be the sharpest, but smallest in size. Since the printer can change resolution as needed the images with higher density per inch would create more detailed images. ---------------------------------------------------- It is the combination of resolution (DPI) and the physical size of the image (in inches or cm) that determines the total number of pixels. Or it is the total number of pixels divided by the resolution (DPI) that determines the size of the image. All of these values are related and work together. You must understand the final use of your image to determine the proper combination. If you are using images for the WEB or for use on a monitor, then the only resolution you need to worry about is 75 DPI. All monitors will consistently display such images. If you are going to use the image for print, then you will want 150 DPI or even higher resolution to get the best quality. You can freely change the resolutions without any loss in quality as long as you remember the associated size change involved.
	Digital Resolution vs. WEB Use vs. Print Use Mega Pixel Camera (@75 DPI) WEB @75 DPI Print @ 150 DPI Printed Size 4 MP 2272x 1920 31.55" x 23.66" @ 150 DPI ---> 15.14 " x 12.8" 3 MP 2048 x 1536 28.44" x 21.33" @ 150 DPI ---> 13.65 " x 10.24" 2 MP 1600 x 1200 22.22" x 16.66" @ 150 DPI ---> 10.6 " x 8.0" 1 MP 1260 x 960 17.5" x 13.33" @ 150 DPI ---> 8.4 " x 6.4" Listed are current camera capture ratios that are widely available for under $1000. They can be used at full size although they're much too large for most WEB pages or computer display as they capture at the same resolution that the monitor displays. For print use, if one wants to use all the original pixels, then one must reduce the size of each image by a factor of two and increase the resolution to 150 DPI (also a factor of 2). The final column shows the idealised print size for a given camera capture ratio. At these sizes all of the original pixels will be used in the printed copy. From actual use I have found these numbers to be a bit optimistic. It is often necessary to reduce original image size some while adjusting the histogram, color correcting and sharpening. (More on these things later.) Anyway this offers a rough guide to the level of resolution you may want to consider depending on your desire to print, and what size you wish to print. A 1 or 2 mega pixel camera is totally sufficient for virtually all WEB use. Images taken from those cameras are reduced in size before use, so you end up throwing away a large part of the original information in most cases. For print use, you may wish to consider the higher resolution cameras or even slide scanners.