The issue of digital image resolution is a source of widespread confusion. This may be due, at least in part, to the fact that it is expressed in different ways depending on the context. To understand why this is so, it helps to know that all of the different expressions are intended to provide a way to determine the ocular resolution presented to the human eye when the image is viewed. In other words, "resolution" has clear and definite meaning only when the image is being viewed. Let's take a look at camera resolution, image resolution, monitor resolution and printer resolution.
Camera resolution is normally expressed as the total number of pixels in the image it producesgenerally in the millions so, Megapixels. This is really a shorthand expression based on the fact that the typical camera has a 4:3 aspect ratio, meaning the width has four pixels for every three in the height. From the total pixels and aspect ratio one can calculate the pixel dimensions of the image. Indeed, the only reason to state the camera resolution is to provide a basis for predicting the image resolution. But you don't need to calculate, just consult the Owner's Manual or take a picture and check the pixel dimensions of the resulting image. And that brings us to...
Image resolution is expressed as its pixel dimensions. A 3.1 Megapixel camera provides an image 2048px wide x 1536px high. A 20 Megapixel camera provides an image 5164px wide x 3873px high. Image resolution cannot usefully be expressed as total Megapixels because the image may be cropped and no longer have the standard 4:3 aspect ratio. (If you were thinking 72ppi or 300ppi, stop! Those have no meaning at all until we have a viewing device.) Note that the image pixel dimensions have no relation to ocular resolution or physical size until we choose a device for viewing the image. And that bring us to...
Monitor resolution is also expressed in pixel dimensions but this time there is a physical size involved, usually expressed as the diagonal measurement of the screen. (I'm positive this is done just to confuse us because the diagonal tells us nothing about the size of the screen unless we know the aspect ratio, which we have to get from the pixel dimensions. Why not just tell us the screen width and height in inches?) With pixel dimensions and screen size, we can now relate the information we have to the ocular resolution presented to our eyeballs.
If we divide the screen width in pixels by the screen width in inches, we get pixels per inch (ppi) and that tells us how far apart the pixels arethe pixel pitch. The old heavy glass cathode ray tubes were nominally 72ppi while LCD screens are nominally 100ppi. (Microsoft still uses 72ppi as a norm for MS Word, which is why you have to zoom to 140% to see your documents life-size.) Our individual visual acuity determines how far away from the screen we can be and still see separate pixels; the resulting ocular resolution is the angle made by the lines from the two pixels to our eye.
When you open a digital image in your browser, it will map itself pixel for pixel to the monitor pixels. If it has fewer pixels than the window in which it has been opened, the full image will display at a size corresponding to the monitor's pixels per inch. If it has more pixels than its window, the browser will normally reduce the number of image pixels to match the window pixels so the entire image is visible. Before browsers were set to do this as a default, images larger than the window required scrolling to see the parts outside the window. There are two things to take away from this:
And that brings us to...
Printer resolution is typically expressed in dots per inch (dpi) where multiple dots may be needed to represent a pixel. So you want at least a 1200 dpi printer to print 300 ppi images. Yes, we are now at the place where the ppi setting of an image is relevant. Although sometimes referred to as the image resolution, it's best to remember that the ppi setting of an image does not in any way change the image; its only function is as a printer instruction. It will determine the size and relative quality of the printed image; an image 3000 pixels wide will be 10 inches wide if printed at 300 ppi and 20 inches wide if printed at 150 ppi. For most inkjet printers, 150 ppi will result in a decent jpeg print. Counterintuitively, line art benefits from higher settings, e.g., 600 ppi. (Professional printing generally starts at 300 ppi for colored photographs and goes up from there, depending on the desired quality of the product.)
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