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I'm excited by the explosion in digital photography. But as a computer and digital imaging professional, I ask myself how does the digital technology affect the pictures? And how can I educate my fellow photo enthusiasts while inflicting the minimum amount of crossed eyes and hair pulling?
I intend to explore here what I see as some essential tech specs. I'll try to keep it very basic, but I warn you – I'm going to be tossing about a few computer terms and doing some arithmetic. Pretty scary stuff.
First, do you know what a pixel is? It's short for picture element, a computer term used by geeks who don't mind confusing the uninitiated. A digital image is a rectangular array, hundreds and thousands of rows and columns of lots of tiny little dots, the pixels. Each pixel is just a single dot of color. Each pixel is just one color.
You’ve looked close at a picture in a newspaper and seen the individual dots of ink that make up the picture, right?
So a digital image on a computer is made up of a lot of these pixels, tiny dots of color. Today’s computer technology can display a maximum of 16,777,216 different colors, which the computer industry refers to as true color.
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Why such a seemingly odd number of colors? For those inclined towards the geeky side: The amount of computer memory required to contain 16,777,216 different unsigned integer values (0 to 16,777,215) is 3 bytes = 24 bits (1 byte = 8 bits; 1 bit can either be 0 or 1 <- the binary numbering system): 224 = 16,777,216 Computers handle chunks of memory better if those chunks are aligned on "even" boundaries, such as 2 or 4 or 8 bytes, so most computers use 4 bytes (32 bits) for each pixel. Only 3 of the 4 bytes used for each pixel are used to specify the color of the pixel. The number "0" tells the computer to paint the pixel black, and "16,777,215" paints it white. I can hear some wannabe geek saying, "My scanner does 36- or 48-bit color." Or, "My RAW files are 12- or 16-bit per channel," which is the same as 36- or 48-bit color - your computer has 3 color channels, Red, Green and Blue. Yes, your device may give you 12 bits per channel/36-bit color or 16 bits per channel/48 bit color, but once you see it on a commercially available computer screen or any other display, all you get is 24-bit color! There are some ideas why 12 or 16 bits per channel might be useful, which I would like to explore in a future article. Don’t be upset by this 24-bit color resolution "limitation," because 16,777,216 different colors is probably more than your naked eye can distinguish; most mere mortals can’t see the difference in white = "16,777,215" and slightly off white = "16,777,214".
If we had graphics controllers and monitors that would give us 12 bits per channel/36 bits to describe the color of each pixel, that would be more than 68 billion different colors. 16 bits per channel/48 bits would be more than 281 trillion. I don't think computers and printers will ever go there, and if they do, my eyes will never know the difference. |
So, my first basic and essential recommendation: Make sure the graphics controller (or video or display controller) in your computer is set to is set to its maximum 16,777,216 colors or true color. If you are using a standard monitor, I also recommend that you set your monitor refresh rate, which is how often the computer redraws the screen, to 85Hz. That will minimize flicker and is easier on the eyes.
Digital camera makers brag about how many megapixels their cameras feature. A megapixel is 1 million pixels. The number of megapixels quoted for a camera is the total number of pixels in the computer chip (“CCD” or “CMOS”) in the camera, but you don’t get all those pixels in the image. For example, the Nikon Coolpix 5700 is advertised with a 5.0 Megapixel CCD and a maximum resolution of maximum resolution of 2,560 x 1,920 pixels. But 2,560 times 1,920 = 4,915,200 pixels, not 5 million.
At 4 bytes per pixel, 1 megapixel requires 3.8 megabytes (3.8 MB) of computer memory storage. (Because of the arrangement of computer memory in binary, 1 MB of memory is not exactly equal to 1 million bytes. 1 MB is actually equal to 220 = 1,048,576 bytes, so 4 million bytes divided by 1,048,576 bytes per MB = 3.8 MB.) (That’s uncompressed image storage – some image file formats such as JPEG/JPG can compress the file size, but that’s a whole ‘nother ball of wax. When a 1 megapixel image is loaded into video memory and every pixel is displayed on the screen it still takes up 3.8 MB in video memory.)
To make a photo realistic image, you see you need, oh, gadzillions of pixels, each one so small that you can’t see it individually. How many pixels is a gadzillion – how many pixels do you really need?
How many pixels you need depends on what you intend to do with the picture, how you intend to display it. And it also depends on your individual tastes, what you’re willing to accept.
These days the typical computer screen and digital projector might be set for a typical 640 by 480 pixels – that’s 640 pixels in each row across the screen and 480 rows of pixels down the screen:
Or the typical computer screen could be set to 800 by 600 pixels, or maybe 1,024 by 768, or other resolutions. The output resolution of many digital projectors is 1,024 by 768 pixels. Now:
So based on megapixels alone, if you’re only intending to show your digital images on your computer, on a web page or e-mail to friends, you don’t need an expensive digital camera with lots of megapixel power. You don’t even need much more than a megapixel!
If you want print quality, now we start talking mucho megapixels. Magazine-quality printed images run around 250 to 300 dots per inch. A digital pixel and a printed dot are not the same thing, but many people use the terms "dots per inch" (dpi) and "pixels per inch" (ppi)interchangably. I think of "dots per inch" as a printer specification, how many dots of ink in an area; and ":pixels per inch" is how closely together the pixels are printed. But for my calculations here if I rudely equate a digital pixel with a printer dot, then for magazine print quality:
So if you want nice prints, you may be able to get as large as 8 by 10 inches with a 5 or 6-megapixel digital camera. If you want larger prints, you may be waiting for a digital camera that doesn’t exist yet!
You can settle for less than 250 or 300 dots per inch if you like. Nikon advertises the aforementioned Coolpix 5700 with, "Print images as large as 16 inches x 20 inches and beyond with outstanding image quality." A 16 by 20-inch print at 2,560 by 1,920 pixels is printed at 120 to 128 pixels per inch. You would have to judge for yourself if that quality is acceptable.
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