| Why Megapixels are Important and Unimportant at the Same Time
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Author: Joseph Hamilton Added: December 15, 2006
A megapixel, simple put, is one million pixels, recorded by your digital camera's image sensor, which is the equivalent of the light sensitive film in film cameras.
So let's even the score a bit and compare apples to apples. Your computer screen displays images at a resolution of 72 dpi. So in order to evenly compare a printed image to a displayed image we need to do some calculations.
Let's take an 8x10 image at 300 dpi, double the minimum standard. So if we multiply 8 inches times 10 inches we end up with 80 inches, not too hard. Then we will multiply 80 inches times 300 dots per inches, or dpi and we get 24000 dots. But that's not how dpi is calculated 300dpi actually means 300x300 dpi in both ther vertical and horizontal directions, so that means 9,000 dots per inch.
So how many megapixels is that? Well a 3 megapixel image is 2048x1536 in resolution, or 3,145,728 pixels. 9000 dots per inch equals 7.2 megapixels. So that means I need a 7 megapixel camera to print an 8x10? Let's find out.
Where can I get a printer that prints at 9000 dots per inch? Do they exist? Let's find out. So I printed out an image on my HP deskjet k60 at full resolution, which the manufacturer claims is 2400 x1200 dpi. So that means we should be seeing 2,880,000 pixels per inch.
To test this I scanned it using my scanner and pulled it into photoshop to examine the print pattern and the true dots per inch.
The first thing I noticed is that the dots coming out of printer are not in a perfect matrix, they are scattered and diffused in a strategic pattern.
As was expected, all the dots are either, cyan magenta or yellow.
How many dots were in an inch? I took 1 milliter samples and counted the dots. There were roughly 20 dots per milliter in each direction, 400 dots total.
A milliter is 1/100 of an inch, multiply 400 x 100 and you get 40000 dots per inch.
What? what happened to 2,880,000 dots per inch?
Now, I was using plain paper, so I would expect the dots to be a little bit smaller on photo paper, possibly yielding 150 dpi of print resolution. Bear in mind also, that I am taking an average, you may get slightly better or slightly worse results with other printers.
Let me also say, the reason why the printer manufacturers say they can print that many dots per inch is because, in some places the printer will actually print a dot on top of a dot to get a certain color. But this is NOT a resolution enhancement because the dots are on top of each other.
So the bottom line? How many megapixels do we need to print an 8x10 with this kind of printer? This printer will print 200 dpi or 200x200 = 40000 dots total. Multiply that by 80 inches and you get 80 x 40000 = 3200000.
3.2 megapixels, to print out an 8x10 at 200 dpi. if you want to print at 150 dpi you are going to need 1.8 megapixels and if you want to print at 100 dpi you will need .8 megapixels
If you don't believe this article, do the calculations for yourself and repeat my experiment to see if what I said was true, maybe I made a mistake. I probably didn't though, we all know that big companies love to "pull the wool over our eyes".
Joseph S Hamilton
Digital Photography
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