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TechBusters: Double Megapixel = Double Print Size

Monday, August 20th, 2012 by | Filed Under: Digital Photography

Welcome to TechBusters from WorldStart, where I expose the truth behind technology myths and sayings.

In this article I’ll talk about digital cameras and higher megapixel counts. Do you know anyone who got a new digital camera and said something like, “This 12mp (Megapixel) camera is so much better; it can print images over twice as big compared to my old 5mp camera.”

Guess what? WRONG!

Why is this false? I know you may be thinking, “Tim… 5×2 = 10 so 12 would be more then twice as big!” well I’m sorry to say your 3rd grade math teacher is very upset with you right now. When you print an image the pixels need to fill a two dimensional object, like a rectangle, so the area is the length multiplied by the width, not the length plus the width…

Let’s say that rectangle is 4×6 – well that’s 24 square inches of space. Now let’s say you want “twice as big”, so that’s 8×12 which is 96 square inches! You would need four times as many pixels to fill the 8×12 print compared to the 4×6!

So what amount of megapixels do you need to double the printed image size?

  • Twice the print size of a 2 megapixel image is 4 megapixels.
  • Twice the print size of a 4 megapixel image is 16 megapixels.
  • Twice the print size of a 8 megapixel image is 64 megapixels.
  • Twice the print size of a 12 megapixel image is 144 megapixels.

So why don’t we all need 144 megapixel cameras? It all comes down to print size, viewing distance, and DPI. DPI is how many dots are in a square inch of space, and the reference for “photo” has been about 300 dpi viewed at arms length. To print a 4×6 at 300 dpi you only need 2.16 megapixels (Multiple the length x 300 then multiply the width x 300, then multiply those two numbers together – example: 4×300 = 1200, 6×300 = 1800, 1200×18000 = 2,160,000 pixels). To print a 300 dpi 8×10 you need 7.2 megapixels. Since most people don’t print images much bigger then 8×10 the need for very high pixel counts isn’t there yet.


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5 Responses to “TechBusters: Double Megapixel = Double Print Size”

  1. Daniel L McCray says:

    What you neglected to say in this article is if you crop a photo hard and the blow it up you WILL want those extra megapixels

  2. richtea says:

    300 dpi is, of course, the highest quality & not absolutely necessary; 150 dpi would do quite nicely, too, providing for a larger print size

  3. Michael Zaret says:

    My math seems to say that 8×12 is 4 times as much area as 4×6. The non-math eyeball will also experience it as 4x bigger.

  4. Allan Poe says:

    Excellent article. For the first time, I feel I have a good grasp of pixel count, dpi and image size.

  5. Allan Poe says:

    I have to disagree a bit with your terminology. When someone says “twice as big”, this normally implies twice the area, not twice the dimensions. Example. If I were to say an area is twice the size of a football field I am saying the new area is twice the area of a football field, not twice each dimension.

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