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How many megapixels do you need?
In buying a digital camera, the first question one will run into is "how many megapixels" do I need? As this is the most visible advertised feature in digital cameras these days, it's important to understand what you truly need.
How large do you want to print your photos?
When it comes down to it, deciding on the resolution for your digital camera is as simple as determining what you would like to do with the printing. If all you want to do is capture basic snapshots that are intended to print as a 4" x 6" (4x6) photo, then it is possible to get away with even a 1.0 - 2.2 Megapixel (MP) camera, depending on the final output quality desired.
However, most of us like to be prepared for the fact that someday we may capture a photo that deserves "blowing up" / enlarging or even cropping. This is where high-resolution digital cameras come to shine.
|Print Size||Minimum Resolution
|4 x 6 inch||1.0 MP||2.2 MP +|
|5 x 7 inch||1.4 MP||3.2 MP +|
|8 x 10 inch||3.2 MP||7.2 MP +|
|11 x 14 inch||6.2 MP||13.9 MP +|
|13 x 17 inch||8.8 MP||20.0 MP +|
|20 x 30 inch||24.0 MP (*)||54.0 MP +|
- The above table indicates the achievable print sizes straight from the camera. Given the techniques shown on other pages (using Photoshop or IrfanView), one can achieve print sizes greater than what is shown above.
- The data in the above table is easy to calculate. One takes the print size and multiplies the
two dimensions (in inches) to get the area in square inches. Multiplying the area by 0.04
for minimum resolution and 0.09 for best resolution will give the megapixel
counts stated above.
Where do those 0.04 and 0.09 come from? It is generally accepted that the minimum resolution in a print that is suitable for close viewing is about 200 DPI (dots per inch). So, for a one-inch square, we would need 200 dots x 200 dots = 40,000 dots = 0.04 megapixels. Similarly, the resolution at which the human eye would have a hard time "seeing the dots" is at or about 300 DPI, which corresponds to 300 x 300 = 90,000 dots or 0.09 megapixels.
- NOTE *: With very large prints (eg. 20x30 inch prints), one can get away with much lower effective resolution (than say 200 DPI) because the viewer will often see the print from a distance. Therefore, the rule of thumb suggesting a 200 DPI minimum is not as realistic. It is possible to get reasonable prints at this size from 10 megapixel cameras, for example. One can also take advantage of some excellent image editing software packages that are able to interpolate (up-res or up-scale) "natural" images to much greater resolutions very effectively (e.g. Genuine Fractals).
Why do some Canon cameras set the resolution to 180 DPI?
For the longest time, I tried to figure out why the Canon 10d generated photos that were tagged with a resolution of 180 DPI. Why not 300? After giving it some more thought, it likely that they wanted the 6.0 megapixel camera to automatically imply a print size of 11 x 14 inches for advertising purposes (but notice that this requires a lower resolution than the 300 DPI that is standard for printing).