If the image is humongous you will may experience frustration when you try to sharing it on Facebook or another social site it will take longer to upload your picture.
When more megapixels are good
The only time when extra pixels on a smartphone camera might be useful is if you might want to crop in on an image to reveal detail in the distance. The extra megapixels will give you a little extra flexibility when it comes to cropping in on your image.
These extra pixels are even more crucial when the camera zoom is poor. Some compact cameras and all DSLRs have optical zooms, which mean that the focal length of the lens increases. Other compacts, and nearly all smartphones have digital zooms. In that case the camera shoots the photo and them crops it in to create the close up.
The Samsung Galaxy S4 Zoom has a 10x optical zoom - thanks to which it looks more like a compact camera that a smartphone.
How many megapixels do you need?
Ken Rockwell explains how to figure out how many megapixels you really need. If you want a super-sharp print you really need to be able to print at 300 DPI (dots per inch). He explains that you can calculate how many pixels you need this way:
Long print dimension in inches = 4 x (square root of megapixels)
Long print dimension in centimeters = 10 x (square root of megapixels)
For example, the square root of four (megapixels) is two. 4 x 2 is 8in. Thus the biggest print you can make without losing sharpness as seen through a magnifier from a 4MP camera is 6 x 8in (15x20cm). From a 16MP camera you could go to 12 x 16in (30x45cm).
Following Rockwell's calculations, this means the 8MP iPhone 5s can give you a [square route of 8 is 2.8284: 10 x 2.8284 = 28.28427] 28cm long, or [4 x 2.8 = 11.3136] 11in long shot.
In comparison, a 13MP phone camera could give you [square route of 13 is 3.6055: 10 x 3.6055 = 36.055] 36cm long, or [4 x 3.6055 = 14.422] 14in long reproduction of your shot.
As a point of comparison, the length of an A4 page is 29.7cm or 11.692in, a tiny bit longer than the 8MP shot, but smaller than the 13MP shot. We think it's unlikely anyone will want to print camera-phone photos out at a size bigger than an A4 sheet so 8MP seems quite adequate. (US letter sized paper is 27.94cm, suggesting that the 8MP shot is ideal for that market).
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