Is having more megapixels always a good thing? I think not and here’s why.
Large files mean more memory cards, more time to transfer the files to your computer, more computing power and time to process them and more storage space to back them up. All this for extra resolution that you will actually be using on a very small percentage of your images.
Do you really need more than 16 or 20 Mpix to make good quality prints? The highest dot-per-inch (DPI) resolution for high quality prints is 300 DPI but 220 DPI is the more commonly used resolution for high-quality prints, 180 DPI is enough for most purposes as it is basically what the human eye can resolve at arm’s length. Given this, here are some common print sizes and the resolution needed to achieve them:
8″ x 10″ at 180 DPI = 1440 x 1800 = 2.5 Mpix
10″ x 14″ at 180 DPI = 1800 x 2520 = 4.5 Mpix
16″ x 20″ at 180 DPI = 2880 x 3600 = 10.3 Mpix
20″ x 24″ at 180 DPI = 3600 x 4320= 15.5 Mpix
24″ x 30″ at 180 DPI = 4320 x 5400 = 23.3 Mpix
Keep in mind that larger prints are normally viewed from farther away so they can be done with a lower DPI like 150. Giant roadside billboards can be printed at 10 or 12 DPI because they are viewed from a long distance! So as you can see, a 16 Mpix camera is enough for most photographers. A 20 or 24 Mpix gives you some extra latitude for cropping while still achieving a nice “full resolution” 20″ x 24″ print.
Speaking of cropping, many photographers like to say that the higher resolution of their full-frame sensor allows them to crop and gain the same benefit as the longer equivalent focal length you get with a cropped sensor, but is that really true? Let’s take a Nikon D810 with a 36 Mpix FF sensor as an example. If you crop it to the equivalent of an APS-C sensor with a 1.5 crop factor, you are actually getting only 36 / (1.5^2) or 36/2.25 = 16 Mpix as compared to the average 20 or 24 Mpix APS-C sensors. If you crop it to the equivalent of Micro Four-Thirds (M4/3) sensor, you get 36/(2^2) or 36 /4 = 9 Mpix, compared to the 16 or 20 M4/3 sensors. So you end up with a lot less resolution than you would by using a cropped sensor in the first place.
So you really have to keep in mind what you do with your photos and what disadvantages higher-resolution sensors will bring to your photography before you spring for the latest super high-res cameras.