Saturday, June 25, 2011
From a purely technical standpoint the number of MPs depends on how you plan to use the photos. If you are planning to sell prints it depends on how big you plan to print and how finely detailed you want the prints. I’ve printed 18’ X 24” posters shot with my XTi which is 10MP. The jump to 15 MP is only 50% and 18 MP is only 20% more than 15 MP (80% more than a 10 MP camera).
The average human eye can't resolve smaller than 100 dpi without the aid of a magnifying glass. Most photos are printed between 150 dpi/ppi and 300 dpi/ppi. At 240 ppi a 10MP image will print 10.8" x 16.2". If it is a highly detailed image and you want to print at 300 ppi it will only print 8.64" x 12.96". If it is low detail and you are comfortable with printing at 150 ppi you can make a 17.28" x 25.92" print. Portraits can be printed at 150 ppi since seeing every pore in the skin is rarely flattering except perhaps to newborns.
My ill-fated G-10 was 14 MP and a 240 ppi print from that would give me a 13.8" x 18.4" print (note the different aspect ratio) but the trade off with the smaller sensor was that cramming in all those photosites onto a tiny compact sensor resulted in noise problems at any ISO above 400. Canon gave up and went back to 10 MP for the G-11 & G-12 because of that. For comparison purposes though, the 14 MPs from the G-10 approximates a 15 MP camera.
My 7D is 18 MP and it will print 14.4" x 21.6" at 240 ppi. That's only 3.6" longer on the short side and 5.4" on the long side larger than 10 MP at the same 240 ppi. That's because the 80% larger is area, not linear. In linear terms 18 MP is only about 30% larger than 10 MP at the same print resolution. For most purposes a 10-12 MP camera is the threshold (roughly equal to 35mm film) but you can do fine work with even less.
FWIW Kirk Tuck, portrait/commercial photographer of considerable repute in Austin, TX, author of several photography books and a popular blog still shoots portraits occasionally with an old 6 MP camera. I have printed the above photo at 30" x 40". It was shot with my 10D, a 6 MP camera. You can’t see every vein in the leaves or count the flakes of snow but that wasn’t my objective in making the image. In the end, photography is more about content and vision than pixels.