TLDR summary: 16MP is probably enough for what most people do with their images *** I often see discussions related to the sensor resolution on digital camera forums and I've been meaning to write an article on a forum giving my take on the subject. I’m not a professional photographer and never have been; I would describe myself as an amateur with a good technical background. What I am is an IT professional with years of experience, a good chunk of which was spend working for a company that provided digital imaging and printing solutions for large corporations to resell to both professionals and end users. My role was to develop and build the back-end server systems that handled the front-end web services, storage and processing of the images. As a result I know my way around (although I am certainly not an expert) a variety of professional printing solutions from wet film minilabs (C41b chemistry), inkjet minilabs through to warehouse printing labs. So anyway; onto the main point of the discussion, how many megapixels do you need? My first caveat is that I am not considering the usage case whereby the source image has been heavily cropped to produce the final image. This has the potential to change the maths in favour larger megapixel sensors. Secondly I am aware that some printing solutions can be configured to accept and input resolution of greater than 300ppi, it’s just that most aren’t configured this way (you may have a friendly lab owner that might do this for you). Viewing on the screen For viewing on a screen; the highest resolution commonly available to consumers is "Ultra High Definition" 3840 X 2160 which is roughly 8MP. So unless you are pixel peeping, there is really no benefit going much over 8MP, you only risk increasing noise unnecessarily. This is probably why certain mobile phones such as the Sony Z series 20MP sensors have a default output of 8MP, it also makes a lot of sense of the Sony Alpha 7S and its 12MP output. Lab Printed Images Printing images is a little more complex, although the maths is pretty simple. The first thing to know is that most lab solutions have a default input resolution of 300ppi. This would mean that any camera image with an output resolution of greater than 300ppi will be down-sampled either before reaching the lab or by the lab during the processing. To make the sums simple, let’s consider a 12MP output sensor with a 4/3 aspect ratio. This would produce an image with a resolution of 4000x3000. For a 300ppi input file your maximum image size without a reduction in output quality would be 13.33”x10”. Anything less than 13.33”x10” is fine, down-sampling is not likely to reduce the quality of the output. Anything more than 13.33”x10” will result in a reduction of potential quality of the output. If you have a m43 camera such as the Olympus E-M10 with a 16MP sensor, then your maximum output resolution is 4608x3456 which should be good for 15.36”x11.52” before you lose any output quality from the printing process. In reality you can push the image much further than this and still get good results. Quite a few years ago I printed a 6MP test image from a Fujifilm FinePix 6900 (this is a 3.3MP sensor interpolated output to 6MP) on a professional inkjet printer at 4ftx3ft which was nicely laminated for me by the lab. The poster looks great to my eyes (I still have it at home); sure if you stand 6 inches away from it you can see the resolution isn’t infinite, but standing a few feet away it looks great. That’s at roughly 60dpi output… Home Inkjet Printing Just a few words about home inkjet printers, which often advertise fantastic resolutions of 4800dpi or greater. It’s important to realise that an inkjet printer has a fixed number of colours available; typically 5 for a basic model (black and nothing should be considered as colours in this case), with 9/10 colour models commonly available (my Canon inkjet has CYM + Light CYM + Black + Grey + None + Text Black). With Inkjet dpi it’s important to remember that there is a big difference between input ppi and output dpi. With an input of 300ppi and an output of 4800dpi, each input pixel will be mapped to an output consisting of 16x16 dots of ink. That’s 256 individual dots of ink, each with a possible of 9 different values in the case of my Canon printer. That’s an argument for increasing the input resolution to 600ppi in order to obtain a better quality output. You might get some mileage out of this, but remember the downside will be reducing the dots of ink per pixel on the output. Summary *** For Aunty Agnes who prints 6x4 for the family album and views images on a 1080pHD screen, 2-3MP is really all that is required. For enthusiasts and many professionals, I suspect that 16MP is more than enough for what most people do with their images. Any more than 16MP will be tech jewellery (I love tech jewellery). For professionals, users who print very large format or someone who like to heavily crop images, a higher MP output might be more appropriate. It’s worth remembering that the difference between 16MP (E-M10) and 36MP (A7R) is 4608x3456 vs. 7360 x 4912 which equates to 15.36”x11.52” vs. 24.53”x 16.37” at 300ppi input resolution to a printing device. To me this doesn’t look as big a difference as the number would suggest, but that’s how headline specifications and the real world relate.