I'm assuming you received a request from someone to provide images at "300 ppi". I get these from time to time.
Images don't have "ppi" (or sometimes called "dpi"). Digital images simply have "resolution" described as the number of pixels wide by the number of pixels high.
PPI and DPI stands for "pixels per inch" (or "dots per inch" as it's sometimes called) and refers to a resolution at a specific display size. But since you can take the image from your camera and display it at ANY SIZE you want, the image wont really have "PPI" until you pick a physical output size to use when displaying the image. The camera cannot know in advance how large that will be. You might take two images in a row, decide to print one as a 4x6" and print another as a 16x20" -- the camera doesn't know what you plan to do with the image -- and more importantly, it shouldn't have to.
Suppose an image is (I'll make up easy numbers) 300 pixels wide by 200 pixels high. If you were to print that image in a tiny size... say 1.5" wide by 1" high, then you'd be printing at 200 ppi because that's how many pixels you can line up in just one inch. If you decided print that very same image 3" wide by 2" high then that very same image would now be 100 ppi -- and yet you didn't change the data out of the camera at all... all you changed was the size of the output media.
The true resolution of your camera sensor is 5184 x 3456. So when someone says they need an image at 300ppi, what you need to say is "What physical size?" (in inches -- since it's pixels per INCH and not pixels per centimeter, etc.)
The PPI is handled by your computer software when exporting a copy of your image for THAT SPECIFIC USE of the image. You don't want to alter your original image data -- you want to create a copy of the image to give to that person who wants to display it at their specific image dimensions and ppi.
One other thing... there's also a notion of the image ratio. Your camera natively produces a 3:2 image -- which means the sensor is 50% wider than it is high. A 4x6" print maintains that same image ratio. But notice if you look at other sizes such as a 5x7 or an 8x10, the ratio is no longer maintained. That means you have to manually crop the image to the ratio you plan to use when you print (or otherwise display) the image.
The bottom line is that only an output display has a notion of "ppi"... you can "print" at a given PPI or you can display on a monitor at a given PPI, but the image out of the camera has no such notion because it's not being displayed yet.
The process of cropping, resampling, and exporting an image so that it will have some specific PPI when displayed at it's intended size is all handled by the software on your computer -- and not by your camera.
BTW, you can set meta-data in an image with a PPI value. It's not real data -- just meta-data. It "suggests" to a printer (which could completely ignore this data if it wants to) the PPI that you want to use when printing. In other words if my image happens to have 1800 x 1200 resolution and has meta-data set to 300 dpi, then a printer who doesn't over-ride the data will end up with a 6" x 4" image.
Tim Campbell
5D III, 5D IV, 60Da
