Originally Posted by lilybelle
I thought pixels and resolution were related?
Resolution is directly proportional to pixel density
, not the raw number of pixels. This is because resolution and pixel density are intrinsic properties of an image of particular magnification, but the number of pixels is an extrinsic property. Let's crunch through the gory details, since this is probably a question in several peoples' minds ...
Your camera has 7.1 Megapixels, and they are probably arranged in a rectangular area with a height-to-width ratio of 3-to-4 (pretty standard). So if you take a picture with it, and expand the photo so the longest dimension is 10", this will create a picture which is 7.5"x10", or 75 square inches of area. Since all 7.1 Megapixels cover this area evenly, this provides coverage of 94667 pixels per square inch. This means there are 308 pixels covering one inch of linear dimension (pixel density), which is reported as 308 dpi (dots per inch), which is a unit of resolution. Of course, if you expand the image further, it decreases the resolution, since the same number of pixels need to cover a larger area. This is why the size
of the photos you want to take should impact your choice of camera.
Your eye's limit of resolution is measured in terms of angles. This is because everything you see, no matter how far away, is projected onto your retina which is a fixed
distance from your eye's lens. The retina also has a fixed density of rods and cones, which results in the eye's resolution limit. For images 10" from your face, this angular resolution is about 1/60 of a degree. This means your eye can discern two dots of light which are 0.002909" from each other. This is the same as a resolution of 344 dpi.