Imaging Misconceptions: Resolution

Every few weeks I come across questions on my regular photography forums which indicate a deep misunderstanding of what is actually occuring under the hood of some popular piece of imaging software.

The most common misconception I see is that a photographic image has an inate physical size, or DPI.

The graphical part of a photographic image is comprised of a grid of pixels; samples of light taken regularly across the dimensions of the image sensor. As far as the raw data goes, that’s it. There is nothing here measured in real world, physical units.

Whatever constraints you build from here on in are entirely devised by the software you are using. If Photoshop is telling you that the image has a DPI of 300, or a width of 10”, then it is merely a guide for the realisation of these pixels. It is not a fact drawn from the imaging data; it is one piece of user generated metadata for visualisation purposes.

One cannot change the dimensions of a dimensionless quantity like a pixel.