Using a trick hidden in the Accessibility features of your Mac you can zoom in to a portion of your Mac's screen. With El Capitan you can use this zoom feature as a magnifying glass, allowing you to see the screen normally while also seeing a zoomed view in a corner. This can be useful for artists or anyone who needs to focus on a portion of their screen. But it isn't useful for revealing more detail in an image.
Check out How To Zoom In On Your Mac’s Screen at YouTube for closed captioning and more options.
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This is a great tip. I will be using it a lot.
It motivated me to check my iPhone/iPad for the same kind of functionality.
Life with my 'i' devices is better already.