Modern mirrorless cameras have the advantage of reading out the on-sensor data and showing you an image on the back of the camera, called 'Live View'. Compacts and smartphones are also technically mirrorless and work in much the same way. But I’m old enough to remember a time before Live View and I’m sure many of you who shot on film before digital came took over will be familiar, too.
The viewfinders DSLRs are optical so you’re looking directly through the lens via a series of prisms and mirrors in the camera body. If it’s really bright you’ll be able to see the scene clearly, but if it’s dark, it’ll be much harder to make out what is in your frame. You also can’t rely on how bright or dark the viewfinder is to give you any indication of how your exposure will turn out, you need to know your settings, read the lightmeter and check your shots regularly to make sure they’re all good.
