Get to Know Your Gear: A Camera Viewfinder Explained for Beginners
Amazon.com : 1.5X Camera Viewfinder, DSLR Fixed Focus Eyepiece Magnifier Viewfinder Eyecup Magnifying for Nikon, for Canon APS-C, for Sony, for Fujifilm, for Kodak, for Sigma, for Leika R, for Olympus :
Inside Camera's Viewfinder - Omnilargess Photography Academy
What Is a Viewfinder and How Is It Used When Taking Photos?
What is the Camera Viewfinder? (And How Does it Work?)