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SLR-- Single Lens Reflex-  You look through the camera main lens.

EOS-- Electro-Optical System--  Nothing new, been around for about 30 years. Focusing

Like yesterdays film cameras  SLR means targeting your subject directly through the lens.  Inside the camera there's a mirror that allows the user to see what the film will see.  When you press the shutter button, this mirror will fold up and allow the film to be exposed.  No different on digital cameras, the action is the same. Except the image instead of film exposure, it is exposed to a digital image sensor and saved as digital data.  When first introduced, the digital image was only 1.3 megapixel, now up to 200+ megapixel.  So much attention to megapixel, people would buy the most megapixel they could afford not considering the lens quality, plastic lenses in place of highly refined and expensive glass made no difference in pixel quality, as it was the lens that was equally important.  But very costly.  Cameras having both high megapixel and quality lenses can cost 50k or more!  But most of us do just fine with are cellcam. I have several old analog and new digital cameras but I just use the one on my cellphone most of the time.

Up until a few years ago, digital cameras could not capture the warm texture of analog cameras using cellulose acetate film.  Also, what brought the digital camera to the forefront is the cost of silver which is used in the making of film.