[sassan]

Good job Don. I was going to give the same exact link.

And yes, we have come through a long way as we stand today with SLR.

Stephen if you like to have a first hand experience on the SLRs and you have enough friends with older cameras or have access to a local shop with variety of second hand stuff, check the F line of Nikon, from the father F to F-2 with various prisms, legendary F-3, modern F4 or F5 then the D line. You see for yourself how much is achieved from the days when meter and diaphragm was not / mechanically coupled to the days that put the battery in and leave the camera in hand of 4 year old for many excellent pictures...

To make a long story short;

-The fenestra cameras with a pin hole and sensitive material on the other end (Includeing a real human painter) was the Renaissance in image creation.
-Lens added the sharpness and speed, when shutter was inside the lens or next to rear end. So you could not use this lens to see the subject unless you had a large format with option of changing the back.
-Box camera made photography mobile and easy when 2 mirrors at 45 degree one on either side of body, helped with landscape and portrait view finder a better choice than guess estimate.
-Range finders and Twin Lens Reflex TLR made the next break through for focusing.
-This was further enhance with split image screen or a similar focusing assist on TLR. Still what you see was not what you get (Paradox error). Exposure was a different story back then.
-SLRs did that. The ingenuity was to use 45 degree mirror plus a pentaprism that made image not only upside, right but Right to left corrected. A huge breakthrough for its time. This could happened because the shutter was now a curtain just in front of film, far away from lens and therefore same light passing through lens could be used for both being seen by photographer and be recorded on the film, when mirror is lifted up and curtains mover one after the other at a certain adjustable gap that determines shutter speed. Obviously the added benefit was also that lenses could be changed over the same body, initially when everyone was Innocent on a 42mm screw thread (Universal mount) and then bayonet as manufacturer got greedy and more self-fish to let their lens be used over some one else's body. Gush I miss my father's Zeiss Ikon Electorwhatever camera of those dates that was forbidden for me and was enjoyed by me when Dad wasn't around).
-Then Fancy things start to happen, lens coupling F and meter, an image that would not make you dizzy once subject or camera starts to move, made view finder very close to final result on film i.e. 100% reproduction on film, of what you see in view finder (WYSIWYG), F-3 is a good example.
-Then a computer chips inside the camera could make logical thinking for right exposure based on several options set by photographer.
-Manual focusing got auto
-And lastly and Mostly, film got replaced by sensors and you have the pleasure of DSLR photography at fraction of cost and labor otherwise needed to produce an equal image.

Hope this helped.