The SLR (single lens reflex) sets the lens wide-open for composing and focusing. This act delivers the brightest possible viewfinder image. We need this brighter image because the SLR sports a “roof prism” in the viewfinder’s optical path. The roof prism causes the image forming rays to reflect off prism surfaces 5 times. This is necessary to deliver to the viewfinder an image that is not upside down and not flipped left for right. For this correct image we forfeit image brightness so a wide open iris is a must. Additionally, the edges of the viewfinder image are seriously vignetted. A Fresnel field lens is placed atop the focusing screen. The Fresnel with its concentric circles, mitigates the vignette. With the lens wide-open, depth-of-field is shallow making focusing super critical.
When the shutter button is depressed for actual picture taking, the reflex (bend back) mirror swings out of the way, now the image forming rays can play on the imaging chip or film. In most situations, the iris will be stopped down because of the brightness of the scene will likely yield an over-exposure. This act increases depth-of-field thus the likelihood of a tack sharp picture is enhanced.
The SLR design gained popularity starting in the late 1950’s. Before the SLR our cameras were mostly all manual. We focused manually wide-open or used a range finder device to estimate subject distance. We then manually set the lens for focus using a sliding lens adjustment with a distance scale. In other words the camera has come a long way. The biggest advance being, on-board systems that focuses and set exposure via computer logic.
Us gray-hairs are grateful for the automation but we remember what we once made all setting manually. We used our light meter to gage exposure and we manually set focus, aperture, and shutter. We still need these skills when using lens systems that are not directly wired to the camera body. This happens with many application like using super wide-angles or bellows or tubes for close-up work. We open the iris wide for manual focusing and close down for the actual exposure.
When eating a watermelon, don’t let the seeds stop you. Just spit the seeds out and keep on champing.