When digital cameras came on scene, SLR was pretty much the preferred standard, although some never did give up their rangefinders. Mirrorless technology just took a long time to catch up. Despite the DSLR bodies being larger, heavier, more expensive, and subject to mirror slap vibration, some of us (me) prefer the confidence I have from actually seeing through the lens, and will never give up the mirrored cameras.

And it's a known fact that digital viewfinders cause hair loss, insomnia, excessive ear wax and infertility. :-D

Boentgru,

This was written in a hurry, so feel free to reply or PM for more details.

The mirror box (the “R” in SLR) stands for Reflex, and stands in the pathway between the optical lens and the optical viewfinder. It reflexes the incoming light from the big lens up to the viewfinder through a prism to provide two important functions;
1. To project an image the eye can detect and perceive as an image for composition and focusing.
2. To flip the image 180 degrees so it looks upright and as the naked eye would perceive the scene.
In most older 35 mm cameras a film frame could be exposed many times (multiple exposures, or double exposures), simply be re-cocking the shutter release mechanism without advancing the film. The mirror has little to do with this function, except provide a visual presentation of the scene before exposure.

If you were to open the back of your 35 mm camera, with no film, and advance the shutter release one-step, you could see the shutter curtain advance one-step. The curtain is usually a black fabric material with a very precise gap cut into the curtain. There may be up to 3 gaps in DSLR bodies. The first gap is called First Curtain, the second gap is Second Curtain, and the third is called “B”. B = Bulb, where you control how long the shutter is opened for long exposures.

In 35 mm cameras, with a mirror, when the mirror was down, light would be reflected into the prism and to the optical eyepiece. Because of the nature of a prism, light could be directed into a photo sensor to measure light and calculate exposure. In modern DSLR cameras, the same process occurs, and in some, the mirror is translucent to allow light to pass through the mirror and onto the photo sensor for auto-focus and exposure calculations.
Recently, manufactures have begun to eliminate the optical viewfinder, prism, and mirror; this relies on the LCD panel on the back of the camera to compose, focus, and capture the scene. This is fine for people who want to take “snap shots”, and use only point an shoot cameras, but for serious photographers it may well fall flat on its nose.
Imagine, holding a DSLR camera up close to your face to view the LCD panel, with a 300 mm telephoto lens attached and holding the camera still enough to compose the photo before pressing the shutter button. The image in the LCD will jump with the slightest movement. For those who have “Live View” as a function, select it with any lens attached, then magnify the LCD image 10x.

DIN/ASA/ISO were essentially the same with different value systems. DIN was a European exposure sensitivity value, expressed with two numeric values. The DIN system was inspired by Scheiner's system, but the sensitivities were represented as the base 10 logarithm of the sensitivity multiplied by 10, similar to decibels. Thus an increase of 20° (and not 19°) represented a hundredfold increase in sensitivity, and a difference of 3° was much closer to the base 10 logarithm of 2 (0.30103…).
log(2) = 0.30103 = 3/10
10


 
ASA
Based on earlier research work by Loyd Ancile Jones (1884–1954) of Kodak and inspired by the systems of Weston film speed ratings[24] and General Electric film values,[26] the American Standards Association (now named ANSI) defined a new method to determine and specify film speeds of black-and-white negative films in 1943. ASA Z38.2.1-1943 was revised in 1946 and 1947 before the standard grew into ASA PH2.5-1954. Originally, ASA values were frequently referred to as American standard speed numbers or ASA exposure-index numbers.

Current system: ISO
The ASA and DIN film speed standards have been combined into the ISO standards since 1974.
The current International Standard for measuring the speed of color negative film is ISO 5800:2001[17] (first published in 1979, revised in November 1987) from the International Organization for Standardization (ISO). Related standards ISO 6:1993[15] (first published in 1974) and ISO 2240:2003[16] (first published in July 1982, revised in September 1994, and corrected in October 2003) define scales for speeds of black-and-white negative film and color reversal film, respectively.

The sensitivity of film is one of the three parts of the equal lateral triangle of exposure, this consists of ISO, Aperture, and Shutter Duration. The degree of light sensitivity for a photosensitive device controls the degree of exposure, and the noise factor (grain) depending on the temperature of the device. Film, or DSLR sensitivity has nothing to do with color quality, this is dependent on the manufacturer of the light sensitivity device, hens the difference between Chroma and Monochrome.
The only optimum setting is in the eye of the beholder, and what he/she is trying to capture; art, or journalism.

Michael G

My experience with photography goes back 50 years, using film of course, and almost exclusively with 35 mm SLRs, mostly Pentax. So, I try to understand the new technologies using what I understand of the old. Two fundamentals elude me, however.
I don’t understand still using the mirror box associated with SLRs. Film can be exposed only once. So a mirror is useful in formatting, focusing and metering the actual scene and then redirecting the light to the film for the exposure. But with digital sensors the metering and formatting functions can be done simultaneously with the capture. So what advantage is there to having the complexity, size and cost of a mirror box when a mirrorless layout avoids all that?
Secondly, a fundamental parameter of exposure was the film and its speed rating (DIN/ASA/ISO) which could be selected based on speed requirements, grain structure (fineness) and color qualities. But in today’s digital cameras the sensor is fixed and not selectable. So why do we still input a speed rating? Surely there must be one rating which is optimum for the sensor fitted.
I invite responses which would educate me in these areas and relate them to my understanding of film photography.