Boentgru Loc: Boston, Massachusetts, USA

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.

Armadillo Loc: Ventura, CA

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

amehta Loc: Boston

On the question of the mirror, you are correct that the digital image sensor can be used for metering, autofocus, and viewing, and these days many camera do exactly that. For the best performance, though, a dedicated AF system and metering system, as well as an optical viewfinder, still work better. The difference is getting smaller, and there are now "pro" or "semi-pro" level mirrorless cameras.

With the sensitivity of the sensor, the electrical settings used in the sensor are not fixed. While there is an optimum sensitivity, the sensor hardware can amplify the image signal to increase the sensitivity, but this also amplifies the noise. The result is similar to the grain of high ISO film.

SharpShooter Loc: NorCal

Boentgru, welcome to the Hog.
Everything you have said has been kicked around, and the technologies are always getting better.
But, mere mortals as myself, I don't even begin to worry about such things. I pays my money. I gets my cameras, and I shoots the best I can.
I let engineers that work at places like Canon, and even Nikon probably has a couple of'em, worry about such things. They actually get paid to worry about that stuff!
So, When the top-end, pro flagship models come out w/o mirror boxes, I'll just assume the tech has finally arrived.
So Boentgru, until then I just keeps paying my money and don't stress about such trivial matters. Good luck. ;)
SS

OddJobber Loc: Portland, OR

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

marki3rd Loc: Columbus, Indiana

Lets keep it simple. To me the most important reason to keep the mirror in SLR Cameras is that it allows an Optical Viewfinder, which I much prefer to electronic imaging viewfinders.

To your second question - (keeping it simple) with film, as you stated, the photographer selected films with different ASA/ISO speeds in order to obtain the best performance for the job at hand (light sensitivity, grain, etc.) With electronics sensors, very much the same criteria applies. The light sensitivity is controlled with a bias voltage and raising the sensitivity adds inherent degradation to the image. However, there are times when gaining the added light sensitivity is more important than the incremental degrading of the resultant image obtained. So, like with film, the photographer decides on a set of trade-offs what ISO to use. There is no sensitivity setting that would be a reasonable universal setting - period.

SharpShooter Loc: NorCal

Ahhh, but Mark, that's where it gets really sweet. Unlike film, or fussy DSLR shooters, I can put my DSLR on Auto ISO, and the camera will decide my ISO/film for me, and all I have to do is take my really expensive DSLR and P&S(pun intended)!! :lol: :thumbup:
SS

boberic Loc: Quiet Corner, Connecticut. Ex long Islander

I would love to hear ( I can't because of excessive ear wax ) rather see the science behind this.

Armadillo Loc: Ventura, CA

SS,

Have you totally given up creativity? :roll:

Michael G

marki3rd Loc: Columbus, Indiana

Sure, you can run on auto-pilot and never have to worry about the trade offs I talked about. However, I was addressing the suggestion that it should be possible to have a fixed ISO sensitivity which never changes (an impossible idea). Running on auto-pilot as you suggest (jokingly?) doesn't change the fact that the camera is not always using the same ISO - i.e. the ISO is changeable/variable.

busted_shutter

Fine for average or well-lit situations, but you know this already...auto not so good in those low-lit moody locales tho.

SharpShooter Loc: NorCal

Mark, let me explain, I was only half joking, and half dead serious. I will never give up the control I can have, which is why I will not give up a DSLR until I can no longer hold one up. And by then, hopefully my phone will be as good. :lol:
For example, I shot Jazz dance, where the light changes every 15 seconds, from blinding to real dark. Many immediately say, just crank it up to 6400, the camera can handle it. Yes, the camera handles it by producing noise. Since there is movement, I need at least 150th sec. to be borderline. So on AV, the camera will shoot wide-open when it's dark, as well as at 6400 or so, but with noise. But when there is a lot of light, the camera will always pick the LOWEST ISO and the f-stop to go with it. So that shot is at ISO 200 and VERY clean. WAY more clean than at 6400, no matter WHAT camera you use. At 6400, no matter how GOOD your camera is, it's never gonna be as good as ISO 100. I don't usually find noise to be creative, and when I do, it's better to introduce it in post, than to try and take it out, and loose all sharpness.

But the choice is mine, to know when to use which.
Mark, does that make any sense at all? :wink:
SS

Pablo8 Loc: Nottingham UK.

So a mirror is useful in formatting, focusing and metering the actual scene and then redirecting the light to the film for the exposure.
************************************************
The mirror does not re-direct the light to the film. The mirror flips out of the 'Light-Path', for the film exposure to take place. I thought that was basic 'schoolboy' knowledge.I certainly learned it in 1957.

bull drink water Loc: pontiac mi.

when I used film,i shot with 200 asa hi-speed ectachrome and got slides that appeared tack sharp of a living room 48x48 size screen at 10-15 feet. how many mp's and what kind of lens would I need to get a print from a digital camera that size?

PhotoArtsLA Loc: Boynton Beach

Well, I'd just add, having shot film Pentax and film Nikon cameras... the initial question is a bit in error. It is possible to multiply expose (multiple exposures on the same film frame) on a pro film camera. Pentax never ascended to the pro ranks, except maybe with the Pentax 6x7, responsible for much of Yoshikazu Shirakawa's "Eternal America" coffee table book.