Modern Classic SLRs Series :
Olympus OM2(n) SLR camera - Other Issues Part IV

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Metering system used in OM2(n) Bodies

The most significant achievement of OM designers successfully incorporated into the OM2(n) body was a revolutionary metering system called: TTL Direct "Off-the-Film" Light Measuring - NOT just TTL flash that is so commonly found in every modern electronic SLR today BUT the proprietary TTL OTF metering is also apply to ambient light ! A Olympus developed Centralized Control System that employed in OM2(n) body that give the camera instant superiority over any other 35mm SLR during its time.

Comparing technologies used in the metering system for a 25 years old OM2(n) with any of a Y2K's Autofocus SLR such as one of those top of the line AF models from Canon, Nikon or Minolta, you will find great similarity among them.

However, the "IDM"* (Integrated Direct Metering) system's awesome -5.5EV (120 sec at ASA100 50mm f1.4). is still unrivaled even by today's standard.

* "IDM" a term used by many other newer publications which is similar to the original term used "TTL Direct "OTF" Light Measuring".

Note: The original OM2 metering range can go up to 60 seconds in auto mode, while one of the refinement made to the OM2n is, the range has been boosted to 120 seconds and making camera carries with the most sensitive metering range in the market.

I hope this illustration would help you figure out how advance and superior is the metering system used and those genius who brought us this remarkable camera seriously deserve some great respect.

Perhaps, some of you may just associate the term "TTL OTF" metering with flash exposure control that is so widely and popularly used in today's 35mm AF SLRs - but very few realized it was a technology first pioneered by Olympus and shown in the OM2(n). But even if you know the TTL flash was first pioneered by Olympus, but I would assume MOST non-OM users wouldn't have notice Olympus's original TTL Direct "OTF" Light Measuring system developed 2-1/2 decades ago was still the only * method so different from any other system.

Olympus's design the OM2 measures available light that passes through the camera lens and hits either the film plane itself or the special computer generated random digital pattern shutter curtain (Simulating most common photographic scenes) at the precise moment when the mirror flicks up and the exposure takes place.

For accuracy and versatility it is infinitely superior to conventional systems that measure the light at the pentaprism before the mirror is raised and then go blind during the vital moment when the exposure is in progress. The same system handle the flash exposure control, which is also TTL OTF (Through the lens, Off the Film Plane). This has opened up enormous photographic possibilities using flash which previously would have required tedious setup and calculation of exposures. * The Pentax LX, 1980 used a similar concept but different approach to avoid copyright infringement.

The metering system used also illustrate excellent practical application of microcomputer in SLR development and brought a new dimension to many aspect in camera operations such as exposure control. The Olympus way is to utilize a Centralized Control System that puts flash, motor drive and every other function directly under the command of the camera and assures Total Mode Automation at the touch of a single switch.

<<-- (44k JPEG File) An ENLARGED Illustration of flow chart how CPU control camera operations.

There are few other points and advantages relate to this revolutionary "TTL Direct Light Measuring System" where Olympus explained quite well in their early literature Firstly, on "MANUAL" mode, OM-2 works in exactly the same way as the OM-1, and has exactly the same performance. Switch it to "AUTO" and its light measuring and shutter control mechanisms function quite differently. In fact the OM-2 functions in automatic mode very differently from any other automatic camera - the Olympus' Method makes conventional "memory device" of most electronically controlled cameras obsolete and in fact, the OM2(n) was used to be the only system that can insure correct exposure for each frame individually even during motor drive operation at a high speed film advance rate of five frames per second (5 fps). Because it has no memory device, the TTL Direct Light Measuring Method operates accurately in far dimmer light than other systems. 100ASA film can be exposed for approximately 60 seconds at F1.2 for earlier OM2 and 120 seconds for OM2n !

The highly reflective random digital pattern on the center of the OM-2 shutter curtain will, while at HIGH shutter speeds, the SBC sensors will pick up light reflected from this curtain, and consequently give a center-weighted reading. N O T E: <<<<-- The random pattern first shutter curtain behind the reflex mirror when being lock out of the way. Early OM2 bodies has a different type - a more evenly distributed pattern than OM2(n).

This is the most suitable for bright conditions when there is often a high contrast in light intensity over the picture area. On the other hand, at LOW shutter speeds, in dim light conditions where contrast is usually small, the sensors read directly and evenly off the whole film surface, giving an overall average reading. The type of reading changes gradually between shutter speeds of 1/60 and 1/15 of a second, insuring the optimum exposure selection in all conditions.

Another advantage is, as you can notice, the OM2(n) has no eyepiece shutter incorporated Because Olympus in the opinion that the TTL Direct Light Measuring Method excludes all possibility of stray light leaking through the camera eyepiece and distorting the exposure reading, because the reading is made during the exposure, when the mirror is in the raised position and automatically cuts off all light from the eyepiece. What about the twin Cds cells reside near the eyepiece ?

Wouldn't it be affected say, with the camera set at AUTO, facing downwards with the eyepiece exposed to bright sky or strong sunlight ? Possible, but the reading provided by the Cds is merely used for viewfinder exposure display and not the actual film taking aperture used which handles by the SBCs and thus, it is not affected. The two light sensitive SBC *(Silicon Blue Cell) sensors that measure light reflected directly off the actual film, this method bypasses the mirror completely, to permit light measurement during the period the film is being exposed. Even there are changes in the light levels, the metering cells will respond instantly to changes in the light intensity during exposure, and feed back the information on real time to the shutter control mechanism. The system is somehow different from using memory circuit first designed by Pentax to store the data prior to exposure, the system is still widely used in many today's conventional electronically controlled cameras.

* OFF-TOPIC SUPPLEMENT: Over the years, there are a few types of metering cells used for SLRs, the more commonly used cells are: Cds, SPCs, GPDs and SPDs. Photocells generates varying electrical characteristic (e.g. current, voltage or resistance) when light is incident upon them. OM-1's Cds (Cadmium Sulfide (Cell) is a light dependent resistor rather than a cell, it is current-modulating light-sensing cell that was quite popular with lots of older cameras exposure metering system and external metering devices. However, it does not generate electrical current when lights hits them as with SPCs. Instead, current is being allowed to pass through the circuit when light hits the resistors.

Yes, theoretically, the data used for critical exposure stored for exposure control can be considered outdated because it is not "real time" as compared with the Olympus. Further, since most camera's metering cell(s) is usually residing near the eyepiece and has to pass through the focusing screen, conventional auto camera that uses clear type focusing screen etc. may not able to provide a accurate reading; the Olympus way of reading only the amount light actually reaching the film plane, accuracy of exposure reading is far superior.

<<<<--- A...Errr.. Sexy Cat on the Walk (33k Jpeg) Copyright © 2000. Philip Chong ®. An OM2n for Fashion Show ? Why not ? In fact, it was taken by a friend who is an EOS1n owner but had a hand on with his first experience with an OM2n together with a fast Zuiko 100mm f2.0 lens.

In the same way it applies when used with any of the Olympus dedicated automatic flash units that would perform similar TTL OTF metering, where the same SBC sensors will measure flash intensity as it builds up. The speed being measured in fractions of 1/10,000 second, it is not possible to close the shutter fast enough when the correct exposure level is reached. Instead the camera sensors link directly to those dedicated flash that are designed to integrate Olympus's TTL flash capability and cut off its light at source. This concept was essentially the most primitive form of modern TTL flash exposure control system, made possible and used for the first time in an Olympus OM2.

A distinctive example of the advantage presents by the TTL Direct Light Measuring is none other than using it with Olympus's remarkable Flash photo Group accessories. Not only in its wealth of units during its time but also in their incomparable performance and modular versatility the group provides the only flash system able to actually enhance 35 SLR capability, instead of curtailing it with restrictions on range, lens openings and overall responsiveness. The Flash Photo Group has spearheaded Olympus into a commanding position of almost a generation ahead of its competitions. In fact, both OM1 and OM2 models have actually helped Olympus positioned among the fore running Japanese camera manufacturers, if not the best. They were also being rated as easily one of the best and most innovative photographic producers during the seventies.

The TTL "OTF" Auto Flash function puts every flash unit entirely under the control of the camera instead of normal auto flash that usually control by the flash unit. The OM-2's SBC (Silicon Blue Cell) light sensors at the base of the mirror box, facing backward towards the shutter curtain, read the electronic flash light just as they read any other light that passes through the lens and hits the film plane during the exposure. When enough light has been gathered determined by the metering circuit, the OM-2's electronic brain cuts off the flash emission directly, in microseconds, through an electrical contact. Because the light is measured at the film plane all extraneous considerations are eliminated.


This gives the photographer complete freedom to use bounce flash, diffused flash, mixed lighting or multiple flash sources, etc. Technically, depending on the guide number * of your flash and distance of the subject away from the camera/flash units, you can also use any lens aperture, thereby gaining far greater control over depth of field and extending the subject distance range enormously. * Guide number is used to express the power output of the flash unit. It indicates the power of a flash in relation to ISO film speed. In TTL flash, the flash output to burst at its maximum output when there is insufficient light gather by the light sensor, in most cases, the flash will blink to indicate maximum output has achieved, underexposure may occurred

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