Pentax LX - Its Integrated Direct Metering system

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One of the strongest asset for the Pentax LX is at its metering system. It is not too unreasonable to quote in terms of sophistication, the Pentax LX has the best metering system found in top rated SLR cameras during the early eighties. Although it cannot perform partial or spot metering like the New Canon F-1 (However, the Canon requires you to change the focusing screen to alter the chosen metering pattern and has proved to be a negative factor refeclted in its sales), but the single metering SPC embedded underneath the main mirror box that extend its function to provide TTL OTF (Through the lens, Off the film plane metering) can easily make what its competitors can offer pale in comparison. This raw metered info is process and work along with the three exposure modes the photographer can choose to operate with the camera: Manual, Aperture Priority AE and TTL Flash AE.

Pentax LX professional SLR camera body
Within, you will find one of the best, powerful and innovative metering system provided in such a compact body.

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It is very much similar to the Nikon F3 in its metering system used and the way of how the meter works - where it also uses a single SPC cell located under the main reflex mirror and facing backward to the film/shutter curtain. The Olympus who pioneered TTL/OTF metering system has great similarities with the way Pentax LX 's IDM metering. But the OM uses two pairs of cells for its metering fucntion. That causes a lampse time in terms of accuracy. OM cameras use one pair inside the mirror box chamber for viewfinder data display while another pair of Silicon Blue cell measures TTL metering. (Read more at Olympus OM1(n) & OM2(n).)

Pentax LX with SPC.jpg (18k)
The New Canon F-1, however has its SPC located next to the focusing screen chamber to take the reading from a split and diverted light path from the focusing screen. While the Pentax LX and the Nikon F3, may sound the same and location is the same, but technological-wise, may be miles apart in what it can perform. What ? May be the exposure range in the automatic mode can give you a clue.

The Pentax LX metering can has a exposure range down to - 6.5 EV (125 sec at f1.2) to EV 20 (1/2000 sec) while the F3 can only perform from EV 1 to 18 (i.e. . f/1.4 at 1 sec to f/11 at 1/2000 sec with 50 mm f/1.4 lens and ISO 100 film). But both cameras also uses the same cell to measure light reflects off the film plane during a flash exposure (TTL OTF flash). How could that be done ? In fact, physically, the SPC in the LX is only slightly more than half the size of the Nikon F3 in comparison. The trick is Pentax's "refined" automatic exposure system with IDM (Integrated Direct Metering). Why I said it as "refined" instead of developed ? Because, it is a refined metering system from the Olympus pioneered OTF metering system, first seen in 1975 with the Olympus OM2.

When you switch to AUTO mode in the OM2, however, the meter circuit controls by two silicon blue sensors in the camera base lined up to read the reflective pattern on the first shutter blind. The pattern is designed to give a center-weighted reading. The meter translates the reading into a value of light intensity at the film surface and adjusts the electronic shutter to a suitable speed to give correct exposure for the film speed set. The metering and shutter speed adjustment is continuous while the shutter is open for longer exposures but then most of the reading is taken from the film surface and is not center-weighted. Thus, the meter reading in the AUTO mode is taken at the shooting aperture. An estimate of its effect is made by another two CdS sensors and is shown in the viewfinder before the shutter is released. Yes, OM2 uses 4 meter cells in total ! That reading is, of course, not continuous.

Illusb.jpg Illus.jpg
Can the refined metering method developed by Pentax overcome this ? As I said earlier, Pentax's exclusive IDM (Integrated Direct Metering) system makes the LX's automatic exposure system the most accurate available on a 35mm SLR camera.
The IDM system measures the actual amount of light reaching the film plane during exposure, regardless of aperture setting or which focusing screen or viewfinder is being used. Even if light conditions change during the brief moment of exposure, shutter speed is adjusted instantaneously to compensate. When the camera is operated in the automatic exposure mode,
the IDM system switches on the instant the mirror flips up and the front shutter curtain begins to move (see illustrations). It calculates the amount of light hitting the film plane and instantaneously controls rear shutter curtain motion for correct exposure. Light values, metered through the actual aperture, are monitored for the duration of the exposure - not the instant before the shutter is released. Thus, shutter speed is correct, even in changing light.

Flash output is also directly measured as it reaches the film plane. Bounce flash, diffused flash, and filtered flash photography are all possible without exposure compensation (with Pentax dedicated TTL Auto Flash units).

Shutter Curtain.jpg (23k)
The front curtain visible in this photo with the specially designed metering pattern on the first curtain.

Compared it with the Olympus's OM2 Shutter curtain which made of silk (cloth).

Flash Illus.jpg
The IDM System is different from conventional TTL systems Unlike conventional through-the-lens metering systems, the Pentax IDM system measures incoming light at the moment of exposure–after mirror flips up. This makes it unnecessary to add a special circuit to store exposure data. IDM circuitry is thus simpler and more reliable.

Also, all IDM metering circuitry, including the metering cell, is located inside the body, not in the pentaprism, so that finder changes do not affect the performance of the metering system. In TTL open-aperture metering, an SPD cell measures light transmitted through the quick-return half-mirror and reflecting off the metering mirror. The IDM system then indicates the photometric measurement in the viewfinder.

In the automatic exposure mode, the moment the front shutter curtain starts moving, the IDM system is switched on. As the shutter curtain moves, the IDM system makes an integral calculation of incoming light and controls movement of the rear shutter curtain for correct exposure.

For automatic exposure in the TTL Auto Flash mode, the IDM system measures only light reflecting off the film plane since the flash goes off after the shutter is fully open. The rear shutter curtain closes after the IDM system determines that enough light has reached the film and terminates the flash.

The full aperture, center-weighted (yes, it only offers center weighted average metering pattern), through-the-lens light metering for both automatic and manual modes enhanced by the Integrated Direct Metering (IDM) system measures light at the film plane via
Silicon Photo Diode (SPD): high-speed measurements for automatic operation are made off fixed pattern on front shutter curtain, slow speed measurements off curtain and film, automatic electronic flash/ambient light measurement with TTL "T"- and "C"-type flash models. Compared with the auto exposure mode, the manual exposure range is tighter but still offering a high width from EV 1 to EV 19 (at ISO 100 with f/1.4 lens). To turn on the meter, simply light touch on the shutter release button, the meter will activate and automatic shutoff after 25 seconds, while the Nikon F3 will turn off by 16 sec, so does the New F-1.

Since the light source is of critical important for metering, and so does the viewfinder brightness for the comfort of viewing and composing, the special all-surface half-mirror in the LX has a reflectance ratio of approximately 85% over the entire surface of the mirror. There are no shadows or ghost images even in close-up shooting or stopped-down metering. In addition, 15 layers of the exclusive Pentax Super-Multi-Coating are applied for better reflectance ratio, clearer and brighter viewfinder image, and more accurate viewfinder color. The Nikon F3 transmits around 8% of the light through the half mirror to the metering cell and generally, many users have complaints over the dimmer image in the viewfinder. Strangely, the most impressive viewfinder brightness were not provided among all these three kingpins in the professional market, although the Canon New F-1's laser matte screen is also very bright. The brightest among all was the Acute-Matte focusing screen developed by Minolta, where its focusing technology is comprised of some 2,500,000 microscopic cells, each of which is a conical micro-lens shaped for better light dispersion and, consequently, a 50% brighter, clearer, higher-contrast image that is easier-and faster-to-focus. This technological breakthrough was so impressive that Hasselblad eventually asked Minolta to design a new focusing screen for the
Hasselblad 500 CM.

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