Pentax LX - Exposure Control
Modern Classic SLRs Series

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The Pentax LX is essentially a hybrid camera, it is considered a mechanical camera in nature, meaning it will still operational even without any battery cells to power the automatic functions. It has the widest range of mechanical shutter speeds to choose from in unexpected situation where the battery cells fail to function normally like a sudden change in extreme of temperature or running out of power. The fact is, when power source is provided, the LX will spring up and comes alive and its awesome IDM will activate to provide manual, automatic exposure control - in manual and both aperture priority and TTL /OTF Flash AE. Open-aperture, center-weighted, through-the-lens light metering for both automatic and manual modes. 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. Manual exposure range from EV 1 to EV 19 (at ISO 100 with f/1.4 lens); stepless automatic exposure range extends to EV -6.5 to EV 20 (125 seconds at f/1.2 to 1/2000 second at f/22, at ISO 100 at normal temperature and humidity). Meter activated by light pressure on shutter release with automatic shut-off after 25 seconds.

Pentax LX professional SLR camera body
Credit: Image(s) above courtesy of some nice folks from DigifanCN®. The group also operates their own active, popular EBAY STORE, trading for many major camera brands and collectibles. Image Copyright © 2008. All rights reserved. Please respect the visual property of the contributing photographer.

Three exposure modes are available in the Pentax LX. Manual, Aperture Priority AE and TTL Flash AE. Prior to the Pentax K-A lens mount, first used on the Pentax Program-A; there is no Pentax camera model that has a Program/Shutter priority mode AE (The same like the Nikon F mount, changed to AI-S, only to see one model, the Nikon FA benefited from the upgrade...Errr.. Pentax users are more luckier). For macrophotography and portraiture where depth-of-field control is extremely critical, the aperture-preferred system of automation always provides the maximum in flexibility, ease of operation and convenience. The LX is equally responsive, however, in fast action situations where by simply opening the lens to its widest aperture, the camera automatically selects the fastest possible shutter speed to capture fast action shots. There is no clear definitive answer of whether shutter priority or aperture priority is better off - although in the options for various exposure control, the New Canon F-1 triumphs in this area, where it offers both - when use in conjunction with various system components to convert the mechanical, match needle meter body into automated camera.

PentaxSuperA.jpg (12k)
Another nice view of the Super Program A, click here. These K-A mount series, along with the M series are the best look models among the Pentaxes.Good choice of material used and well designed exterior look in a compact body, add and projected a very rugged feel.

That made people forgot about the K-A mount bodies are fully auto bodies and have no mechanical shutter within. Personally, that M series and the Pentax LX present the best of mechanical engineering from Pentax.

As explained earlier in the metering section, the IDM and the TTL flash uses a sole SPC to perform all these task. This off the film plane metering method makes the Pentax LX (Along with the Olympus's models) has, theoretically, most accurate and extended metering capabilities (-6.5 EV or 125 sec in auto mode). In the manual mode, there are 16 settings, from 1/2000 sec to 4 sec with B and X. While in TTL Flash automatic exposure, the LX's automatic flash system automatically chooses between the Auto Flash and automatic exposure modes, depending on light conditions and the degree of flash recharge. It provides TTL measurement of ambient and electronic flash lighting directly off film plane with "T"- and "C"-type Pentax flash models providing auto flash sync at "X" and visible flash-ready and exposure check in viewfinder when flash reaches full charge; automatic flash sync and visible viewfinder flash-ready light with other Pentax dedicated flash models.

At the camera front, just beside the lens mount, there are three terminals/contacts: "FP" terminal and "X" terminal, plus contacts for TTL integrated automatic flash control; depending on viewfinder selection, the FA-1 Standard Finder features built-in "X"-sync hotshoe with TTL contacts.

(Refer to the Flash section for more detail).

Optical Path.jpg (37k) Big...but nice.

Diagram illustrating how the light path traveled through the main body and relay vital data to the CPU and viewfinder info display panel for various metering and exposure control.
Red line applies to both open-aperture, Flash metering and during actual exposure off the film plane.

For shutter speeds faster than "X" (1/75 sec.) and bulb shooting, the LX employs a reliable, high-performance mechanical shutter. Combined with the LX's rugged body construction, this shutter operates accurately without batteries, even under extreme conditions of humidity and shock and temperatures down to -30°C (compared to a minimum of -20° for an electronic shutter - Pentax Lab test figure).

Shutter Speed Guide.jpg

A mode:
A Red LED will glow next to the "A" index on the viewfinder scale to indicate overexposure. When this occurs, stop your lens down or change to manual exposure control to obtain acceptable exposures.

A mode: A Red LED will glow next to the "LT -B" index on the scale (Bottom) when there is insufficient light for a proper exposure or when the camera has selected a time exposure longer than 4 seconds in the automatic mode.

Viewfinder diaply.jpg
Yellow LEDs are used along the shutter speed scale between 1/15th second and 4 seconds as a slow speed warning and reminder to use a tripod or supplemental lighting, or to change the lens to a wider aperture setting.

Green LEDs are used along the shutter speed scale between 1 /2000th and 1/30th second to indicate hand-holdable speeds.

Manual Mode with Shutter Speed Index and LED Meter Display Photometric readings from the TTL exposure meter are indicated by 16 LEDs in 3 colors arranged along a scale of possible shutter speeds. To set the correct aperture in the manual mode, first choose a shutter speed, then turn the aperture ring on the lens until the LED matches the blue index tab indicating selected shutter speed. A red LED at either the "A" or "LT-B" position indicates that the selected aperture/shutter speed combination is either over- or underexposed. A red index tab appears above the shutter-speed scale to remind the user that exposure factor is other than 1X.


Over, under or suggested correct exposure reading
display in the viewfinder (
Green LED),
of cause, you can alter either the shutter speed
or aperture for precise setting.
- the manual mode -

Aperture-Priority Automatic Exposure Mode with 3-Color LED Display
When the shutter speed dial is set at "AUTOMATIC", the blue index tab remains at "A", indicating that the automatic exposure mode has been selected. For any aperture setting, an appropriate shutter speed is automatically selected and indicated by an LED light. A red LED at "A" means overexposure beyond the exposure coupling range. "LT-B" means a long exposure time. A yellow warning LED appears at shutter speeds between 1/15 second and 4 seconds, indicating that handheld shooting may cause image blur.


In the aperture priority automation mode, the camera will match the user defined aperture value selected by displaying the permissible shutter speeds match. However in extreme exposure range out of the meter can handle, over or long time exposure is indicated by the red LED. While the Red Index Tab (Very useful) reminds the user that exposure compensation is on.

Recharge & Confirmation Indications for Dedicated Flash With Pentax dedicated TTL Auto Flash Units, like AF400T, 280T, 200T or 080C, a red LED at the "X" position indicates that the flash is fully recharged. When the AF400T, 280T, 200T or 080C units are used in the automatic flash mode, the shutter speed is automatically set at 1/50 second even if the shutter dial is set at "AUTOMATIC." When the flash is successfully discharged, a red LED at "X"
blinks for confirmation. In the manual flash mode, shutter speed should be set slower than 1/75 second ("X").


Flash AE with the LX is much more sophisticated than any of the models in its class, the TTL measurement of ambient and electronic flash lighting directly off film plane with "T"- and "C"-type Pentax flash models providing auto flash sync. For more detail, refer to another section just focus on the Pentax LX's flash capability.In the A mode or if you are not using a dedicated flash, accidental setting to the A mode ("Automatic" in the shutter speed dial) or press the shutter release with the lens cap on or in the dark, an excessively long exposure will result. To reset the camera, move the dial to one of the manual speed settings will close the shutter from the exposure.

Whenever time is limited and light is changing rapidly, you're sure to find the automatic exposure control capabilities of the Pentax LX a useful and welcome feature. Because the LX incorporates a direct off-the-film plane light measurement system,.readings are made continuously to ensure uncanny accuracy even under the most difficult lighting conditions.

Pentax bayonet lenses for Pentax LX professional SLR camera body
To operate the Pentax LX in the automatic exposure control mode, first rotate the shutter dial until the word "Automatic" locks in place opposite the index mark. Set the lens f/stop you desire on the camera lens and activate the metering system by pressing lightly on the shutter release button. The LX will then read the amount of light striking the film plane, select the precise shutter speed required for a proper exposure from the continuously variable range of 1/2000th second down to 125 seconds during normal temperatures. The approximate speed selected is indicated on the viewfinder display. Speed selected by the camera can be changed by merely selecting a different f/stop or by using the exposure compensation dial.

Credit: Image(s) above courtesy of some nice folks from DigifanCN®. The group also operates their own active, popular EBAY STORE, trading for many major camera brands and collectibles. Image Copyright © 2008. All rights reserved. Please respect the visual property of the contributing photographer.


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