Modern Classic SLRs Series :
Pentax LX - The TTL / OTF Flash Metering

 
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Compared with conventional Flash AE, TTL OTF Flash metering can opened up many photographic possibilities. I think Pentax has spent an enormous time and effort to fully explore with the numerous benefits it can offer. Most of these R&D effort are centered around the concept of its IDM system in which Flash AE was one of the main area with intense development.

Unlike Olympus OM system, TTL flash is handled by the same photocell that does the metering in ambient light. This has also enabled the LX be at the fore front of flash exposure control technology among the big five SLR camera manufacturers (it is even ahead of the Nikon F3 !).

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Thus, when used in conjunction with the Pentax AF-280T and AF-400T Automatic Electronic Flash models, your Pentax LX provides the ultimate in operating ease and electronic flash exposure versatility. Because they are fully integrated with your camera's IDM through-the-lens light measurement system, the camera meter is able to read both ambient lighting and electronic flash to assure a proper exposure of any subject within the respective unit's operating range, regardless of the aperture set on the lens.

Another advantage of integrated metering at the film plane is that the camera's non-flash automatic exposure system overrides the flash when flash is not needed, thus eliminating flash failures due to overexposure, even in changing lighting conditions. The only flaw that you can pick on the LX's flash system, perhaps is the slow sync speed of 1/75 sec. Which by today's standard, limiting its broader appeal. Much due to the horizontal traveled shutter curtain, limiting its maximum sync speed allows, as you can note, virtually ALL modern SLRs are using vertical traveled curtain now to boost its sync speed to as high as 1/300 sec (Some came with FP, which can synchronizes up to 1/4000 sec). Cross reference: Nikon F3 provides up to 1/80 sec, the New F-1 maximum is 1/90 sec, Olympuses is 1/60 sec - all with horizontal traveled Shutter mechanism).

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* Note: The LX is designed to accept a wide array of flash models using either the built-in hot shoe of the standard FA-1 viewfinder or the synchronization terminals at the front of the camera body. The "X" terminal is for synchronization of electronic flash or F-type and MF-type bulbs; the "FP" terminal for synchronization with focal plane bulbs.

The integrated flash operation of the LX also eliminates the need for exposure compensation when using filters or close-up accessories and allows you to take full advantage of the bounce flash capabilities of the AF280T and AF-400T while retaining through-the-lens metered flash exposure control.

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Auto Flash Synch/Viewfinder Ready Light After attaching the TTL flash unit to the LX hotshoe or connecting the AF-400T to the camera via its 5P Synch Cord, set the flash unit to "TTL AUTO" operation and switch the unit "ON". Leave the camera's shutter dial set to "Automatic" and the camera will automatically set the shutter speed at "X" (1/75 second) for flash synchronization and illuminate the viewfinder LED at "X" as a visible "ready light." In addition to "Automatic" the camera's shutter speed dial may also be set at "X" for TTL operation. Do not use other settings, which may cause in-proper synchronization .

Pentax LX professional SLR camera body rear section exposed interior
Automatic synchronization at "X" is obtained when viewfinder shutter speed indication is between 1/30th second and LT.B (occasionally flash is obtained also at 1/60 second because of stepless metering). On TTL Auto, both the AF-280T and AF-400T signal confirmation right inside the viewfinder after the picture is taken ensuring that your subject received adequate flash exposure. After a properly exposed photo, the red "X" LED flickers on and off.
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For flash action photography with flash, the viewfinder exposure check can be turned off by a switch on the back of the flash unit.
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NOTE: In addition to "Automatic" the camera's shutter speed dial may also be set at "X" for TTL operation. Do not use other settings, this may cause in-proper synchronization. Automatic synchronization at "X" is obtained when viewfinder shutter speed indication is between 1/30th second and LT.B (occasionally flash is obtained also at 1/60 second because of stepless metering).

How about non-dedicated Flash ? Don't worry, since this camera is designed for the Pros, the LX system also offers you the unique option of synchronization even with non-dedicated electronic flash units with the shutter dial set at "Automatic". In addition, manual speeds from "X" through "B" may be used. On "Automatic" the synchronization range is between 1/30th second and 4 seconds.

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Do not use "Automatic" for flash when the viewfinder indicates a faster speed or the complete frame will not be exposed; be sure to take precaution against picture blur when long exposures are indicated.

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