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<blockquote data-quote="WayneF" data-source="post: 135696" data-attributes="member: 12496">You are right that TTL flash exposure depends on preflash, but the TTL flash system is always two exposures, one for the ambient light, and one for the flash.&nbsp; A double exposure, so to speak.&nbsp; Ambient light might be insignificant level sometimes, but there are always these two factors.In Auto modes (A, P, S), the ambient light reading sets the shutter speed or aperture for the ambient light exposure.&nbsp; Then slightly later, the separate TTL metering fires and meters the preflash, and sets the flash power level for that aperture it discovers is being used. &nbsp; TTL BL mode exposure tries to balance (reduce) the flash level to fit in with the ambient exposure, where TTL mode exposure ignores ambient, and gives full working flash value regardless of any ambient situation.If you want to meter TTL flash in one area, and then move the camera and take the picture in another area, using the previous exposure metering, then the FV Lock feature does that (meters the flash value and holds it until shutter button is triggered).&nbsp;&nbsp; However, it is most useful to bypass getting pictures of the subject blinking from the preflash (and/or commander signals).</blockquote>

[QUOTE="WayneF, post: 135696, member: 12496"] You are right that TTL flash exposure depends on preflash, but the TTL flash system is always two exposures, one for the ambient light, and one for the flash. A double exposure, so to speak. Ambient light might be insignificant level sometimes, but there are always these two factors. In Auto modes (A, P, S), the ambient light reading sets the shutter speed or aperture for the ambient light exposure. Then slightly later, the separate TTL metering fires and meters the preflash, and sets the flash power level for that aperture it discovers is being used. TTL BL mode exposure tries to balance (reduce) the flash level to fit in with the ambient exposure, where TTL mode exposure ignores ambient, and gives full working flash value regardless of any ambient situation. If you want to meter TTL flash in one area, and then move the camera and take the picture in another area, using the previous exposure metering, then the FV Lock feature does that (meters the flash value and holds it until shutter button is triggered). However, it is most useful to bypass getting pictures of the subject blinking from the preflash (and/or commander signals). [/QUOTE]

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