Thanks for the comprehensive answer. A silly final question on the
subject:
In the case of "A" lenses, must the lens be set to "A" for the
pre-flash aperture reading to be detected? No..right?
Jack
--- Godfrey DiGiorgi <[EMAIL PROTECTED]> wrote:
>
> On Dec 23, 2005, at 9:38 AM, Jack Davis wrote:
>
> >> From the archives: It's Godfry's 12/20 contribution to the
> > TTL/Metering? thread:
> >
> > Referring to the D-
> > "...(built-in flash when using lenses that do not support P-TTL
> > metering)..."
> >
> > Maybe there is more to know about the difference between TTL and P-
>
> > TTL.
> > I once asked Pentax that question and was told "P"
> > is simply a reference to Pentax equipment.
>
>
> The full quote is:
>
> >> Do any digital SLR's provide Off The Sensor metering?
> >> I gather not..(?)
> >
> > TTL flash metering is supported by the D (built-in flash when using
>
> > lenses that do not support P-TTL metering) and by the DS/DS2 when
> > using a compatible dedicated external flash unit. Due to the nature
>
> > of TTL flash metering, it must be reading "off the film/sensor"
> > since it is determining light levels dynamically at the time of
> > exposure.
> >
> > The TTL ambient light metering is all pre-exposure, through a
> > sensor system fed by semi-silvered portions of the mirror and
> > sensors above the focusing screen.
>
> [addendum] "... therefore it cannot measure ambient light while
> exposure is actually happening since the mirror is up at that time."
>
> P-TTL stands for "preflash-through the lens". If Pentax wants to
> market it as "Pentax-Through The Lens", fine, but the name then means
>
> nothing, it's just a tag. What P-TTL flash metering does is take an
> ambient light reading and a flash light reading with a pre-flash,
> integrate them, and set the exposure specifics, including flash unit
>
> output, to match.
>
> TTL flash metering is much simpler: a sensor simply reads the flash
> illumination coming through the lens and sends the flash unit a
> quench signal when nominal exposure has been achieved.
>
> P-TTL requires information about the lens maximum aperture and
> working aperture to work because it makes the preflash and ambient
> light readings prior to exposure when the lens is still held wide
> open and the mirror/ambient exposure system is enabled. This requires
>
> KA or KAF mount lenses on the D/DS/DL/DS2 bodies, there is no
> aperture simulator to mechanically transmit aperture information to
> the metering system. With the DS/DS2/DL bodies, the built in flash
> ONLY supports two modes of operation: P-TTL or non-metered full
> output. When a lens which does not have at least the KA electrical
> connections is used with these bodies, the built in flash unit
> defaults to non-metered, full output mode and no pre-flash is fired.
>
> More on the D body below.
>
> TTL flash metering reads the light reflected from the sensor during
> the actual exposure. This requires no information regarding maximum
> lens opening or working lens opening ... it's simply reading the
> light and quenching the flash at the appropriate moment. The D body
> supports TTL flash metering with K mount lenses using the built-in
> flash, as well as P-TTL with KA and KAF mount lenses. The D, DS and
> DS2 bodies support TTL flash operation with K mount lenses only when
>
> used with a dedicated external flash unit like the AF360FGZ. The DL
> does not have the circuitry to support TTL flash metering with K
> mount lenses even with an external flash unit (at least that is the
> best I can make out from the DL owners manual).
>
> So ... KA and KAF lenses are fully compatible with P-TTL and all
> other *ist D, DS, DS2, DL exposure functions. K mount lenses are only
>
> compatible with TTL flash metering with the DSLRs, and only when
> using the D body or DS/DS2 bodies with a dedicated external flash
> unit.
>
> I hope that's clear now.
>
> Godfrey
>
>
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