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
