"Alex W" <alex@(slet)skriverstuen.dk> skrev i en meddelelse
news:40d04351$0$199$edfadb0f@dread12.news.tele.dk...
> > Nogen der har erfaring med subj. til Canon G2?
> >
>
> Ikke til G2 men til andre Canon produkter.
> Der kan den blitz (Sigma EF500 ST) ikke bruges. Canon har et system hvor
et
> lille preflash måler lyset og så fyrer kameraet blitzen rigtigt af. Det
kan
> ovenstående flash ikke.
>
Nogen i gruppen kan måske rette mig hvis jeg tager fejl, men ovenstående
flash burde virke på et G2. Jeg går ud fra, at G2 ligesom Canon andre
modeller, bruger både A-TTL (til flash af EZ-typen), E-TTL (nyeste og mest
avancerede og kræver flash af EX-typen) og Almindelig TTL der virker med
alle flashtyper og altså også Sigma.
FLASH METERING MODES
====================
To understand the TTL, A-TTL and E-TTL metering modes, one must first
understand the function of the *two* light sensors contained in EOS
cameras:
Evaluative Metering Sensor: The Evaluative metering sensor
(positioned above the camera's focusing screen) is used for ambient
metering with all EOS cameras. E-TTL flash readings are also taken
with this sensor. E-TTL is currently available with EOS Elan II/IIE,
Rebel G and EOS IX when used with Speedlites 380EX or 220EX. Note
that the Evaluative metering sensor is blocked during exposure by the
camera's reflex mirror.
TTL Flash Metering Sensor: The TTL flash metering sensor (positioned
in the camera's mirror chamber and aimed at the film plane) is used
for A-TTL and TTL flash readings off the film plane during exposure.
All current EOS cameras feature segmented TTL flash sensors, designed
to bias flash exposure to the area of the picture identified by the AF
system as the main subject. Note that only the TTL flash metering
sensor is active during exposure, and then only in combination with an
A-TTL or TTL Speedlite.
With that background, here's what the flash metering modes do:
Evaluative Through-the-Lens Mode (E-TTL)
E-TTL (prestored evaluative through-the-lens flash exposure control) is a
new flash mode that reads through the lens, but not off the film.
Introduced for the first time in 1995, E-TTL is currently available with
the EOS Elan II/IIE, Rebel G and EOS IX SLR cameras exclusively when used
with Speedlites 380EX or 220EX. Utilizing a preflash fired after the
shutter button has been fully depressed but before the camera's reflex
mirror goes up, E-TTL uses the camera's evaluative metering sensor to
analyze and compare ambient light exposure values with illumination
reflected from the subject by the preflash. This data is used to
calculate and store the flash output required for optimum exposure of the
main subject (identified by the autofocus system), while maintaining a
subtle balance between foreground and background. Though transparent to
the user, E-TTL represents Canon's most advanced flash exposure control
system to date.
Advanced Through-the-Lens Mode (A-TTL)
The predecessor to E-TTL, A-TTL (Advanced Through-the-Lens) is used with
Speedlite 540 EZ when set for direct flash in the camera's Program mode.
(TTL flash exposure control is available at other times.) Like E-TTL,
A-TTL reads through the lens and concentrates its sensitivity on the area
of the picture covered by the active focusing point. Unlike E-TTL, A-TTL
controls flash exposure with a dedicated sensor that reads flash
illumination reflected from the surface of the film during exposure.
A-TTL also uses a preflash, but unlike E-TTL, the preflash is used to
calculate an aperture value based on the distance that light must travel
from the flash to the subject. In Program mode, the camera compares the
distance-based aperture data to the ambient-based aperture data calculated
by the camera's normal metering system, and selects the smaller aperture
value of the two. This method ensures accurate exposure of the subject in
any lighting condition. Speedlites equipped with A-TTL are fully
compatible with all EOS cameras.
Through-the-Lens Mode (TTL)
TTL (Through-The-Lens) is the automatic flash exposure control mode used
by the small pop-up flash units built into some EOS cameras. It is also
available with Speedlites 380EX and 220EX when used with EOS cameras other
than the Elan II/IIE, Rebel G and EOS IX. Additionally, TTL is the
standard flash mode for Macro Ring Lite ML-3, Speedlite 200E, Speedlite
480EG and Speedlite 540EZ (except when set for direct flash in the
camera's Program mode as outlined above). TTL is identical to A-TTL in
almost every way, except there is no preflash. It can be used in every
camera exposure mode. In Program mode, TTL always selects an aperture
value based on the level of available light.
--
MVH
John K Hansen
www.jkhansen.dk
(Fjern NOPE fra E-mail adresse)