Exposure Control; Flash - Canon EOS 40D White Paper

Exposure Control

The EOS 40D uses the same 35-zone metering sensor as in the Metering Sensor
EOS 30D. The metering modes are evaluative, partial (approximately
9% of viewfinder area), spot (approximately 3.8% of viewfinder
area), and center-weighted average metering.
Previously, during automatic AF point selection and AF point-centered
evaluative metering, there was a tendency for slightly excessive compensation. The
compensation is now looser so that exposure is more consistent.
The E-TTL II autoexposure algorithm of the EOS 40D is the same as the EOS-1D Mark III's
E-TTL II autoexposure algorithm. By optimizing the method of using lens distance
information, exposure becomes more accurate for highly reflective subjects. With the
EOS 30D, the selected metering mode was used to meter the ambient light. This sometimes
resulted in inaccurate exposures when the selected metering mode was not evaluative
metering. Therefore, as with the EOS-1D Mark III, evaluative metering is now used at all
times to meter the ambient light.
The EOS 40D uses the same exposure control system as the EOS 30D, with eleven AE
modes and manual mode. While based on the EOS 30D's shutter unit, the 40D's shutter
is now more reliable with a contactless switch instead of a contact switch for running
the 1st and 2nd curtains. Shutter durability is approximately 100,000 cycles, the same
as with the EOS 30D's shutter. With a fully-charged Battery Pack BP-511A, the shutter
supports a bulb exposure as long as about 2.5 hours.

Flash

A new feature, shared with the EOS-1D Mark III, and a considerable convenience, is that
both built-in flash and external Speedlite settings can be set on the EOS 40D from the
camera with the menu screen. With external, EX-series Speedlites, this includes Flash
mode, sync setting, FEB, flash exposure compensation, E-TTL II, zoom, wireless flash,
and clear settings. The EOS 40D can make communication settings as well, such as
communication channel, flash group, and flash brightness
External flash Custom Function
when the Speedlite 580EX II is used. Also, with the 580EX II
attached to the camera, one can set or cancel the Speedlite's
Custom Function settings (C.Fn-0 to 13) with the camera. With
an EX-series Speedlite other than the 580EX II, the camera can-
not be used to set the Speedlite's Custom Functions.
The flash unit in the EOS 40D is the same as the unit in the EOS 30D. Its Guide Number
is 43/13 at ISO 100 in feet/meters. Flash coverage is effective for focal lengths 18mm
and higher, with some limitations based on the size of the lens. If the built-in flash is
fired rapidly in succession, the flash firing will be stopped to prevent heat damage to
the built-in flash cover and fresnel lens (diffuser). In the case of the EOS Digital Rebel
XTi, which has similar specifications, if the built-in flash is fired 20 times successively at
10 sec. or shorter intervals, the firing will be stopped. With the EOS 40D, the flash control
is more precise, so the flash stoppage is less likely to occur than with the EOS Digital
Rebel XTi.
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