Reference Guide

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A-TTL FLASH AE
A-TTL stands for Advanced-Through The Lens Flash Auto Exposure.
It's the standard flash mode with the 300 EZ, 420 EZ, and 430 EZ
Speedlites. A-TTL's unique feature is a small pre-exposure flash
burst which measures approximate subject distance. Unlike other
systems, A-TTL can provide confirmation of correct flash exposure
before the picture is taken, even in bounce flash photography.
A-TTL can also provide totally automatic fill-in flash exposure
if desired. However, many other options are possible, permitting
the photographer to control the amount of automation according to
the type of photograph he/she wishes to make.
For example, the ratio of flash exposure to available light is
automatically set according to the level of available light. But
when the 430 EZ is used with the EOS 630 or EOS-l, the automatic
ratio can be overridden up to +/- 3 stops in 1/3 step increments.
Both shutter speed and aperture are automatically set when the
camera is set to the Program Mode. But by selecting Shutter or
Aperture Priority, the photographer can control either setting
while achieving automatic fill-flash, even in low light.
BASIS (AF SENSOR FOR EOS CAMERAS)
The BASIS sensor is the heart of every EOS camera. Its job is to
gather focusing information to be calculated by the camera's
microprocessor network which then controls the focusing motor in
each EF lens. BASIS stands for Base-Stored-Image-Sensor,
referring to the sensor's electrical accumulation method which
differs significantly from conventional CCD (Charge Coupled
Device) designs used by most other AF SLRs.
Actually, due to the nature of TTL phase-detection AF, there
are always at least 2 sensor arrays in every BASIS chip. In the
case of the EOS-l, there are 4 arrays, 2 to detect horizontal
lines and 2 for vertical lines. Each array consists of a large
number of individual sensing elements.
The operational difference between BASIS and CCD designs is that
BASIS contains individual amplifiers and peak-voltage monitors
for each sensing element. BASIS preserves signal integrity by
amplifying each electrical signal before it is read out,
improving the quality of information that is provided to the AF
calculation circuit. This results in unrivaled signal clarity (80
dB peak to thermal noise ratio with the original BASIS design).
Automatic Gain Control (AGC) is carried out with the aid of the
peak-voltage monitors, thus providing appropriate signal
amplification according to the level of incident light striking
each element.