Advanced Flash Technologies
Technology - Technology
These days most systems offer flash metering capabilities beyond pure TTL flash metering. This page will offer an overview of the current state of flash metering technologies.

Multi-Sensor TTL Flash Metering

As mentioned in the TTL chapter the standard flash metering works by measuring the reflected light from the film during the actual exposure. The classic implementation is just one flash metering sensor with a center-weighted characteristics. This may sound simple but you can usually make a good guess how the flash metering system reacts in certain situations and compensate manually if needed. Multiple sensors work pretty much the same like normal matrix/segment metering systems ambient light - the image (usually just a center portion) is divided into different portions and the camera tries to calculate weighting factors dependent on the the metered light distribution - usually the brightest image portion gets the highest weighting factor assuming that the main subject (closest to the camera) reflects the most light when illuminated by the flash.

Sample Scene
TTL flash metering DURING exposure



Please note that the scene above would probably result in an underexposed image because of the white clothes.

AF-coupled Multi-Sensor TTL Flash Metering

AF coupled metering goes one step further. Instead of relying purely on the metering pattern the selected AF sensor(s) is also taken into account. So if your camera has e.g. 3 AF sensors and the left-most sensor is active the left portion of the image gets a higher weighting factors for the final metering. This may sound smart on the first view but a recomposition of the scene would naturally fool the flash metering system so you should know how the system works here.

Sample Scene
AF coupled TTL flash metering DURING exposure



The AF sensor coupling may help to reduce the underexposure risk in the image above because the selected AF point to a relatively dark spot (= different to the TTL flash metering result).

Balanced TTL Flash Metering

Balanced flash metering is the current the state of the art. Modern SLRs fire a pre-flash when you press the shutter-release button, right before actual exposure starts. Usually you'll not even notice this pre-flash. The std. metering pattern (also used for ambient light) (*) interprets the light reflected from the subject and "balances" this data with the information from the TTL flash metering sensors during the exposure.
 
(*) except Nikon - they use the TTL flash metering sensors to interpret the light reflected from the shutter

Sample Scene
pre-flash std. maxtrix metering taken PRIOR of the actual exposure
+
TTL flash metering
DURING
exposure

  + 

or

Focus Distance coupled

As the name implies the focus distance to the subject is obtained from the lens. Combined with the metering pattern taken from the pre-flash the data is used to make an educated guess about the reflection characteristics. The metering results from the classic flash TTL metering is either ignored or at least less weighted. The distance information requires in-lens logic which needs to transfer the data to the camera so with older lenses you may not be able to take advantage of this option.

Focal length (subject magnification) coupled

In conjunction with or parallel to the focus distance the focal length can be used to calculate the depth-of-field around the subject. I'm not really sure how this information could be used - the weighting factor for the scene background may be either reduced or increased depending on the object magnification or pure focal length data.

System-specific Overview

Please note that this is a pure overview. It can't really be used to assess the quality of the combo. The integrating logic (e.g. database-driven, fuzzy logic, etc.) is as important as the individual technologies. Unfortunately the supported features per camera aren't overly well documented so take this overview as a rough guidance only so no guarantees for correctness/completeness whatsoever!

Brand
TTL
Multiple TTL
Sensors
AF Sensor
coupled
TTL
balanced
Distance
Information
coupled
Focal
Length
coupled
Supported
Flash Units
Supported
Lenses
Supported
Cameras
Canon
A-TTL
Yes (**)
Yes (**)
No (*)
No
No
EZ type
All
All EOS cameras
except digital SLRs
E-TTL
Yes
Yes
Yes
No
No
EX type
All
current SLRs
starting with the EOS 50/Elan II
Minolta
xi TTL
Yes
(**)
Yes
(**)
No
No
No
xi type
All
xi series
Balanced TTL
Yes
(**)
Yes
(**)
Yes
(**)
No
Yes
(**)
HS type
All
si series plus Maxxum/Dynax 9
ADI
Yes
Yes
Yes
Yes
Yes
HS (D) type
D type
Maxxum/Dynax 4/5/7
Nikon
Balanced Matrix TTL
Yes
No
Yes
(**)
No
No
SB-25 and higher
All
all (?) Nikon SLRs
3D Balanced Matrix TTL
Yes
No
Yes
Yes
No
SB-25 and higher
D type
newer Nikon SLRs with std.
3D Matrix metering
Pentax
digital TTL
No
No
No
No
No
FTZ type
All
all Pentax AF SLRs
P-TTL
No
No
Yes
Yes
Yes
FGZ type
FA/F? type
MZ-S & MZ-6/L

* = The light is balanced with an external sensor located on the flash unit - no TTL balanced metering though.
** = may not be present on older base class models

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