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