Canon, Nikon and Pentax each have their own TTL flash metering systems, respectively E-TTL, i-TTL, and P-TTL. They all use a weak preflash that is metered and used to calculate flash power level required for correct exposure, but I suspect there might be differences in implementation details.
What are the nuances by which the modern TTL systems differ, and in which situations do strengths and weaknesses of these metering algorithms show?
I already know that each system is brand-specific and uses different hot-shoe contacts, so no need to mention that.
Answer
Canon E-TTL
E-TTL stands for "Evaluative Through the Lens" and was introduced in 1995.
A low power preflash is fired immediately before shutter opens and its reflectance is measured to determine correct flash exposure. Entire frame is analyzed by the same evaluative exposure metering system as ambient exposure, area under active AF point is given more emphasis in calculations.
If the ambient light level is above 10 EV (Exposure Value), flash exposure is automatically dialed down, unless the feature, called "Auto Fill Flash Reduction" (a.k.a. "automatic reduction of flash output"), is disabled. At 10EV, the compensation is -0.5EV, increased by -0.5EV for each additional EV of light, capped at -2EV compensation for light levels 13EV and above.
Exposure compensation that has been set on the body is not applied to flash exposure; you have to use flash exposure compensation to affect flash exposure.
Immediate preflash before real exposure can cause subjects to close their eyes, so you can use FEL (Flash Exposure Lock) to perform preflash metering in advance.
E-TTL can also be used to control and meter wireless flashes. There are 4 channels to separate multiple photographers in same location. Flashes can be arranged into two or three groups (depending on master flash); the groups are metered separately by a quick succession of preflashes. Exposure ratios for groups A and B and exposure compensation for group C can be controlled on master flash (always in group A).
E-TTL II
Most significant advancement is that evaluative flash metering is no longer based on assumption that an active autofocus point covers the subject. All changes are in camera body, any flash supporting E-TTL (such as all Canon EX flashes) can also be used by in E-TTL II.
There are two modes for weighting the zones (selectable in camera Custom Functions) - evaluative and averaged. Evaluative mode uses pre-flash analysis information to determine weights for the zones - the zones with slight differences from ambient light are selected for flash exposure calculation; high difference areas are disregarded as possibly highly reflective to avoid underexposure. In averaged mode, results from metering zones from the middle of frame (area AF ellipse) are averaged equally and the rest of frame is ignored.
If such an EF lens is used that provides distance information, that information is used to determine a ballpark exposure which is then used to refine calculations. There are several exceptions when the distance information is ignored: macro flash, wireless flash, bounce flash (whenever flash head is not straight nor using slight downward tilt).
Nikon iTTL
Introduced in 2003. Similar to E-TTL, ambient and flash metering is performed during preflash immediately before mirror rises and shutter opens. Reflected light is measured (lens wide open) with 5-segment flash sensor in center of frame, and required flash power is calculated, using also data from RGB sensor concerning data on light and shadow areas, subject distance (from lens), reflectance, color temperature.
Originally, iTTL flash exposure metering was completely separate of ambient exposure metering (using only the same hardware); more recent bodies, starting from D3 and D300, automatically underexpose ambient exposure in bright light when flash is turned on to prevent overexposure in areas lit by flash.
Exposure compensation is applied to both ambient and flash exposure; if only ambient exposure compensation is needed, this can be counter-compensated with flash exposure compensation.
The flashes can again be divided into up to three groups, each of which is metered by a separate preflash. In iTTL, the computed required power level is immediately sent back to the group. All flashes in same group fire at same level. Note that since the groups are metered individually, there is no handling for situations where same spot is lit by several groups; the camera instructs the groups to fire with slight underexposure to cope with that. Each group can be instructed from master flash to fire in TTL (you can also adjust flash exposure compensation for the group) or at a manual power setting.
Also, there are 4 channels available to enable multiple photographers use each their own flashes.
TTL-BL is a separate mode for fill flash. For successful operation, it requires that subject is darker than background, and works to brighten it to balance with the background.
TTL-FP is Nikon's term for high speed sync.
Similar to FEL in E-TTL, FV Lock can be used to perform preflash metering in advance.
Pentax P-TTL
First appearance in 2001. Similar to others, P-TTL meters a low power preflash with lens wide open before opening shutter and calculates needed flash power level.
There does not seem to be any way for triggering preflash metering manually.
With a P-TTL flash turned on, exposure compensation is applied to both ambient and flash exposure. However, there is a quirk - when using positive exposure compensation, shutter time is capped at "handholdable value" - roughly 1/(1.5 x focal length) seconds. When shutter time reaches that value, further positive compensation only affects flash exposure. The cap on shutter time is applied even if it means underexposure without any compensation - camera hopes flash to fill in the gaps. In Program mode, aperture does not change when compensation is adjusted.
There are 4 wireless channels; when you use pop-up flash as the controller, a flash has to be paired with camera body (on hot shoe) to make the body use the same channel. The flashes cannot be arranged into groups available to be controlled from camera or controller flash; exposure compensation has to be set on each flash separately.
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