TTL is te acronym for "Through the lens". It refers to any function that performs some calculation based on "real" information received from the image, as it arrives to the sensor. Film cameras could include built-in light metering, which was indicated as TTL. In the same way, autofocus function works in TTL mode, using real image data.
In flashes, TTL refers to the automated setting as per ambient conditions and the subject lighting. Since its itroduction in the Canon T90 and flash 300TL in 1985, the camera is able to modify the flash setting to increase or reduce light intensity, if needed. Of course, both camera and flash must be compatible and share a common standard.
From this point, you may find different TTL working modes in flashes:
- A-TTL (Advanced TTL), based on an infrared preflash to calculate distance to the subject (and approximate object size, based on amount of reflected light). The camera controls the amount of flash light to be produced just by the adjustment of the flash duration, at full power, looking for a balance between subject and background illumination.
- E-TTL (Evaluative TTL), it uses a white light preflash, so that overall illumination can be evaluated, in order to decide required flash duration and intensity. Flashes with E-TTL can typically also work as A-TTL, in cases when the light is pointed to some other direction (bounced flash).
- E-TTL II: New generation, it considers also the possibility that the subject is NOT in focus at the time of metering. It introduces light averaging in the calculations.
- I-TTL (Intelligent TTL): From Nikon, it is based on the reading of several light sensors (typically, five) distributed over the image sensor, and it can include data on the colour temperature, together with distance and general illumination.
- P-TTL (Preflash TTL): From Pentax, it opens fully the diaphragm (aperture) when making the preflash metering and lighting evaluation.
- Nikon has also a proprietary work mode called CLS (Creative Lighting System), which combines the concept of i-TTL with advanced features such as flash exposition block, wireless triggering and simultaneous use of several flash units.
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