TTL - (English: Trough The Lens) - a system for measuring exposure and focusing based on the light that has passed through the lens. In computer technology, TTL is the time to live of packets transmitted over a network connection.

All Canon EOS cameras (including modern ones) have AF that uses TTL (Through The Lens - through the lens) SIR technology (stands for Secondary Image Registration Through the Lens) using a CMOS sensor. Multi-based means more than one AF point, and CT (CrossType) = Cross sensor type (in 30D and 550D only center type, all 9 in 40D and 50D are cross type). With each successive new camera model, Canon makes updates to the algorithms used by the DIGIC processor that processes the AF data.

TTL metering

There are two types of technology TTL: from measurement exposure at the operating aperture value or at a fully open relative aperture. The first method (English Stop Down Metering) is less accurate, since less light enters the photoresistor through a closed aperture. The technical implementation of this technology is the simplest and is used for lenses that are not equipped with a jumping diaphragm, as well as in filming equipment.

First cameras with a TTL exposure meter, for example, the Pentax Spotmatic and Canon Pellix measured exposure at the working value of the jumping aperture, closed with a repeater combined with the metering button. The overwhelming majority of domestic cinematographic equipment was equipped with TTL-exposure meters measuring exposure in the same way.

TTL and flash

Supports ADI-TTL mode

Supports ADI-TTL auto exposure mode.
ADI-TTL (Advanced Distance Integration TTL) is an algorithm developed by Minolta used in Sony and Minolta cameras. When calculating the flash output, information about the distance to the subject being photographed is used.
ADI-TTL is only used when directing the flash towards the subject.

D-TTL mode support

Supports D-TTL auto exposure mode
D-TTL is based on matrix exposure metering. In this mode, flash output is calculated to maximize the balance between the subject and the background illumination. During metering, a series of discreet flashes of varying power are fired. The final calculation is made taking into account such parameters as the sensitivity of the photographic film (or photomatrix), the aperture value, focal length and distance to the object being shot.
D-TTL is used in Nikon cameras.

E-TTL mode support

Supports E-TTL auto exposure mode.
In the E-TTL (Evaluative TTL) mode, the exposure is estimated from a preliminary light pulse of low power. Flash operation in E-TTL mode is visually no different from normal operation, the preliminary pulse occurs very quickly and the human eye is not able to notice it.
E-TTL is used in Canon cameras.

Supports E-TTL II mode

Supports E-TTL II auto exposure mode.
E-TTL II is an improved version of E-TTL (see “E-TTL Mode Support”). The new version uses information from light sensors both before and after the pre-flash. In addition, when calculating the required flash output, information about the distance to the subject is used (when such information is available).
E-TTL is used in Canon cameras.

P-TTL mode support

Supports P-TTL auto exposure mode.
In P-TTL mode, the flash pre-pulse is used to determine the exposure.
P-TTL is used in Pentax cameras.

S-TTL mode support

Supports S-TTL auto exposure mode.
S-TTL was developed by Sigma specifically for their cameras. This mode uses a flash pre-burst to estimate exposure.

TTL mode support

Support TTL auto exposure mode.
The abbreviation TTL (Through The Lens) means that the exposure calculation measures the amount of light that passed through the lens and hit the film or photosensitive sensor.
TTL automation works as follows: when the shutter is triggered, the flash is ignited, and special sensors in the camera capture the light that has passed through the lens. This data is used to calculate the flash duration required to obtain a high-quality photo. After this time, the flash lamp turns off.

I-TTL mode support

Supports i-TTL auto exposure mode
i-TTL is an evolution of D-TTL (see “D-TTL Mode Support”), it will include all D-TTL functions and also support wireless multi-flash control.
i-TTL is used in Nikon cameras.

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