What is the difference between TTL and E-TTL

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TTL

Canon introduced the TTL flash metering with the T90 in 1980s. After which, it has incorporated this feature as a standard with the EOS line of film cameras. The Canon T90 is the only non-EOS camera capable of using Canon’s TTL flash metering system.

TTL flash metering works by measuring the pulse of flash-generated light bouncing back off the subject and entering the lens. It actually measures this light reflecting off the surface of the film itself, in real time, by using an off the film (OTF) sensor. The light from the flash bulb is quenched when the sensor determines enough light that has been produced to achieve a satisfactory flash exposure to get a mid-toned subject. Given the fact that digital cameras do not have film. Hence, digital EOS cameras do not support TTL.

One very interesting thing to note is that the sensor records light reflecting off the surface of the film itself. Therefore, it will react differently to film with very different reflective properties. All EOS cameras with TTL flash are calibrated to work with the emulsion characteristics of typical colour print film. As a result, there may be very subtle differences in flash metering when you use slide film, since slide film has very narrow exposure tolerances (latitude).

EOS cameras that support TTL flash :

T90 and all EOS film cameras except the EF-M, EOS 300x and EOS 30v.

All digital cameras including the EOS digital series - do not support TTL.

All “E” series speedlites support TTL flash.

TTL metering is more but it can still be fooled. For example, a highly reflective subject or a subject in white surroundings can result in a lot of light reflecting back, so the resulting picture may well be underexposed as the camera quenches the flash too soon. An off-centre subject poses similar problems. Another issue is that the flash metering occurs while the shutter is open, so the camera can’t accurately factor flash in with ambient light metering.

Canon refined TTL control on their multiple focus point cameras by adding a feature they call AIM, (“Advanced Integrated Multi-point Control System”) which is basically multiple-segment flash metering. This lets the camera bias the flash exposure to the currently selected focus point, thereby increasing the chances of getting accurate flash exposure for off-centre subjects.
The AIM system means that it’s best to rely on selecting off-centre focus points for flash photography rather than using the centre point and then recomposing the image (unless you use FEL).

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E-TTL

Canon introduced the E-TTL (“evaluative through the lens”) flash technology in the launched of EOS 50 in 1995. E-TTL fires a low-power preflash of known brightness from the main bulb to determine correct flash exposure. It measures the reflectance of the scene with the preflash, and then calculates proper flash output to achieve a mid-toned subject, based on that data. Even though it uses a preflash, but it does not suffer from A-TTL’s drawbacks for two reasons.

Firstly, the E-TTL preflash occurs immediately before the shutter opens and not when the shutter release is pressed halfway. Unlike the A-TTL preflash, the E-TTL preflash is actually used to determine flash exposure and isn’t fired during the ambient (existing) metering stage. Some users may be surprised to learn that E-TTL actually fires a prefire flash before the main flash at all. Using regular settings the process happens so quickly that the preflash is difficult to notice, though you might catch glimpse of it before the mirror blackout - an exception being second-curtain sync.

Secondly, the preflash light is analyzed by the same evaluative metering system that the camera uses to meter ambient light. This means it meters through the lens and is harder to fool than external sensors.

In addition, it is not confused by bounced light and does not read anything off the surface of the film like the TTL.
E-TTL is generally superior to TTL and A-TTL when it comes to fill flash. The E-TTL algorithms are usually better at applying subtle and natural fill flash light to daylight photographs. E-TTL exposure is also linked to the current AF focus point, which in theory results in finer-grained exposure biasing than most multiple-zone TTL flash sensor systems.

Unfortunately, E-TTL is not a perfect flash system. Many users have reported serious problems with wildly varying exposure when using an E-TTL flash unit with earlier model Canon DSLRs, particularly the D30 and D60. Some of these problems stem from the users focusing and recomposing and failing to use the flash exposure lock (FEL) feature, which sets the wrong area around which the flash will meter. The main problem appears to stem from the way in which E-TTL on these bodies biases flash exposure heavily to the focus point. Others set their lens to manual focus once focus had been achieved, since the camera uses a centre-weighted average metering pattern for flash metering when in manual focus.

The EOS 10D has revised E-TTL algorithms which rely on centre-weighted average metering for E-TTL flash, even if the lens is set to auto focus mode. And E-TTL II, which analyzes all metering zones before and after the preflash for improved flash metering. Later digital cameras seem less vulnerable to metering errors caused by bright highlights.