Guide to SLR TTL flash modes

Guide to SLR TTL flash modes

TTL is used to describe any flash system that meters exposure Through The Lens.

What is the d ifference between the Nikon TTL flash types?

TTL - With film SLR cameras, light passed through the lens and reflected by the film is continually measured by the flash sensor during exposure until the correct exposure has been achieved. This system is referred to as a TTL flash metering system (see the image of a TTL flash sensor system below).

D-TTL - Digital camera sensors do not reflect enough light for the TTL sensor to use during exposure so metering is performed using light from the *Monitor pre-flash reflected from the shutter curtain before the exposure, the end of the exposure is detected by the camera metering system. This system is referred to as a D-TTL flash metering system.
*What is ''Monitor pre-flash' (or 'pre-flash')?
The Speedlight fires a series of pre-flashes to 'test' the scene, using the flash meter it can then calculate exposure just before firing the main flash to illuminate the subject during the actual exposure time. The pre-flashes are lower power than the main flash. Nikon use the term DX as a suffix for Speedlight names that support the pre-flash function which is an essential element of D-TTL operation.

i-TTL - Is an evolution of D-TTL that incorporates more accurate systems for pre-flash. Metering is performed using light from the pre-flash reflected from the shutter curtain before exposure as with D-TTL. However the pre-flash fired is much stronger and of much shorter duration than D-TTL to achieve greater accuracy using the flash sensor.

i-TTL Speedlights are compatible with Nikon's Creative Lighting System (CLS) when used with compatible Digital SLR cameras (D2H and D70) offering new functionality that is not compatible with non i-TTL Speedlights.

What is the difference between the Nikon TTL flash modes?

Standard TTL flash - TTL
The TTL sensor monitors the degree of flash illumination exposed on the film surface during exposure, the camera's exposure metering is not used in the exposure calculation.

Matrix **balanced fill-flash - TTL +
In this technique the camera's Matrix light meter adjusts ambient exposure and the TTL sensor adjusts exposure during flash. This is done to balance the flash illumination so that it doesn't overpower the foreground subject. Pre-flash and distance information from the lens are not used. 'Spot' and 'Centre weighted' versions of of balanced fill-flash are available depending on the chosen camera metering mode .
**What is 'balanced' flash?
In this mode the camera uses both the TTL flash sensor and the camera exposure meter to balance the exposure for foreground and background. The resulting exposure produces the most natural balance possible.

3D Multi-Sensor balanced fill-flash - TTL +
The Speedlight fires a series of pre-flashes, just before the first shutter curtain moves. The monitor pre-flashes are detected by the cameras TTL sensor, analysed for brightness and contrast, then integrated with distance information from 'D' or 'G' type lens which is required for this kind of function. It is called 3D because the final exposure is calculated from camera meter reading, information from the monitor pre-flash reading and lens distance information.
This mode accomodates most picture taking picture taking situations including bright subjects, subjects which are off centre and pictures with small subjects against a distant background.

Multi-Sensor balanced fill-flash - TTL +
The same as 3D Multi-Sensor Balanced fill-flash above but if the lens is a non 'D' or 'G' type lens then distance information is not communicated. There are two types of Multi-Sensor balanced fill-flash, one uses monitor pre-flash and the other does not, the type activated depends on the Speedlight and lens in use.

Automatic balanced fill-flash
This is a generic term for automatic flash power control to balance the brightness of the subject in the foreground with the background using the TTL Multi-Sensor (see image of '5 segment TTL Multi-Sensor' below).

For information on Speedlight compatibility and a key for Speedlight LCD indicator icons Click Here.


TTL flash sensor
5 segment TTL Multi-Sensor system
The image on the left shows the latest flash Multi-Sensor (sensor with five red segments). The sensor is located under the reflex mirror facing the film (or shutter), reflected light is detected from the film (or shutter if a digital camera).

The optical microlens above it directs light from five segments of the image area to the flash sensor which sends this information to the camera processor to be used in the exposure calculation.

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