ttl, dttl, ittl...

I would like to know, on a technical base, what's the difference between ttl, dttl and ittl !

Any information or links about that ?

Thanks

Claude

Hi Claude,

first things first: I can not tell you the difference between D-TTL and i-TTL.

From the technical point of view I can only write about the step between the TTL and the DSLR used D-(E-)TTL.

The standard and classic TTL:

During the exposure when the curtain is open the incoming light is reflected by the film surface and measured by a sensor which is in general positioned below the mirror.

The camera monitors the amount of light for the exposure. When enough light has hit the film surface the camera shuts down the flash. The intensity of the flash light is therefore only controlled by it’s burning duration.

Nikon D-TTL, Canon E-TTL:

The problem is that the surface of the image sensor of a digital camera is different to film surface. The light reflection of the sensor during exposure is no more usable for measuring.

Nikons D-TTL and Canons E-TTL solve the problem the following way.

Before the exposure a pre-flash is emitted. The curtain is still closed. The curtain is coloured like a standard Kodak grey card. First the mirror goes up and the curtain reflects the pre-flash which is now measured by the light sensor. From the given value, the camera now calculates the required amount of strobe light and opens the curtain and fires the calculated main flash.

The classic TTL just measures the amount of light in general. D-TTL and E-TTL have measuring capability for different zones in the picture. So ambient light is taken into account. Also a bright sunlight in one area of the picture is taken into consideration for the flash intensity calculation to avoid underexposure as old TTL would have done.

D-TTL and i-TTL: I don’t know what the advantage is. But it is a shame that i-TTL is unable to fall back to D-TTL compatibility. Makes it even more difficult for underwater flash and housing builders.

Canons E-TTL and E-TTL II are compatible. What I’ve read E-TTL II just takes the subject distance (delivered from the lens focus position) into account. There are no special E-TTL II flashes around. Every Canon E-TTL flash also gives E-TTL II on a certain camera and lens.

The only DSLR I know which supports old TTL and works with amphibian strobes is the Fuji S2Pro. The camera reads the amount of light off the sensor during exposure. So no pre-flash required. Very nice.

adieu,

Julian

Thanks Julian for this detailed answer...

I've got the information about iTTL : it's like Canon eTTL-II (it assess the subject distance) - but's right : no backward compatibility :roll:

mfg

Claude

Hi Claude,

If you're new here, try doing a search on "DTTL" and you will get a lot of results.

Here's my personal "treatise" on the subject of DTTL and iTTL:

http://www.wetpixel.com/PNphpBB2-viewtopic...ight-dttl.phtml

Cheers

James