Definitive Metz 54 E-TTL Information

Started Feb 6, 2004 | Discussions thread
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pbleic Veteran Member • Posts: 3,699
Definitive Metz 54 E-TTL Information

I made the call to Les Brown today. He is the Product Manager and technical guru on the Metz line at Bogen (Bogen being the parent company of Metz). Think of him as the Chuck Westfall of Metz. I will do my best to paraphrase our 15 minute conversation:

Bottom line - the E-TTL "pre-flash" works, in part, through a small sensor located at the bottom left of the flash head (light generating part).

Per Les, E-TTL is a very complex system involving measurement of WB and reflectivity (including light balance, backlighting, etc.). Many measurements are taken of both the ambient light and the pre-flash - comparisons are made and proper flash intensity and duration determined. The requirements for exchange of information with the Canon system is highly complex. In order to optimize this process, Metz put a small sensor on the flash HEAD (NOT the Automatic Flash sensor by the red infrared light). The sensor reads the pre-flash data, communicates it to the E-TTL software in the camera. The camera combines this information with the TTL ambient metering and computes a proper flash duration and intensity. The shutter opens and the flash goes off. The TTL meter in the camera is active at this point, ensuring a proper exposure.

Because the sensor is on the flash head, the Metz doesn't work well with an Omnibounce, or in > 60 degree bounces (this is per Les), as the sensor won't get a proper read of the scene.

There should be no further argument about this. Perhaps I got some of the exact details slightly wrong, but the basic stuff is right - the sensor that reads the pre-flash on the Metz is NOT the TTL sensor; it is the one on the flash.

It is perhaps because of this that the E-TTL of the Metz may even be better than the Canon flash (my words, not Les').

Finally, Les said that digital cameras are driving the market towards fully automatic flashes.

Appendix from Photonotes.org: The standard sequence of E-TTL with a Canon flash. To compare with the above.

The usual E-TTL sequence of operations, not counting the optional flash exposure lock (FEL) feature or wireless operation, is as follows:

When the shutter release is pressed halfway the current ambient light levels are metered by the camera as usual. Shutter speed and aperture are set by the camera or user depending on the current mode - PIC (icon) modes or P, Av, Tv or M.

When the shutter release is pressed all the way the flash unit immediately fires a low-power preflash from the main flash tube. (ie: white light)

The reflected light from this preflash is analyzed by the same evaluative metering system that the camera uses for metering ambient light levels. The appropriate power output (ie: flash duration) of the flash is determined and stored in memory. The entire sensor area is evaluated and compared to the ambient metering, and the area around the active focus point is emphasized. If you are in manual focus mode then either the central focus point or averaged metering is used.

If the photo is being taken under bright lighting conditions (10 EV or brighter), auto fill reduction is applied (unless it has been disabled by a custom function, as is possible on some bodies) and the flash output is decreased by anywhere from 0.5 to 2 stops. However, the E-TTL auto fill reduction algorithm has never been published to my knowledge, so nobody outside Canon knows exactly how it works.

The mirror flips up and the shutter opens, exposing the film - or sensor chip if it’s a digital camera.

The flash tube is then fired at the previously determined power level to illuminate the scene. Start time of the flash burst depends on whether first or second curtain sync has been set. The OTF sensor in the camera, if present, is not used in E-TTL mode.

The shutter stays open for the full duration of the shutter speed time.

The mirror flips down and the shutter closes. If the flash unit has a flash exposure confirmation light and if the flash metering was deemed adequate then the light glows.

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Paul

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