Monday 14 July 2008


For further photography-related information check out my compendium of tips.

These are some brief notes on E-TTL II. For an in-depth read on Canon's flash systems read NK Guy's magnum opus at Photonotes.

Evaluative through-the-lens II (E-TTL II for short) is the latest incarnation of Canon's automatic flash exposure control system. I currently use a Canon EOS 5D with 580 EX II Speedlite which is based upon this system - it's a match made-in-heaven for flash photography. Previously I used a Canon EOS 5 with 540 EZ Speedlite (based upon A-TTL, where A = advanced) which was far less reliable and required me to constantly adjust flash exposure compensation (FEC).

The system is a 'black box' in that Canon publish only limited information about the algorithms that control flash exposure. Their attitude is switch it on and use it. I would urge you to do the same. Get an empirical feel for how the flash system works and then make adjustments as you wish. I did, however, find the following information on the Canon Professional Network website regarding fill-in flash (see my post regarding exposure values, EV).

"The output of the fill-in flash depends on the shooting conditions. With lower light levels (below about EV 10), you get a flash output just as if you were shooting a subject at the same distance indoors. Above EV 10, the flash output is gradually reduced, to a maximum of -1.5 stops (-2 stops with E-TTL autoflash) at EV 13 and above. This auto flash output reduction helps to create a better balance between the daylight and the flash illumination in bright sunlight."

When shooting indoors I still tend to use some flash exposure compensation (at least 2/3 stop) to keep the light contribution from the flash very subtle but the shot will still look good without. Check out my most recent gallery of wedding photography - most of the shots used flash, both indoors and outdoors (the Speedlite was set to high-speed sync outdoors).

With E-TTL II, the camera transmits the lens focal length, exposure control mode, aperture and image sensor size. With more recent lenses focusing distance data is also communicated - even if the Speedlite is attached via an off-camera shoe cord (although for this to work accurately your flash unit must be a similar distance from the subject as if it were mounted directly on the camera - a flash-bracket is fine). Distance data is not used if flash is bounced or used wirelessly.

Metering for flash uses the same system as for ambient light metering and is linked to the current auto-focus point. The camera fires a pre-flash which allows it to meter the scene for flash exposure. To see this, put the flash in second-curtain sync and then set a long exposure time.

Flash exposure locking is available for situations in which the camera is fooled and you're not confident using FEC. These are the same situations that can cause problems metering for exposure - when the overall tone of the shot is not medium-grey.

Check out more of my photography here: UK photographer

No comments: