All Canon EX Speedlites offer automatic
control of flash exposure, but only a few high end models have adjustable manual
power control. The manual control feature is desirable, since flash exposure
can sometimes be fooled by highly reflective objects in the metering field.
This is particularly so when using a combination of E-TTL and
manual focus.
One way, described in a separate
document, is to force the flash into TTL mode or even manual mode by
insulating certain pins on the hotshoe. This document details another, zero
cost method to achieve full power flash by mis-using the Flash Exposure Lock
(FEL) feature of the E-TTL system. The method described here
works with a hotshoe mounted E-TTL flash, as well as for wireless E-TTL controlled
remote slaves. However, the technique requires E-TTL, so it is not suitable
for Type B cameras nor for non E-TTL flash units.
Power up your flash and camera. Now
completely interrupt the light path between the flash and the lens. That's
right! Either wrap the flash with foil, or leave the lens cap covering the
lens.
Next push FEL and watch for a "flash underexposure" warning. Uncap your flash
and lens, and take your picture. Voila! Full power flash. Just be sure to release
the shutter before the 16 second FEL timeout. If this is not long enough, either
push FEL again, or hold the shutter button half pressed.
This technique is called "blind FEL" since
the preflash emitted by the flash is prevented from being sensed by the camera
meter. Accordingly, the camera drives the flash unit(s) to maximum power.
Of course, shooting with full power flash means relying on Guide Number calculations
or an external flash meter. But this also gives you the means to achieve full
control of flash exposure. For instance, use an aperture that is wider than
the guide number indicates for positive flash exposure compensation. Always
be sure to proof check your results before committing to an important shoot.
By misusing flash exposure lock (FEL), it is also possible to force an EX Speedlite to MINIMUM power. Point the camera directly at a reflection of the flash, so that the E-TTL sensor is blinded when you activate FEL.
Depending on your camera and the setting selected, you might need to use
either the CENTRE metering cell, or the metering cell over on OFF-CENTRE
autofocus sensor.
As usual, these techniques might work differently on your camera. Practise
with your own gear before an important shoot.
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