The Canon Speedlite Transmitter ST-E2 can be used to trigger up to two slave groups with different power outputs.
The Hold button is a very useful feature to prevent an accidental change in your settings
It has four different channels which is more than enough for almost every photographer.
It is very light compared to some of the other Canon accessories as it weighs less than 1/4 of a pound (only 100g without the battery).
You can control the lighting ratio between speedlites up to an 8 : 1 ratio.
One of its more under-used benefits is that the ST-E2 can be used as a stand-alone focus assist in situations where your Canon camera is struggling to focus on it's own.
This is a great feature when you are shooting in low-light situations.
This accessory uses infra-red pulses to trigger the remote flashes rather than radio waves like the Canon Speedlite 600EX-RT.
It has an outside shooting range rated at about 30 feet and indoor about 45 feet.
With indoor shooting you have the added advantage of the infrared pulses bouncing off walls to give you more flexibility in placement of your speedlite.
Otherwise your speedlite must be in direct line-of-sight view from your transmitter.
Some units ship with the 2CR5 battery and some don't, so pay attention when you do your shopping.
If the ST-E2 sits idle for 90 seconds, the Save Energy kicks in and turns off the power to prolong battery life.
A gentle touch to the camera shutter button wakes it up instantly.
I got a real kick out of the illustration on page 26 of the ST-E2 manual. It's a good illustration on how to use the Canon ST-E2 but the subject's pose might be difficult to achieve.
Here is the link to the Canon Speedlite ST-E2 Manual PDF
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