ST-E2 flash ratio, need explanation?

<p>I've got 2 - 420EX Speedlites and I'm bidding on a Yongnuo ST-E2 to control both 420's. It's my understanding that I can use these in eTTL mode and let the controller trigger both flashes. I've read the instruction manual for the Canon ST-E2 and I don't understand what ratio means. Does ratio control the flash output or is something else?<br>

At the suggestion of forum members I picked up a $25 set of 3 16 channel receivers and 1 transmitter and have been experimenting with my 420EX's and a 420EZ, got a lot to learn. I'm hoping the ST-E2 simplifies things because I won't be in Manual mode, is this correct?</p>

<p>(Canon's) ST-E2 will let you set the flash ratio or the light output between two EX Speedlites, in E-TTL mode. I don't think the EZ flash is E-TTL compatible so I don't know how this fits into the equetion. In this way you'll be working in E-TTL mode whereby the total flash output and the ratio between flashes is automatically controlled,as contrary to your manual receivers + transmitter.<br>

Personally I would prefer E-TTL with not too many flash units (2 or so), when the number of flash units rises I have the feeling that I have more controle with a manual setting so in that case I go manual but YMMV. </p>

<p>Yes, the ST-E2 can control two groups of slave flash units "A" and "B", you simply set each flash unit to “slave” and then either group “A” or “B”, the ST-E2 does not manage group “C”. Then the ST-E2 automatically manages both the flash output exposure of all flashes using ETTL and the ratio between the two slave flash unit groups A and B. For example, let's say you are taking a portrait shot with a main flash on one side of the subject and a fill flash on the other. Traditionally you might position the flashes at different distances from the subject with the main flash closer, or you might wind down the exposure manually on the fill> But with the ST-E2 you could set up one slave flash as group "A" on the left, and another slave flash "B" at the same distance on the right. Now you can adjust the flash ratio any way you like to control the main light and fill light, you can even switch which side the main and fill lights are on by just setting the ratio and without moving (or even approaching) the flashes themselves. It all works very well because ETTL is controlling the exposure while the ration is controlling the difference in light output between the slave flashes in the "A" group vs. the flashes in the "B" group.<br>

That said, in my experience with Canon flash you need to understand clearly how it all works; how the different exposure modes work, how Av is fill for example but P is full. Best is “M” but you need to understand it. Likewise there are Custom Function settings such as “Flash sync speed in Av mode” (if you use that) which have a big impact. Custom Function settings such as “ETTL Flash Metering” which sets Evaluative or Average also have impact. Also the ISO and flash compensation mater a great deal. I find that once you figure it out then one set of settings tends to do really well in almost all situations. For example; ISO 400, at +2/3 Flash compensation with Flash Meter set to “Average” in M mode with a shutter at 1/60^th can consistently work great on one model of camera but will come out underexposed on another. I use the ST-E2 on a 20D, 5D II and 1D MK III and they all work differently but once I get it down for each one through experimentation then I am good to go and get very consistent results no matter which one I use because I have preferred settings for each. My advice would be that if you are having problems keep researching and keep at it.</p>

<p>I think you'll have better luck using an ST-e2 to control the 420EXs than simple transmitters and recievers, especially since there is no way to manually set the power output on the 420EX. ie. it's all or none, you can only vary output through ETTL (on camera), or wireless slave (off). </p>

<p>Peter has written a good explanation, but the best way to learn how to refine your output is to play with your setup (w/ the ST-E2), and teach yourself to see how the flashes will play the light across the scene.</p>

<p>Mark, I'd Google to see if anyone has had any problems with the YN ST-E2 and your camera model. I had to quit using mine when I found out the T3i I had just bought wouldn't fire the unit.</p>

<p>The ratios control flash power. A 1:1 ratio means that both A and B are exposing at the same level. This isn't the same as firing at the same level. A 1:2 ratio means that A is half the exposure that B is. A 2:1 ratio means that B is half the exposure that A is. There are a few other things that you can set, but it all resets when you turn the YN ST-E2 off and on.</p>

<p>Actually, the RATIO really does refer to power. A 1:2 ratio means the second unit is putting out twice the power of the first unit.</p>

<p>As for exposure, ALL the exposure math is done by the camera. First, there is a preflash (metering flash). Then, based upon the exposure the camera will command a total output of some output, and the flashes will fire to meet that output, biased by the set firing power ratio. Flash exposure based upon each individual flash is simply not possible, considering that there is only one metering flash and one observer (the camera).</p>

<p>In my experience, firing two flashes based upon ratio is as complex as I want to get in casual situations. Even then, figuring out the right way to balance ratios is very much a trial and error affair. Not to mention controlling shadows . . . .Add anything more than two flashes I find that so much trial and error is involved with the ratios and flash placement that you may as well be fireing everything in manual. But the ST-E2 is a great device. . . I have gotten very good results with it.</p>