Can someone explain to me why TTL exposure, such as Olympus RC, is better than slave sTTL?

I recently stitched from an Inon (sTTL metering) to a Backscatter strobe (Olympus RC metering). While I normally shoot manual I have noticed RC works better than sTTL for the wide-angle work that I do. I would now like to understand:  why?

Unless I am missing something the process is functionally similar:

sTTL:

1. Camera is not aware of external strobe. 

2. Camera flash fires a single weak preflash. Strobe attempts to mimic the duration/strength of the pre flash. 

3. Camera calculates exposure/required flash duration assuming the more powerful strobes pre-flash came from its own flash. I assume the camera is measuring both pre flash and ambient light, and allowing for metering area (matrix, spot, etc). 

4. Camera fires flash for main exposure. Strobe attempts to mimic duration/strength of camera flash.

RC/TTL:

1. Camera is aware of external strobe. 

2. Strobe fires a weak pre-flash, or more likely, multiple pre-flashes.

3. Camera calculates exposure based on the strobes pre-flash. This likely includes pre-flash and ambient light, and allows for metering area. 

4. Camera fires strobe for actual exposure.

My understanding is that a mirrorless camera will not stop the flash or strobe when correct exposure is reached on the sensor in step 4 - the flash duration is set during step 3. Correct me if I'm wrong.

During my reading on this I see a lot of vague comments such as ‘in TTL the camera talks to the strobe’, 'in TTL the camera is aware of the strobe and is properly metering the scene' or ‘sTTL is only based off pre flash strength’ but no actual concrete answer. So - what am I missing? Why is RC better, or alternatively, do you find both are similar?

The only partial answer I’ve seen is that in sTTL the camera is firing blind: it is firing its own flash but seeing the light from the more powerful strobe and assumes it's from its own flash, while the sTTL strobe does not know what camera, flash or lens is being used, and is following a generic ratio/protocol in deciding what output should match the weaker camera flash. This can lead to underexposure in the final image due to the pre flash being too bright. I also assume it may lead to overexposure when the preflash was too weak/the subject too distant, but this would apply to both sTTL and RC. 

Hopefully someone can come along and explain the key differences between sTTL and RC to me (particularly in wide angle with a bright or dark background) as if I’m 5 years old.  :-) 

Cheers

Rohan

Edited 1 hour ago1 hr by Tassie Rohan

The RC protocol sends optical signals to the strobe to tell it how long to fire for so the camera can actually communicate to the strobe during the exposure. The exposure is set purely on the pre-flash image in both protocols. In theory the sTTL should do the same job as it is trying to mimic the durations of the pre-flash and the calculated main flash length, but in practice it seems it doesn't necessarily work that way.

Another point is that RC mode tells the strobe to emit a pre-flash, which the camera reads. The pre-flash power is known by the strobe and all the camera needs to do is to send a multiplier for the pre-flash duration to get the exposure right, so if for example if the pre-flash power is 1/32 and you needs 4 stops more power in the main exposure the strobe needs to fire at 1/32 x 2 x2 x2 x2 = 1/2 power . In standard TTL there are assumptions about how powerful the pre-flash is as a percentage of the strobes full power as the camera has no idea about how much power the UW strobe actually has..

There are also issues with measuring the power of the pre-flash in the UW strobe as the duration may not be in the optimum range for triggering the UW strobe and it may not match the duration of the trigger signal from the camera. An RC UW strobe though knows exactly what power the pre-flash was it can then can do the calculations based on the communication signals from the camera to work out how long (how much power) the strobe needs to fire for in the main exposure.