Thank you so much! This was very helpful.

A poor description of a rather useful flash automation feature. TTL is of best use with at least two flashes mounted off camera and with a trigger of some sort in the camera’s hot shoe. The key light flash and, say, the background flash, are assigned separate groups to permit independent adjustment from the camera position-a major feature. The camera’s computer causes the trigger to use either optical or radio signals (one or the other) causing each flash to fire a low power flash but not at the same time. The camera measures each pre-flash and uses the measurements along with shutter, aperture, ISO, distance, type of flash (possibly, lack of published data makes me unsure) and whatever the camera type may consider. Then, the camera commands each flash to fire at the needed power level to give the calculated exposure. It is not necessary to set the camera to seriously underexpose for ambient conditions as you did. TTL can augment and enhance the ambient light conditions quite nicely. If you then decide to brighten the key and dim the background, each can be adjusted independently if they are on different groups. In fact , a group can include two or more flashes. Each can be a slightly different distance but that’s not going to be compensated for, all in the same group will fire at the same power, a more distant flash will contribute light on the subject it is aimed at depending on its distance. I wrote all this in hopes of helping people understand TTL, which I think was your aim.

Dear me, I thought I was King Ramble, but I've being pushed into second here, the video starts at 12:42.

I disagree with Mr. Kilner and say this is a good explanation of the basic design intention of TTL flash photography. Using two flashes goes beyond the scope of this study. Meanwhile, I am an inquisitive student of flash photography, and a seasoned electronics engineer with understanding of basic lighting, inverse square law, yada yada. I own a Sony a6000 and a6400 and a new Godox speedlight ("S" type, bought recently (brand new) for reason of compatibility with Sony alpha cameras) model V860iiiS. While my amateur photography experience is with fully manual cameras and an old "thyristor bounce" Vivitar speedlight, I look forward to modern equipment that throttled the "LIGHT PUMP" with THROUGH THE CAMERA'S LENS monitoring for correct flash exposure. (My Vivitar's feedback method excluded the camera, as the speedlight had its own shorter feedback loop; i.e., feedback element (sensor eye) is in the speedlight body, itself. Moreover, the hot shoe connections are primitive, by today's standards.) So TTL seemed to work OK with Sony a6000+Godox V860iiiS ... until I installed a NEUTRAL DENSITY FILTER on the camera's lens. This, unexpectedly, caused my TTL flash photographs to be UNDEREXPOSED. That was not supposed to happen! ONE EXPLANATION of my illumination failure is that my speedlight had hit its limit, as a light pump. There are also possible design faults in the Godox item, which I suspect is the case. I will repeat my tests, before the end of 2022, with hopes of publishing here. Again, my speedlight is a pip-squeak for outdoor flash photography, which is how I used it in the aforementioned failure. I realize that all flashes have their limitations, as light pumps. So I will repeat my testing in a non-stressed situation, such as that used by Alex Silva in this fine, interesting video. UPDATE: I retested, and indeed, Godox V860iii TTL works with Sony a6000+ ND4 filter (which attenuates light to 25% of normal transmission). My earlier problem was repeatable.. if I pushed the Godox beyond its ability. WITHIN the operable range of the Godox V860iii, TTL worked, to compensate for an ND4 filter installed on the camera's lens. It is imperative to realize the quantitative value of any ND filter, and that artificial light must be multiplied by that same factor (here 4x) in order to keep a target illuminated.

Just a friendly comment meant for PD. Honestly man, your unpreparedness for the video really took me away from the whole experience. There was a few times where you can tell you didnt plan out how you were going to teach a particular point and it shows. A few suggestions for a better viewer experience will hopefully help; 1. Give us faster cuts. Say your point fast and try a few takes if you stutter a little bit. Taking the extra few minutes for better dialogue would help so much 2. Overall planning. Know what you want to talk about and when. Chronological order 3. The demonstration was extremely underwhelming. You could of demonstrated different settings for example 4. and as a nit pick, the background behind you could be so much better Again not to discourage but to build from. I wouldnt type this out if i knew you dont have what it takes but you do. Keep it up man. Cheers