TTL, external strobes, optical triggering - how does it work?

I am hoping for a somewhat generalized answer, but if it matters, my specifics are Sony a7rIV, Inon Z240 Type 3 strobes, fiber optic triggering, and a (yet-to-be-purchased) UW Technics TTL trigger. Generally shooting WA and CFWA - no macro.

I have been shooting for several years now with an Olympus m43 and always in full manual. Now I have a new camera and I was thinking that I should get a TTL trigger, rather than a manual for "just in case" I might want to shoot using TTL control. But, I don't understand how TTL actually works for a setup like I would have.

Can anyone point me to some tutorial/video/explanation of how TTL control works using optical triggering and external strobes?

Questions that come to mind are:

- Does TTL always and inherently use a pre-flash? I ask because I have seen some stuff in that past that seemed to imply that some setups could do TTL without doing a pre-flash. I really don't understand how that would work.

- Does the external strobe do its own metering when you use it in TTL mode? Or is that the strobe CAN do its own metering, OR the camera can do the metering and tell the strobe how to fire?

- if the strobe does its own metering, how does it and the camera coordinate to ensure that the camera's aperture/SS/ISO works with the strobe's flash? (again, when triggered via optical cable)

- if the strobe does its own metering, how does it know what to meter against? Often, it will not be pointed directly at the subject, or even very close.

- I watched a video about Canon speed-lites. They showed one that does TTL and one that is manual. The TTL one has 5 pins in the hot shoe. The manual one has only 1 pin. If 5 pins are needed for the camera to do TTL control of the flash, how can the camera do TTL control of an external strobe that only has one fiber optic cable connection?

Thanks for any help.

I expect I will generally still shoot in Manual. But, I was thinking that some scenarios might work out better if I can shoot in A mode, using TTL. Especially with the camera's ability to set a minimum shutter speed. It seems like I could control depth of field and motion blur that way, and then let the camera sort out the ISO and how much strobe is needed to get a good exposure. I read another thread and some discussion about how it can be somewhat random because of how the camera decides what part of the scene to meter against. But, it sounds like using an FEL control (or FV) might be a way to actually control that. Also, I think my camera has a setting that I set to lock exposure to the focus point. So, I can use AF-C with Tracking and expect it to meter against whatever focus point I have locked onto (which actually seems quite ideal).

So, I am eager to try some TTL shooting and see how it works out, but I would really like to understand how it is supposed to work.

Following. I'd really like to know most of this too.

In general, with optical TTl the camera flash sends out a signal, that sends out a strobe pre-flash. The camera metering system reads that flash and then sends the appropriate length flash to the strobe which fires the same length strobe flash. Some strobes can do TTL via a slave function, a light sensor on the strobe looks at the scene then sets the strobe power (actually pulse length). 

Here is a good tutorial

http://www.inon.jp/technicalguide/sttl-auto.html

The TTL board understands the Sony protocols for making flash strength choices and controls a dumb strobe correctly.  Unlike film era or electrical sync TTL, of course Optical TTL has only a one-way connection, the strobe can listen to the camera but not talk to it.

Bill

With Z-240, you have two options for TTL - S-TTL and External Auto. In the latter mode, the strobe does not use a pre-flash, and instead uses a sensor on its front to measure reflected light from the subject and cut off illumination when it deems the exposure to have been sufficient - kind of like film TTL in the bad old days. See Inon's guide here for full explanation: http://www.inon.jp/technicalguide/externalauto.html

To go over your questions,

39 minutes ago, stuartv said:

- Does TTL always and inherently use a pre-flash? I ask because I have seen some stuff in that past that seemed to imply that some setups could do TTL without doing a pre-flash. I really don't understand how that would work.

S-TTL always uses a pre-flash, External Auto works without pre-flash. Note that Inon's current generation of strobes (Z-330/D-200/S-2000) does not support External Auto anymore; it is only present on Z-240 and D-2000.

41 minutes ago, stuartv said:

- Does the external strobe do its own metering when you use it in TTL mode? Or is that the strobe CAN do its own metering, OR the camera can do the metering and tell the strobe how to fire?

In external auto mode, the strobe does its own metering. In S-TTL mode, camera meters off the strobe's pre-flash then tells it how long to fire on the main flash.

41 minutes ago, stuartv said:

- if the strobe does its own metering, how does it and the camera coordinate to ensure that the camera's aperture/SS/ISO works with the strobe's flash? (again, when triggered via optical cable)

Per Inon's guide, you set the camera to ISO 100 or ISO 200 and a certain aperture, then use the EV knob on the strobe to match the camera's aperture.

43 minutes ago, stuartv said:

- if the strobe does its own metering, how does it know what to meter against? Often, it will not be pointed directly at the subject, or even very close.

It doesn't. That's probably part of the reason they dropped this feature from newer models.

43 minutes ago, stuartv said:

I expect I will generally still shoot in Manual. But, I was thinking that some scenarios might work out better if I can shoot in A mode, using TTL. Especially with the camera's ability to set a minimum shutter speed. It seems like I could control depth of field and motion blur that way, and then let the camera sort out the ISO and how much strobe is needed to get a good exposure.

I shoot a Sony A6300 with two SeaFrogs ST-100 Pro strobes in TTL using camera manual mode, auto ISO and center metering mode. This way I set the aperture for depth of field and the camera dials in ISO to properly expose the background and flash power to properly expose the foreground. Obviously this requires the foreground subject to be in the center, and while this can be worked around using FEL, I've never found it limiting. A7R IV should work in a similar fashion.

Thank you, @bvanant and @Barmaglot!!

Those 2 articles were just what I needed!

With the “lock exposure to focus point” option on my new camera, I am definitely going to try some TTL shooting. If it really works like they claim, that could really help me.

I don't have that feature on my A6300, only manual flash exposure lock which I've never used underwater, but I'm not sure how well it will work. On my A6300, I have three flash metering modes - wide, center and spot, where wide meters the whole frame, spot meters just the center, and center measures the whole frame but emphasizes the center. Of these three, I find center to work the best, as with wide it tends to overexpose the foreground by trying to light the background with flash, and with spot, it underexposes the background, but center produces a good balance. I don't know how exactly the 'lock exposure to focus point' feature in A7R IV works, but I suspect that it's closer to spot (except with flexible location) than to center. It might work well if you manually set the shutter speed, aperture and ISO for proper background exposure and then just let the camera figure out how to light up your focus point, but I might be totally off base here.

The other advantage of a TTL trigger unique to Sony at least on prior models, if the camera thinks an external Sony strobe is attached and will allow a higher flash sync speed (of 1/250) than the 1/160 that manual strobes connected via sync cables or dumb flash triggers are restricted to.  This is unique to Sony and the proper TTL trigger will allow this higher sync speed.

As to how useful TTL is - this will depend on the camera - it makes the decisions on how long to fire the strobe for.  The strobe interprets this signal my mimicking the pulse length.  Sony is reported to be reasonable with this.  Some cameras are fooled by large areas of water in the frame and TTL is least reliable for wide angle shooting

20 hours ago, Barmaglot said:

It might work well if you manually set the shutter speed, aperture and ISO for proper background exposure and then just let the camera figure out how to light up your focus point, but I might be totally off base here.

That is exactly what I was thinking (to try).

7 hours ago, ChrisRoss said:

The other advantage of a TTL trigger unique to Sony at least on prior models, if the camera thinks an external Sony strobe is attached and will allow a higher flash sync speed (of 1/250) than the 1/160 that manual strobes connected via sync cables or dumb flash triggers are restricted to.  This is unique to Sony and the proper TTL trigger will allow this higher sync speed.

As to how useful TTL is - this will depend on the camera - it makes the decisions on how long to fire the strobe for.  The strobe interprets this signal my mimicking the pulse length.  Sony is reported to be reasonable with this.  Some cameras are fooled by large areas of water in the frame and TTL is least reliable for wide angle shooting

I asked the guy at Reef Photo about this. He tested the Nauticam manual flash trigger and the UW Technics for me. He said at 1/200, the manual trigger just started to show a little bit of a band across the top. With the UW Technics, the image was clean at 1/250. I.e. just what you said.

yes that is what should happen - some weirdness from the way Sony configures their flash protocol.

On 12/18/2019 at 1:02 AM, ChrisRoss said:

yes that is what should happen - some weirdness from the way Sony configures their flash protocol.

Obviously the underwater Technics product has got electronics with faster response times if you can get from 1/160 to 1/250 this makes the A6X series much more worth considering

4 hours ago, Interceptor121 said:

Obviously the underwater Technics product has got electronics with faster response times if you can get from 1/160 to 1/250 this makes the A6X series much more worth considering

Unfortunately this is only valid for A7/A9 series cameras. I just double-checked with my A6300 and the UW-Technics converter, and in manual/shutter priority modes, my shutter speed is restricted to 1/160 - speeds above that simply aren't shown. Flash modes available with the converter attached are 'fill flash', 'slow sync', 'rear sync' and 'wireless', with 'flash auto' and 'flash off' greyed out. Oddly, in auto mode, the 'flash off' function is available, along with 'flash auto' and 'fill flash' (but not any of the other modes), and if I set the camera to auto mode, set the flash to off, then switch to other PASM modes, I can choose any shutter speed I want - but the camera won't fire the flash regardless of what mode I pick in the menus.

Ok A6XXX stays as it was lol[emoji23]


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