I have two Godox flashes, a V850 and a TT350. Both have built in optical slave triggers with modes S1 and S2.

S1 is a plain dumb slave, it fires when it detects a bright flash of light.

S2 ignores the pre flash and fires with the main flash.

Both operate as described, what I don't understand is, what is S2 actually useful for? The preflash (at least on my camera) is used for metering, so all s2 mode really seems to achieve is to screw up the metering by making the main flash far bigger than the camera expects.

What am I missing?

  • 3
    For what it's worth, it's not just Godox — modes called S1 and S2 with the latter ignoring preflash seems to be almost universal across every flash brand I can think of. – mattdm Nov 9 '19 at 10:15
  • Thanks Matt, I wasn't sure if that was Godox specific or more widely used. – Joseph Rogers Nov 9 '19 at 11:18
  • @mattdm, except for Sigma, where I believe they use SD(igital) for S2 and SF(ilm) for S1. – inkista Nov 11 '19 at 8:41

Many built-in flashes have no mode in which there is no preflash. This means that in S1, your external flash will be triggered then, and may fire too soon to be included in the exposure at all. So S2 is needed to even work.

You're right that the flash won't be factored into the metering in this case. You'll have to factor it in yourself, using manual exposure settings (my preference when working with flash) or using EV compensation.

If it's any consolation, TTL metering systems are often not smart enough to do what you want with external flash anyway (especially if your setup involves lighting different parts of the scene differently), so you probably would need to intervene anyway.

All that said, these days, radio triggers are so cheap and are just plain objectively better, so in most cases you'd want to use that instead of either optical trigger mode.

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What am I missing?

That it's a timing issue.

The S1/S2 modes are "dumb"; there's no communication between the camera and the flash going on. A simple sensor on the flash detects when a flash burst has gone off, and then fires the flash. As you noted, S1 fires on the first burst sensed; S2 on the second burst sensed.

With flash, the camera cannot meter how much light flash adds in the scene until a flash burst is in the scene. That's why TTL uses a pre-burst.

But that TTL metering pre-burst also trips the sensor. If you're using TTL, and your flash is in S1, it trips the flash early: on the preflash, not during the main burst.

...all s2 mode really seems to achieve is to screw up the metering by making the main flash far bigger than the camera expects.

S2 doesn't affect the flash power at all. Only the timing of when the flash fires. S1/S2 are always manual triggering modes where the power level on the flash must be set in M mode, directly on the flash itself.

If you want to use TTL power control over an off-camera flash you must use a "smart" optical or radio triggering system, or 3rd-party TTL radio triggers that can fully communicate with the hotshoe of the flash.

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  • Not all preflashes are used for TTL metering. Red-eye reduction, for example, also uses preflashes. So does low light AF assist with flashes that do not have a separate near IR emitter. Even when flashes are set using manual power, these preflashes need to be ignored by a "dumb slave."
  • Not all "dumb slaves" are used as key ("main flash") or even fill lights. If the "dumb slave" is being used to light a white backdrop, for example, it has a minimal effect on the exposure of a subject in front of the backdrop, which might be lit by TTL flashes.
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The mode where "the preflash is used for metering" is TTL mode.

Triggering a metering preflash on an optical slave is (generally, maybe there are exceptions) not supported (the slave flash would need to know far more about what the camera expects of the preflash, so it might be difficult to implement).

Optical slave setups are, apart from some proprietary systems, NOT intended to be used with TTL mode. You will need to take control of your flash power yourself, by using manual power settings and/or appropriate aperture selection, determining the appropriate settings by experimentation and/or guide number calculation.

For off camera TTL operation, radio control systems are the state of the art solution.

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