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I am considering buying an off-brand Yongnuo flash for simple portraits in low-light conditions, but I am a bit confused by the terminology. As far as I understand, I can either use a hot shoe to use the flash directly on my camera, use a PC cord to link it to the camera's port or use a wireless trigger.
But is it also possible to trigger the remote flash using the built-in flash on my camera? Or is this only reserved to TTL-compatible flashes?
Most Yongnuo flash units have a built in slave mode that will trigger the flash to fire when it sees any bright flash of light. This includes both TTL and non-TTL Yongnuo flash units. (the YN685 does not have any optical trigger but uses the YN622 radio system instead)
The ETTL "pre-flash" used in the built-in flash of most cameras can cause a regular optical slave to fire too early. Yongnuo flash units have a second slave mode to deal with this:
S1 works with any normal manual flash as a master. (no ETTL preflash)
S2 ignores the preflash and will synce with your camera's flash properly.
From the YN560 manual
In general, some flashes have a non-TTL optical trigger mode.
Metz calls this function Servo Mode (implemented in e.g. the 52 AF-1 digital) - the flash is only triggered, but not controlled by a (positive) change in light intensity.
So if you set it to 1/2 and fire any other flash (or point a torch at the sensor), it will fire with 1/2. Set your internal flash to wireless transmission and 1/4 for all groups, and it will still fire with 1/2. Set the external flash to 1/8 and fire your internal flash normally (TTL or manual; it does not matter) and your external flash will get triggered and fire at 1/8.
I bet that other manufacturers have implementations of the function, too - but maybe they won't feature it as prominently or call it something else. And I bet that basically, most flashes just use the optical TTL system and switch off the TTL-part.
So, basically:
Personal note: In my experience, the optical triggers in compact flashes are not very reliable: outdoors, they tend to get triggered at random by the sun (or not at all by anything), and even indoors, many of them are quite tricky to set up. If I were to build a flash kit from the start now, I'd go with radio controlled wireless triggers.
I am probably repeating the basic idea that it's being already answered by @Mark Sowsun. Yes. If the flash has an optical slave mode, yes.
But I want to answer this question more deeply.
It also possible to trigger the remote flash using the built-in flash on my camera?
Depends. @flolilolilo commented on this.
The optical slave works when it sees a considerable burst of light. But potentially will not fire if:
The flash is receiving light from another source; a window, a lamp.
It has no direct path from your camera's flash; if it is off the flash's path; if something is blocking the path.
It is too far.
The sensor of the flash is not pointing to the camera.
Basically, you are restricted in a ton of ways.
Even if you manage to trigger the external flash using the built-in flash is a really bad idea for some reasons.
As explained before, the flash should receive a good amount of light from this triggering flash. And this built-in flash is then already illuminating your subject with that flat light. That is not the point of using an external flash. Any potential artistic setup is then ruined.
The RC603 remote controllers are really cheap, and the freedom they give you is really worth it. If the flash has a built-in receiver you only need one RC603. A remote trigger is one of the best inversion you can make.
But is it also possible to trigger the remote flash using the built-in flash on my camera?
Yes.
Or is this only reserved to TTL-compatible flashes?
No, it's not. There are two manual-only single-pin Yongnuo models that can be triggered as a CLS or wireless eTTL slave with TTL/HSS, etc.: the YN-560EX, and YN-510EX.
All the flashes in the Yongnuo lineup that can be "smart" optical slaves have EX in their names. But most will be the TTL models, because the difference between "smart" and "dumb" optical slaving systems is the ability to communicate TTL (and HSS, and settings changes, and...)
You can, of course, also use the S1/S2 modes on most of the Yongnuo flashes that aren't EX-designated, like the YN-560III/IV, etc. (pretty much everything except the YN-685). But this is a manual-only triggering system, and you cannot have TTL, HSS, or settings control, and you'll have to set the power level on the flash itself. But it will respond to any flash burst: even that of the built-in flash on a P&S compact camera.
The S1 mode fires on the the first burst it sees, the S2 mode fires on the second burst it sees (to skip firing early on a TTL metering pre-burst).
