I have currently a Nikon D850 along with a SB-700 flash, whose main
purpose is to shoot cosplays in events. I'm happy that using it on the
hotshoe would get better photos than without it, but still I'm not
satisfied. So I'd like to experiment to having it off-camera.
Now, I've read [various] ways to trigger the flash off-camera, and I
still am at a loss about which one should I use. I want to preserve
the HSS and, if possible, the TTL capabilities of the SB-700.
Then, the three options are:
- i-TTL cable
- Nikon's optical CLS/AWL system
- i-TTL radio triggers
Methods like S1/S2 "dumb" optical slaving, PC/3.5mm/2.5mm sync cables, or manual radio triggers do not do TTL and HSS.
A TTL cable is basically an extension cable for the hotshoe/flash foot connection. All four of the signals on the Nikon hotshoe are communicated to the flash. But the majority of these cables are created for flash bracket use, and may be limited in length. They're a good solution if you want to hold the flash in your left hand, or on a monopod, but not necessarily for a studio-style setup.
Nikon's CLS/AWL system is built into their speedlights, and into the pop-up flash of many of their prosumer camera bodies, which often makes it convenient. But the D850 does not have a pop-up flash, so as you figured out, you'd need another SB-700 or SU-800 or other CLS optical commander unit on the hotshoe to control an off-camera speedlight. And optical has the disadvantages of requiring line of sight and having reduced reliability/range in brighter ambient situations (e.g., outside in bright daylight). Line of sight means you can't hide your light behind solid objects—the sensor on the side of the SB-700 has to "see" the master signal on your camera's hotshoe. And placing lights behind you can be problematic unless you can swivel the unit on the hotshoe. Which is why we like radio.
Radio triggering is usually the most robust and reliable system for off-camera flash triggering, so that's why it's preferred, and why an SB-5000 costs a bomb. 3rd party solutions (e.g., Yongnuo YN-622N, Godox X1R-N) though can be quite a bit cheaper.
Which solution you want to use is up to you and how you envision using the flash. When I'm roaming the halls of Comic-Con, I used to use an 8" softbox and a TTL cable, but I now use the 8" softbox with radio triggers, simply because it's more convenient now that my Godox TT685 eliminates carrying a separate radio receiver (and batteries for the receiver), and works in TTL/HSS with both my Panasonic GX7 and Fuji X100T. I will carry a TTL cable as backup, though, in case there's radio interference.