Is there a 3rd party flash which is both capable of being used off the camera using optical transmission and radio transmission?
Short answer: The Yongnuo YN600EX-RT II can be used both optically and via radio as either an on-camera master or as an off-camera slave.
Pretty much any third party flash that is capable of optical wireless communication can also be used for radio wireless communication by attaching an appropriate radio transmitter to the camera's hot shoe and a matching receiver to the flash's hot shoe. This would include flashes such as the Yongnuo YN568EX II or the Nissin Di866 II.
Here's the deal: There are plenty of third party flashes with built-in radio receivers capable of radio communication. The problem is that many of them use different radio protocols. Just because a flash has a radio receiver doesn't mean it will work with all radio transmitters. Far from it.
Before we go any further, let's establish a few definitions:
- Wireless can refer to either optical or radio communication between the camera and flash.
- Optical wireless communications uses pulses of light (either visible or near-infrared) for the master unit or transmitter on the camera to communicate with off-camera slaves.
- Radio wireless communications use encoded radio signals, usually in the 2.4GHz band, for the master unit or radio trigger on the camera to communicate with off-camera flashes.
- Protocol is the 'language' that each wireless system (both optical and radio) uses to communicate. Even when two different radio systems use the same radio frequency, they often use a different protocol to transmit the information over that radio frequency.
Though most radio systems use the 2.4GHz band, they use different protocols for the signals transmitted within that band.
Canon has the RT system. Godox has their own X1 system (as well as a different older system). Yongnuo has the manual YN560/RF605/RF603 system, the E-TTL YN622 system, as well as some products that are compatible with the Canon RT system. Most Yongnuo YN622 units made since the end of 2014 can receive (but not transmit) signals from YN560/RF605/RF603 transmitters but the reverse is not true - YN560/RF605/RF603 units can only receive signals from YN560/RF605/RF603 transmitters. There are a few others as well.
As long as you stay within the same radio system, though, there are plenty of third party flashes that will work with your EOS 700D. With the 700D, any radio system, including Canon's own RT system, will require a radio transmitter attached to the camera. As of May 2017 no Canon body has a built-in RT radio transmitter. Most recent Canon bodies with a pop-up flash do have limited ability to control remote flashes optically. This would include any Canon flash with an 'EX' in the model name as well as any third party flash with 'Canon wireless (optical) E-TTL' compatibility.
There are disadvantages with using optical wireless communications instead of radio communications, though:
- Distance limitations. Most optical systems, especially when using a weak built-in popup flash as the controller, are much more range limited than radio systems.
- Positioning limitations. Most of the optical controllers only cover an area about as wide as a 24mm lens on a FF camera. If the remote flash is further to the right or left it may not receive any optical signal, even if it is only a few feet from the camera! Radios transmit in all directions from the camera.
- Line-of-sight requirements. In addition to being in the "cone" of light transmitted by the master, off camera flashes must have a clear line-of-sight to the master with the optical receiver on the flash pointed in the direction of the master. This inhibits being able to place optically controlled flashes inside modifiers, placing them behind objects in the scene, etc. Radio systems are not limited to line-of-sight and can even be used on the other side of walls and other obstructions (although the obstructions may reduce the range somewhat).
- Difficulty with bright ambient light. Especially outside under sunlight, the power of optical wireless control is very limited. Again, especially with a relatively weak built-in popup flash, the master just doesn't have much power to cut through the bright sunlight and the receivers can't detect the weak signal from the master over the very bright sunlight. Radios work just as well in bright sunlight as they do in a dark studio.
About the only advantage optical control has over radio control for you is that you don't need an on-camera radio transmitter attached to your camera's hot shoe. (But you'll probably soon discover that you'll need a more powerful on-camera optical master attached to your camera's hot shoe to get the optical system to work the way you want, if it will even work then.)
For these reasons, it is probably better to start by just choosing a radio system and then stick with that system. Optical control of off camera flash is fast becoming old technology that is only included in current products to allow them to work with cameras and other flashes that were made before the use of radio communication became more prevalent to control off camera flashes.
Yongnuo is probably the cheapest way to go. You can choose to use the YN600EX-RT or YN600EX-RT II along with a YN-E3-RT radio transmitter and also have cross-compatibility with most Canon RT flashes. Or you can choose to use the YN622C system that includes several newer flashes with built in YN622C receivers. Note that you can still use other Canon E-TTL flashes, including RT flashes, with the YN622 system but you'll need a YN622 receiver attached to the hot shoe of the RT flash.
For the price of a single Canon 600EX-RT II you can buy two YN600EX-RT II flashes and a YN-E3-RT controller. For about the price of the Canon ST-E3-RT controller you'd need for the 600EX-RT you can buy 2-3 more Yongnuo flashes. (Or if you choose to use a YN-E3-RT controller with a Canon 600EX-RT II the money you save on the controller would almost allow you to buy a couple of YN600EX-RT II units.) The advantage of the RT system is you won't need a receiver on each remote RT flash. If you go the YN622 route, the YN622C-TX controllers and YN-622C II transceivers (they can be either a transmitter or receiver, but don't have the LCD display of the transmit only YN622C-TX) are about $40 each. The advantage of the YN622C system is than any E-TTL compatible flash (Canon or 3rd party) should be controllable via a YN622C receiver attached to its hot shoe. Of course the YN685 has a built in YN622 receiver and doesn't need a separate receiver attached to the hot shoe.
The Godox X1 system is becoming very popular as well. They have integrated a single radio protocol that allows control of their E-TTL flashes, manual only flashes, and small studio flashes. This allows someone who starts out with manual only flashes to grow into E-TTL and studio lights without having to start over with a new radio system. Most of their current models have built in X1 receivers. You'll need an external Godox X1 receiver to use the Godox system with any non-Godox flash, though.