If you can still find a 90EX at the bargain prices they were selling for a while back, it will perform as well as the built-in flash for controlling a 420EX off camera. The 90EX was introduced as a hot shoe mounted flash for the EOS M line, but can be used as an optical controller with any EOS digital camera. It is relatively low powered as external flashes go, but has more power than the built in flashes on Canon's EOS cameras do.
Although the 90EX was intended to be an economical flash option for the original EOS M body that did not have an internal flash, due to the very affordable price many folks were buying them to use as optical controllers on other EOS bodies. For whatever reason, Canon stopped making them and started putting very weak pop-up flashes in subsequent EOS-M models.
You also have the option of using any other flash with the capability of being a Canon E-TTL optical master. That would include such Canon flashes as the discontinued 580EX II or the current 600EX II-RT and 430EX III-RT. The RT flashes can be used as either a radio or optical master flash. There are also a plethora of flashes from third party makers such as Yongnuo, Godox, Neewer, etc. that are capable of being used as master flashes with the Canon optical control system. The key to recognizing most of them is to look for 'E-TTL' and 'Master' somewhere in their description.
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, most systems have moved on to radio control of off camera flashes. The ability to use optical control with products such as Canon's RT flashes is to allow users to use those newer flashes with the older optical system.
If you decide to go to radio wireless, any Canon TTL capable set of transmitter/receiver/transceiver will work. You need to be sure the transmitter and receiver use the same protocol. Different sets that are all compatible with the Canon TTL system use different protocols to communicate with each other. For example, any piece from the Yongnuo YN622 system will work with other YN622 pieces, but not with a Godox unit or even a Yongnuo unit that uses the Canon RT communication protocol. Various third party RT compatible units should work together, but sometimes there are glitches.
All of them have various advantages and disadvantages compared to the others so it usually comes down to what your long term plans are. If you plan on staying with Canon over the long term, a third party transmitter and receiver that uses the 'RT' protocol is probably the way to go. If there's the possibility you may eventually want to mix in other camera systems, or you want to mix speedlights with manual only studio flashes (or manual only speedlights, for that matter), Godox has a system that is somewhat cross brand compatible (on the receiver end - the transmitters must have the pin pattern to match the hot shoe of the camera) and also uses the same protocol for their manual as well as TTL flashes, all the way from the cheapest manual only speedlights to the higher end studio strobes with TTL capability.
