I recently bought cactus 60rf and apparently there is no af assist beam in it, which when i tried to take a picture in the dark it didn't automatically focus.
So i will return this flash to the store and buy Canon 600ex-rt, and my question is, if this speedlight's assist beam will help me focus in extreme low light situations, i mean if will it work properly with my camera (Canon 1100D).
Please answer me as fast as possible,
Yes, it certainly will work... But it's sort of massive overkill to get the top-of-the-line 600EX-RT primarily for it's AF Assist feature! It's a great flash, but the Focus Assist is just one of many things it can do!
If the main thing you need is Focus Assist, an ST-E2 Flash Controller module will do the same thing, and is much smaller and considerably less expensive. (Note: the ST-E3-RT does not have Focus Assist).
If you want a flash with Focus Assist, you really can pick and choose among many of the Canon flashes (as well as a bunch of third party flashes for Canon). Among the Canon flashes, Focus Assist is:
- Near Infrared type (less intrusive), 15 to 30 ft. range on: 600EX-RT, 430EX III-RT, 430EX II, 270EX II & ST-E2
- White light flashes (rapid bursts, more intrusive: 15 to 25 ft. range: 430EX III-RT, 270EX, 90EX.
- White light, continuous LED (serves as video light too). Range 10 to 13 ft.: 320EX .
- N/A: ST-E3-RT.
For other purposes... The RT flashes and ST controller use radio to communicate with each other for multi flash setups. The 600EX-RT and ST-E3-RT can act as masters, while the new 430EX III-RT is slave only. Using radio gives them more range and greater flexibilty of placement.
The 600EX-RT (optionally) and some of the other non-RT flashes and controller use optical (near infrared light) to communicate with each other. The 600EX-RT (and the now discontinued 580EX II, 580EX, 550EX models) and the ST-E2 controller can act as masters. The 430EX II (and discontinued 430EX, 420EX) and 270EX II can be used as slaves only. The optical triggering and control requires line-of-sight setup and arrangements, and has a bit less range than the radio control.
Some of the camera's with built-in flash have optical means of controlling off-camera flashes. I don't think the 1100D is one of them, though.