iphone 5s slow motion questions

[BabaG]

i have one thing i need to do involving slow motion video so it seems like the iphone might be the best way to do it rather than something like buying a video camera. would be different if i did this all the time.

i need to shoot some slow motion of a flame. when i've shot flames before, i've always 'stopped down' to get more detail from the fire, which tends to overexpose if left to auto-exposure. focus is also a problem. this kind of shot would be done against a black background, making focus difficult.

is there an app which anyone knows of which will allow me to:

1. shoot at120fps/720p
2. manually set and lock my image brightness
3. manually set and lock my focus

i'd be looking to do all three at the same time. while i understand there is a lock feature for focus and exposure in the native camera app, i don't know if they can be set separately or are both locked together. i also might want to do something like have lights on, set focus, then turn the lights off for the flame. don't know if the focus setting might be lost when the lights go out.

thanks for any help and recommendations,
BabaG