All though this topic appeared several times before, I havent been able to find specific enough info on the subject.
I am shooting a night scene in a forest, and plan to light it only with the light coming from two practical flashlilghts carried by the main characters (apart from very low-key HMI light for ambiance). I will be shooting on the alexa, and hope to acheive a T stop of about 2-2.8. How much Lumens do I need these flashlilghts to be? is 500 Lumens enough? do I need much more than that?
I also plan to color the flashlights with Storaro Yellow jel which will take up some light...
Using 'lumens' is some what difficult to deal with when estimate 'how much light falls on a particular area'. The reason for it is that the lumen output is usually in terms of the total light 'beam'. So for a flash light beam of say 45 degrees those 500 lumens are spread across such a cone of light. (not to mention some manufacturers just go from 'fundamental principles' rather than actual measured output of their device...)
For Israel, and the rest of the world, the unit of measure to be concerned about is lux, in the US it would be footcandles. This is 'how much light is falling on a particular point'. And is also what meters are calibrated to for reading out the exposure.
Unless the flashlight has a data sheet that lists lux/fc at various distances, you really have to emperically measure it. So, at night, with a light meter that reads out in lux/fc, and some measured distances, you can determine whether there is enough light for your needs.
As for 'night', no matter what the output of the flashlights are and even with some 'low light fill', there are going to be large areas of 'black', in a 'forest' situation. I have read that the Alexa's 'natural' ISO is 800, and I have read some times people go up to 1280 or even 1600. This of course is better than most Film film, but still if you don't want 'blank' blacks, that perhaps shooting just after sun set, would allow for more illumination of the 'background' if that is desired...