# Guessig camera exposure before the shoot

Ok here is a funny one.

Prod wants to use a projector as a light source, and we are not entirely sure about what output power we need.

got the math down to calculate the surface illumination and using the EV formular I kinda think i have something that makes sense but i am not sure

So lets say surface is 250cd/m2 and I am using a alexa at iso800 T2 and 1/50s shutter .

Plopping that into EV formular I get a Ev100 of ~7.643
using the formular to calculate the needed evs for a “proper exposure” if a surface at 250NIT I end up at ~ EV100 of 10.9

EV= log2(F-number^2/time) so log2(4/0.02) = 7.643

EV= log2(NIT*ISO/12.5) (12.5 is a constant that depends on the light meter 12.5 seems to be the most common one) = 10.9

Now we raise the iso from 100 to 800 on the camera so I sustract 3 EV to the camera getting me ~5.64 EV800 ? Wikipedia suggest that you add +1 EV to every iso step but that doesnt add up to me , number should be going down with more light captured, as when I change the shutter speed from 1/50 (0.02s) to (0.08s) which is equivalent from raising iso from 100 to 800 i get a lower EV number so thats what I am going with…

Now the part I an not sure is why does the EV for the surface include ISO but … eh ok I got something mixed up I am sure

Comparing the ev800 5.64 and the ev100 of 10.9 I would assume I am at a difference of 5.3EV

So in my case I would need to stop down the camera by 5.3 Stops

quickly trying this out with a DSLR and a PQ HDR monitor set to 250NIT this seem to add up, at iso100/F2 and 1/200 It is in fact pretty well exposed. (totall ballpark but it seems to make sense)

Now how can I calculate the resulting scene linear float value from it? I would kinda just assume i would get 0.18 from a “perfectly” exposed surface is that what the light meter formular aims for? Mid-grey? if so at iso800 it would be 5.3 stops over so 0.18*2^5.3 =7 scene linear float ?

When I tried it with my DSLR set to NLOG I get a scene linear value of 20, which is ~ 2 Stops off my estimation which is kinda what I expect , maybe there is some kind of offset wirh the 0.18 I am not anticipating?

Does that make any sense?

(I dont really need all this practically its just a mind bender I like to get down)

I was down this path before trying to see what EV cameras in renderers have, and then figured that for example redshift has a slider for it)