Calculating 10 stop exposures

[Ac1dh0u$3.]

Hi there..

Can anyone tell me the best way to work out how long to take a picture with a 10 stop filter attached.

I'm already pretty sure this is wrong but the light meter app on my phones goes up to like F512, so I so I figured if the cameras at F22 then with the 10 stop filter on the end that would be F32? Gives out a time in my dark room of like 2mins, which really cant be right.

So whats the calculation to work out the shutter speed ?

 

[Ac1dh0u$3.]

Thanks for all this information everyone.

I've also found an app on ios which is the same as the one Albaba posted, you take a exposure, put in how long it took without the filter and what filter your going to put on and its tells you how long to do it with the filter. Its called "Longtime"