Quote Originally Posted by Greg Davis View Post
No. It would be 1 sec at f/1.8, if you change to f/9 you will underexpose your film. To answer your question to me: multiply the factors together first, then divide the film speed by that number. For example, if you have two filters, one with a factor of 2, and the other with a factor of 1.6 then:

2 x 1.6 = 3.2

ISO 400 / 3.2 = ISO 125

Since a factor of 2 is one stop, and 1.6 is 2/3 of a stop, this works out to adjusting your exposure 1 2/3 stop total.
So then yes, I had it right. A said 1 sec at f1.8.

I described this, perhaps clumsily, as "effectively" 1 sec at f9. f9 being simply a way of describing the effect of the amount of illumination the factors reduced. For 1 second, the amount of light reaching the film plane at f4 with the extension and filter in place is roughly equivalent to the amount of light reaching it at f9 without the extension and filter...if I understand things correctly.