Changing overall exposure without changing ratio

[engstrom]

Let's say I have a 2 light (key & fill) setup. I turn off the slave, meter the key, and get an exposure. Then I turn off the key, turn on the slave, meter the slave, bump power up/down as needed and meter again. Eventually I end up with the key 1 stop brighter than the slave. Now I meter both with the meter pointed parallel to the lens for my camera exposure.

But what if I have a specific aperture in mind - maybe I have a specific DOF in mind given the subject or maybe I want to shoot at the sweet spot of my lens. Now I have to figure out how much to bump the key light and start the process over again. Depending on the modifiers used on the lights (and the amount of spill and wrap and position of the lights) the percentage amount that each light will contribute to the overall exposure is going to be different right? So do you eventually just get a gut sense of how much to adjust lights based on their setup so you can get both the ratio and overall exposure you want?

[bondarnes]

Originally Posted by engstrom

So do you eventually just get a gut sense of how much to adjust lights based on their setup so you can get both the ratio and overall exposure you want?

The short answer is yes, but remember the third leg of this triangle is ISO. Let's assume you were at ISO 400 and the reading is f8. But you want to use f5.6. Set the ISO to 200 and you are there. ISO 100 would give you f4, etc.

However, I have found it easier to start with the fill light and knowing that I want to shoot at f5.6 with a one stop difference to the main, I would make the fill read f3.5. Then I would turn on the main and meter until the combined lights read f5.6. Less work in my opinion.

[engstrom]

Thanks Don. And thanks for mentioning ISO - it's a pretty obvious third leg but I probably wouldn't have thought to use it if you hadn't mentioned it. Even though I have an "ancient" digital camera it's still capable of changing ISOs - not like the old days with film. :)