Understanding How to Balance Flash with Ambient

When taking flash pictures, I've always heard that you control the power of the flash with aperature, you control the brightness of the ambient scene with shutter speed. I prefer to think about it differently but am not sure I have my head on straight about it.

If I want my ambient scene two stops darker than my primary subject, can I just meter for the ambient scene, reduce that exposure by two stops, and count on ETTL to do it's thing? Does it matter how I dial in ambient whether it happens to be with a wider aperature and faster shutter or narrower aperature and slower shutter?

Anectodally, it seems like it shouldn't matter; the camera sees the scene two stops dark regardless and the flash fires to illuminate the primary subject correctly, no?

i can't be sure but in ETTL i would think that because you are exposing two stops down your flash would pump out more power to illuminate your subject therefore blowing out your subject...at least that is why I would expect i can't say for sure.

Peacefield wrote in post #10235962
When taking flash pictures, I've always heard that you control the power of the flash with aperature, you control the brightness of the ambient scene with shutter speed. I prefer to think about it differently but am not sure I have my head on straight about it.

If I want my ambient scene two stops darker than my primary subject, can I just meter for the ambient scene, reduce that exposure by two stops, and count on ETTL to do it's thing? Does it matter how I dial in ambient whether it happens to be with a wider aperature and faster shutter or narrower aperature and slower shutter?

Anectodally, it seems like it shouldn't matter; the camera sees the scene two stops dark regardless and the flash fires to illuminate the primary subject correctly, no?

Provided that you are shooting in Manual mode on the camera and ettl on the flash your scenario should work. You can do it in other modes with a combination of Exposure Comp, and FEC but you are limited to how far you can go with the EC to get the balance you want. Manual is easier.

It is better to set everything manually. TTL may see the scene differently than you do and set the power "wrong". Sure it will work most of the time and be "close enough". But its better to nail it every time. There may be times you want to overpower the sun in order to drop the background, and there may be times you simply underexpose the background and compensate with the flash. Different situations call for different techniques.

It is kind of a mantra. Shutter for ambient, aperture for flash, But there is also ...power of the flash. Dont forget that it can be changed as well. If you are looking for a specific aperture do maintain proper DOF you may need to work with the power.

Thanks, everyone. And I should've indicated that I'm setting exposure manually.