My Never Ending Journey to Understand ETTL

[Peacefield]

The situation: Working outside, I want to underexpose the general scene, but throw some strobe at my subject. So I want the background a stop or so dark, but the subject lit correctly.

Obviously, underexposing can be done bu turning dowm my my EC or just dialing in what I want in Manual mode. My question is, if EC is stepped down, does ETTL think I also want my flash power stepped down and will throw out less light on my subject, too? Or does it always assume I want a properly lit subject no matter what I'm doing with EC?

Thanks in advance.

[egordon99]

EC is independent of FEC. In Av mode, EC will alter the shutter speed (which alters the "ambient" exposure), and FEC alters the flash power (which alters the "flash" exposure)

[Peacefield]

Thanks for the quick response. So if EC is -2 but FEC is left at zero, background is underexposed but subject is pretty much on?

[egordon99]

Thanks for the quick response. So if EC is -2 but FEC is left at zero, background is underexposed but subject is pretty much on?

Pretty much. You have to realize that a flash picture is made up of two independent exposures. The ambient exposure is determined by the ISO, f-stop, and shutter speed. The flash exposure is determined by the ISO, f-stop, and flash power.

So EC ONLY affects the ambient exposure, and FEC ONLY affects the flash exposure.

[bieber]

Keep in mind that this may or may not actually work, depending on the situation you're in. At 1/250, you need to stop down pretty far to underexpose the background, so you'll need to be pretty close with the flash if you want a properly exposed subject. Your best bet is probably to go to M mode, set 1/250 and an Aperture/ISO combo that gets you the ambient you want, and then add the flash. Keep an eye on the distance scale on the back of your flash, and it will tell you the range you'll need to be in to properly expose the subject at those settings.