I'm trying to understand exposure and flash compensation. Specifically, are they separate or associated?
For example, if I use iTTL flash and reduce exposure by 1 stop without touching flash exposure compensation, is just the background exposure adjusted or is the flash (key) exposure also adjusted down by 1 stop? If so, do I always have to reduce exposure and increase flash exposure if I want to maintain proper flash exposure on my subject while under-exposing background?
#1. "RE: Exposure and Flash Compensation" | In response to Reply # 0blw Nikonian since 18th Jun 2004Mon 22-Oct-12 09:59 PM
There are two completely separate metering operations when we do through-the-lens flash: ambient and flash. You can see the ambient one in the viewfinder. (It's often showing severe underexposure.) This is how the background part of the frame will be rendered. Then there is the flash exposure, which is dominated by the primary subject. This is usually determined by the AF focus point, or at least biased that way. No matter what the ambient exposure, the flash exposure is metered to "properly" expose the main subject.
Each of the metering operations has exposure compensation, and they are cumulative. If you dial in -1 stop of exposure compensation (on the camera), that affects BOTH of them. (I suspect you know this, but in the New to Photography forum, likely few others do.) If you then "reverse" the flash exposure by dialing in +1 stop on the flash, the main subject will be exposed at -1 stop + 1 stop = 0 stops off, while the background (ambient) will be -1 stop.
Used properly, this is another way to make a subject "pop" off a background, even if it otherwise might not, for example if the lens is stopped down, or if it is of similar color to the background.
Note that the above applies to Nikon's iTTL (and older dTTL). I'm pretty sure that Canon's eTTL is set up somewhat differently, although the same net effects can be achieved with either system.
Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
#4. "RE: Exposure and Flash Compensation" | In response to Reply # 1sjcampbl Registered since 10th Oct 2012Wed 24-Oct-12 02:05 PM
> -1 stop + 1 stop = 0 stops off
I think that's what I was after. Thanks. I came from Canon Rebel which I believe is different... -1 stop exposure comp +1 stop flash comp = +1 on subject.
Thanks also for the CLS link.