Additive Flash Exposure Compensation — The Benefit
Note: Originally, this article implied that the additive capability applied to more than the Sony A7RII. That is a misleading suggestion and I’ve removed it.
Flash Exposure Compensation (FEC) is one of the control options when shooting TTL with a speedlite or strobe. My Sony A7RII has a camera firmware based Flash Exposure Compensation(FEC-C) setting accessible via a menu option. The Godox¹ X1T I use with my Godox AD600TTL strobes offers Flash Exposure Compensation (FEC-L) adjustment via the Gr button for each group. For me, an additional benefit is that, with the A7RII, the camera FEC and the x system FEC are additive. That means there is a total of 6EV variation available between the minimum and maximum FEC that can be set.
To demonstrate the benefit I took Manny into the backyard, placed him in the shade with a combination of shade and sunlit background, including some sky. A typical background for shooting portraits, groups and other situations where using a flash for fill or main light is desirable.
The first image in the series that follows was shot at the 0EV ambient exposure meter reading with the camera metering set for Evaluative. For most cameras that’s the default and generally delivers a well exposed image. The exposure data for the image shows a shutter speed above x sync at an F2.8 aperture, at ISO 100. That’s as close as the lens/camera combo will get to x sync without an ND filter. (The exposure settings along with the FEC settings and net EC are included in the caption for each image below.)
Going from 1/400 to 1/6400 on the shutter underexposes the ambient scene 4 EV. The subject, with only ambient lighting looks to be a bit under exposed so being able add some light, even at ambient exposure is beneficial. (first image in series) Adding fill flash at 0FEC adds some lighting character but doesn’t significantly increase the main subject exposure. (second image in series.)
Setting up the sliding scale for FEC
The first time I came across the ability to slide an exposure compensation range was with the Pocket Wizard ControlTL system. It was offered in firmware for the Einstein because the total range for power adjustment was greater than the 6 EV range in the ControlTL firmware. When they introduced the AC add-on for the TT1 it offered options for adjusting how the 6EV range was applied as well.
The A7RII and X1T offer plus and minus 3 EV range each. Since they are additive that means a total of 12 EV from -3/-3 to +3/+3. Its highly unlikely that I’ll need that range, but its available. More practically I can use the camera FEC setting to optimize the 6 EV range available with the X1T controller. Here’s how I did it; I doubt that I’ll want to go more than -1EV net FEC so I set the camera to +2 FECC. Thus, when the X1T is set to -3 FECL the strobe is delivering light that’s 1EV below the ambient exposure. That means that when the X1T is set to +3 FECL the strobe is delivering light that is 5EV greater than ambient exposure. Just where I’d want it to be for a maximum differential when going to the under exposed background limit with most camera/lens configurations. The series of images below show the progression through the range.
The last three images in the series were made with the strobe set to manual power. Full power delivers a main subject exposure close to the +5EV TTL exposure. That means the TTL sliding scale method has delivered to maximum potential. The other two are 1:2 and 1:4 power for reference.
As the last three images show, the same thing can be accomplished using the strobe in Manual mode. So, what benefit does TTL offer? Many contend that, because it meters every exposure there is frame to frame exposure variation. When the main subject dominates the frame that’s not a real concern, at least for me. The benefit, as a working pro, is being able to look at a situation, decide I want the background 2EV under exposed and will need the main subject 1 to 1.3 EV over ambient for optimum separation and main subject lighting. Once I’ve made that determination, I can set the FECL to the proper setting (-.7EV) and then put the camera up to my eye, set the ambient exposure (in M mode) so the meter shows -2EV and make my first image. If my analysis was right, the first image is properly exposed and I can keep shooting. Making minor adjustments as I desire during the shoot.
Can it be done with everything in M mode. Sure. This just eliminates a step for me and let’s me pay more attention to the subject and the result.
¹I’ve decided to use Godox and its naming convention for the X system to simplify things. I have X1Ts controller/triggers from Godox and Adorama, I have XPLOR600TTL strobes from Adorama, I have remote heads and V680s speedlites from Cheetah Light. They are all compatible.