cgardner
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You just need to know the Sunny 16 / Shady 5.6 rule of thumb and limitations of your gear.
The biggest gear limitation is the sensor range of the camera. Most outdoor scenes exceed it. Sometimes that can be remedied by, flash other times flash is not an option and exposure compromises must be made.
The Sunny 16 / Shady 5.6 rule of thumb is based on the sun and shade being constant on clear day; shady side 3 stops below the sunny side. Adjusted for the 1/250th sync of the camera you need to shoot at f/11@ 1/250th @ ISO 100 when what is most important in the scene is in the direct sunlight (sun at your back), and f/4 @ 1/250 @ ISO 100 when your most important content (like faces) are in the shadows (sun in your face). Exposure in-between f/11 and f/4 (i.e. f/10... f/4.5) will blow the highlights of light colored objects to some degree. While less than ideal that can be made non-distracting in shoots by cropping.
In a situation like the backlit hanger what I'd do without flash, or under performing flash, would be to take wide establishing shots outdoors with the sun at my back exposing for the sunlit highlights knowing that most of the scene range would be recorded faithfully. Indoors in the hanger a wide shot exposed for the outdoors will also work to establish the scene related to the outdoor shots. Then for interior shots of the people I'd expose for the open shady side, using shallower DOF and cropping tight enough that the blown highlights in the background didn't become a distraction.
Flash exposure with ETTL-II bodies is based on a baseline ambient reading separate ambient reading taken after full press of the shutter. So if the ambient highlights are blown by the shutter/aperture selection it will affect flash in ETTL mode to some degree. This is the default sequence of the metering in Av / ETTL:
Half press of shutter: Ambient Exposure Lock / Focus Lock
Full press of shutter:
1) Baseline ambient lighting map created for flash calculation
2) Pre-flash fired to create combined ambient + flash map
3) Metering logic subtracts baseline ambient readings from the ambient+pre-flash
What I've discovered with testing is that when the ambient exposure set with shutter and aperture — manually or via EC adjustment in Av mode — ETTL-II will do a very good job of exposing backlight subjects within the limits of the range of the flash at FEC = 0. In the test below shot in Av mode it took - 2 EC to get the sunny parts below clipping on the target...
... but when flash was turned on FEC =0 produced this result
So by default in Av mode EC=0 will bias exposure to made the shaded foreground placed in the middle of the frame more normal looking with detail, but doing so will blow the highlights on the edges. It can do this because it evaluates discrete zones on the viewfinder in evaluative mode, not the average. Overriding the Av default with - 2 EC underexposes the background, but that's what is needed for successful results with flash. You simply need to know the range of your flash and stay within it.
The simplest way to determine max range on a flash in any situation is to fire a ranging shot in M flash mode at 1/1 power. For example after taking the wide ambient only establishing shot inside the hanger without flash what I'd do is expose for the outside, put my flash in M mode at 1/1 and fire off direct and bounced test shots shot. Those tests then tell me how close I need to be for the flash to be effective.
Due to the way flash and ambient are metered Av and ETTL exposures can vary a great deal in a situation like that and its better to use all manual control rather than chase and correct the camera's guesses. That's where the habit of shooting the establishing shots is valuable. They will not always be at the "rule of thumb" f/11 sunny and f/4 shady values, but in shooting the establishing shots you will know what they are. Knowing that, use M mode on the camera and switch between them as needed for detail in the sunny background or detail in the shaded foreground when shooting with the flash.
If for example you could shoot from say 10ft in the range of your flash you could set aperture from the f/11 sunny baseline for full range of detail in the background, f/8 to blow the background by a stop, f/5.6 to blow it by two stops, etc. The more you are able to open the aperture and blow the background highlights the greater the effective range of the flash will be. By the time you get to f/4 (or whatever your shady baseline is) the background will be totally blown out and you won't need any flash at all to expose the shaded faces correctly.
Flash isn't always fill...
Any flash added to an outdoor scene is generically considered "fill flash" but it's actual cause and effect depends on its angle relative to the lens axis. Flash close to the lens which illuminates everything the camera sees adds EVENLY to whatever ambient fill there is. In the hanger situation if the camera is set at f/11 to correctly expose the outside there is ambient fill inside, just not enough of it for the lame camera sensor to record detail in the shadows of the ambient.
If you raise your flash on a bracket or move it off axis on a stand with a radio trigger it's not "fill" anymore in the role of lifting everything the camera sees. Moving a flash off axis changes it's role to "frontal key light" and changes role of the sun to "hair / accent" light. The fill? That still comes from whatever ambient light is bouncing around the space hitting the parts of the scene the camera sees.
Why is this distinction in terminology important? If you think of the flash in a backlit scenario as "key" light rather than "fill" it is easier to conceptualize how to position it and set exposure for it. Natural lighting comes from overhead most of the time and the reason many flash shots look fake is the angle of the flash is too low. Raising it on a bracket or a stand over the heads of subject or above objects so it hits at about a 45° downward angle produces more natural looking results. The flash overlaps the ambient "fill" to create modeling highlights. That being the case you need to set the flash exposure based on the highlights on the front side, not the shadow tone.
In a sunny backlit + flash situation I ideally want the sunlit parts below clipping on skin and white clothing and start by setting shutter and aperture to produce that result with white at 250 eyedropper reading about 1/3 stop below clipping in the playback (one click). As mentioned if shooting in Av mode it usually requires about - 2 EC. Then adding flash from my bracket I will raise flash until I see clipping on the white clothing in front, then reduce the flash by 2/3 stops (two clicks). That simple two step procedure puts the sunny highlights 1/3 stop below clipping and the shaded frontal white highlights 1/3 stop darker than the sunny highlights which still retain detail...
The net result is adding flash in front at the same downward angle the skylight is already modeling the face, blending the two seamlessly with the skylight providing the fill for the frontal shadows and avoiding a blown out background.
So it is possible to get perceptually correct exposure in foreground and background with natural looking results, you just need to know how to use the ambient light to best advantage, the range limits of your flash gear, and how to position it most effectively for natural modeling on the things in front it illuminates. The simplest way to get more natural modeling with flash is raise it on a flash bracket. Bouncing flash is also a way to get a more natural downward angle, but it's less efficient and not possible outdoors, or practical indoors with high /dark ceilings.
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