4 comments from me (I know that my first two comments are partially countered by your later dog example):
- It is impossible to judge if there is little or much spill light from the controller flash compared to the slave flash when you don't use the slave for lightning up a subject. I would suggest that you placed the slave in the door opening, pointing directly away from the camera. This way, the controller would light up the wall around the door opening, and the slave would light up the walls in the hallway. Then it would be possible to compare intensities of the two flashes.
- I will assume that the intensity of the controller intensity is fixed. But the intensity of the slave is dependent on light metering. So if you use camera settings where the slave can deliver adequate light at 1/32 intensity, the difference between the controller intensity will be much smaller than if you use camera settings where the slave has to fire at 1/1 intensity. An ISO of 1600 and a large aperture will probably cause the slave to fire at very low intensity.
- Everybody talks very generally about this as if the light spill is equal for all Sony and Minolta cameras. But do we know that? I have a feeling that the light spill from my a77 is larger than it was from my Minolta 7D. Perhaps some models emits a pure control signal while other models are also intended to contribute actively to the scene lightning. Somehow that fits into my general impression of the a77 being more intended for a point and shoot audience than the 7D was.
- It is strange that Sony doesn't sell an IR pass through filter for the controller flashes. As I have understood it, such a filter solves the problem completely since the flash emits plenty of IR light, the sensor on the flash reacts to IR light, and the camera's image sensor is immune to IR light because of a built in filter. Instead, people have to mess with exposed film in front of the controller flash.