E-TTL should compensate for the difference in flash power to achieve the ratio you are looking for. But it may not be that consistent if your subject is moving or you're changing angles frequently.
Your wording will confuse TS more. TTL flash systems are dumb. What it does most of the time is calculate the "correct" exposure through it's TTL ( through the lens) sensors in the viewfinder
with information like focal length ( angle of coverage) and focusing distance of subject and also a pre-flash to take an ambient reading of the scene and flash mode selected to control flash output to and calculate to give correct exposure based on assumptions programed in flash exposure software.
It does not decide what lighting ratios you want...it cant read your mind. You will have to do it
to get the "look or lighting" you want.
A-TTL autoflash
The ‘A’ stands for ‘advanced’. Here, a short, low-power pre-flash is emitted from the Speedlite as the shutter button is pressed, but before the shutter opens. The pre-flash is reflected from the subject, passes through the camera lens and is reflected up into the camera viewfinder where the brightness of the light is measured by another sensor. This lets the camera determine the aperture needed for the flash exposure. At the same time, the camera takes a normal light reading to determine the aperture needed for the ambient light exposure. The smaller of these two apertures is then set and the main flash fires. The actual flash output is still controlled during the exposure by a sensor in the base of the camera measuring the light reflected from the film. Again, the duration of the flash is controlled by ‘off-the-film’ metering during the actual exposure. The advantage of A-TTL over TTL autoflash is that correct exposure is given up to the maximum range of the Speedlite regardless of the lighting conditions.
E-TTL autoflash
‘E’ is for ‘evaluative’. The flash system shares the light sensors in the viewfinder, which are also used for evaluative metering of the ambient light. As the shutter button is pressed, an ambient light reading is taken. A low power pre-flash is then emitted by the Speedlite. The sensors measure the pre-flash illumination reflected by the subject, but can’t help seeing the ambient light at the same time. So the camera deducts the first ambient light reading from this second reading to get a flash-only reading. This flash reading allows the camera to determine the intensity of the main flash needed for correct exposure. Finally, the shutter opens and the main flash fires at this pre-determined intensity. Unlike TTL and A-TTL autoflash metering, the E-TTL system does not monitor the flash output during the actual exposure.
E-TTL II autoflash
E-TTL autoflash works on the assumption that the subject will be covered by an autofocus (AF) point. However, an AF point does not always cover the area of the subject where you want the flash exposure to be correct. The E-TTL II autoflash system overcomes this problem. When you press the shutter button, the ambient light is metered by each sensor. Then a pre-flash fires and this is also metered by all the sensors. The ambient and flash readings for each sensor are then compared. The area of the scene that shows a significant difference between the readings is the area where the subject is most likely to be. This is because the main subject is likely to be closer to the camera than the rest of the scene and will reflect more of the flash. However, if the difference between the two readings is significantly high, the camera will ignore this area on the basis that the flash is being reflected back to the camera by a very shiny surface, such as a mirror. The pre-flash meter readings from accepted areas is weighted and averaged. It is then compared with the ambient light reading before the main flash output is calculated and stored in memory for the exposure. If the lens is able to provide distance information, this is used to determine the closeness of the subject and any highly reflective areas relative to the background. This information is used to refine the flash exposure. The result is better flash exposure for difficult subjects, such as white wedding dresses.
From original article:
http://cpn.canon-europe.com/content/education/infobank/flash/flash_basics.do
How ETTL II technology works to your advantage
The ability of your camera to intelligently evaluate the photo you are taking and apply flash lighting to it is the result of many years of technology advances and developments. Studio photography with multiple external strobes and lights set and controlled by the photographer, and essentially invisible/unrecognized by the camera, must be metered manually. An on-camera flash, however, has a number of evaluative measurements that the camera and flash manage and that adjust the flash’s intensity and area of coverage (the flash adjusts its area of coverage to match the focal length of the camera’s lens). Canon’s current and most advanced technology today is called ETTL II; the acronym stands for evaluative through-the-lens, meaning the camera is reading the subject through the lens for the most accurate evaluation. Canon’s higher-end flashes, the 580EX II and the 430EX, both use ETTL II technology, and are backwards-compatible if you’ve mounted them on an older camera that doesn’t support ETTL II.
As part of the flash process, ETTL II uses a virtually invisible preflash and emits a very short flash before the actual image-taking flash takes place. This preflash evaluates and gives information to the camera and flash for a proper exposure based on the reading. And, as implied by the ETTL definition, the reading taken during the preflash is done through the lens and read by a sensor within the camera. ETTL is smart enough that if the ambient light is bright enough, it adjusts the flash for fill-flash settings, powered-down from what it would shoot in a darker situation. ETTL II, the second-generation of Canon’s ETTL technology, adds superior metering capabilities and integrated distance data (its ability to meter distance from the camera to the subject) that is communicated between compatible/supporting EF lenses.
There are situations where ETTL II adjusts to accommodate the fact that the flash is in a different position, where if kept the same, ETTL II would produce an inaccurate reading for the image being taken (see 8-3). For example, macro photography often requires that you get very close to the subject where traditional flash metering is difficult to accomplish; however, ETTL II technology can adjust the reading accordingly. Another situation is when you’re performing a bounce flash (aiming your flash against a ceiling or wall instead of your subject), since the flash is (presumably) pointed away from your subject. Finally, and similar to the bounce-flash, ETTL II doesn’t work fully automatically when you’re using a remote/wireless flash because the flash is not attached to the camera and isn’t going to have a relative focal distance to that of the camera’s lens.
Note
What’s the difference between ETTL and ETTL II? While both modes provide a sophisticated evaluative measurement of your subject so that your image exposes optimally, ETTL II features an improved ability for candid, on-the-fly shooting where you’re less likely to have to test your shot or use the Flash Exposure Lock (FEL) setting (see later in this chapter for more on FEL). Generally speaking, it’s better at evaluating the scene and setting a more reliable flash.
From original article:
http://canon-digital-photo-pro.com/flashes/how-ettl-ii-technology-works-to-your-advantage/
