Let's start with simple aperture- and shutter-priority modes, since these are the root behaviors, both historically and technologically. Because these were invented in the film days, back when "ISO adjustment" meant "rewind the film and put a different type in," that left only a single variable in the exposure equation:

• Av mode fixes the aperture in place, requiring the shutter speed to vary to give an appropriate exposure according to the scene metering.
• Tv mode is the opposite: the photographer selects the shutter speed and gives control over the aperture to the camera.

The ranges available flow from physical design limitations in both cases.

Next we come to the early 1980s when programmed exposure modes became popular, owing to the rise of the microcontroller. In these primitive systems, the photographer gave the camera freedom to select both shutter speed and aperture under some criteria, these last being the topic of your question.

The incentive of the camera manufacturers is to deliver good photos, and so they put in limits that seemed sensible to them at the time. As outsiders, we haven't got complete visibility into what these limits are, and even when we do discover them somehow, they vary from one camera to the next, even within a brand.

I point this out because this question is answerable only in general terms when you give vague criteria like "Canon". They've been making cameras so long that there are people now living in retirement homes who weren't yet born at the time they changed their original company name.

Yet, there are general principles that all camera designers must follow if their goal is to deliver high-quality pictures:

1. Camera shake. There has to be a lower limit on shutter speed in order to control the effects of camera shake. For handheld photography, the classic rule of thumb is 1/f in seconds where f is the focal length of the lens in millimeters. That is, a camera with a 60mm lens shouldn't be hand-held with the shutter open more than 1/60s.

   This being a rule of thumb from the days of 35mm film photography, you must adjust for the crop factor on smaller digital sensors. The general form of the rule accounts for the angle subtended by the lens' unsteady axis over the subject during the exposure time, which in turn is a function of the imager size. For a 2× crop, the shutter time has to be half as long to get an equally-sharp image, all else being equal.

   Another way to view this same issue is that a 60mm lens designed for a 35mm camera is "equivalent" to a 120mm designed when put on a 2× crop body. The rule therefore becomes 1/120s maximum shutter open time even though the lens' actual focal length did not change. What did change was the geometry of the rectangular pyramid projected through the lens from the smaller imager.

2. The aperture cannot vary infinitely. If the scene is dark and the camera has opened the lens’ diaphragm as wide as it can go to compensate, P mode gives it no choice but to violate rule #1 to meet the exposure goal. Likewise, with bright scenes, once the lens aperture is stopped down as far as it can go, the camera has to start increasing the shutter speed to avoid overexposure.

3. Designer's choice. Once the hard physical limits are taken care of with the prior rules, there is freedom for the designer to bias the results one way or another. You're right to observe that there are infinitely many possible combinations that all give the same exposure, but the thing you have to realize is that there is no hard-and-fast rule that requires the designer to go one way versus another. It's their choice, and that will likely be guided by expertise and — for lack of a better term — taste.

All of this leaves you with three routes to a concrete answer:

• A huge series of experiments to empirically determine the P mode's criteria for a given camera by running it into all limits and developing a series of response curves.

• Reverse engineering the camera's firmware to extract the actual curves used.

• Getting the manufacturer to reveal these proprietary secrets, somehow.

Until someone does one of the three, general information such as I've given you above is the best you can hope for.

As for A+ mode, all that does is add a single additional degree of freedom to the P mode. The primary limitation on the range will be the noise versus sensitivity curves for the sensor the camera uses.

(Incidentally, I'll use the term "sensor sensitivity" instead of "ISO" going forward since the complicated ISO system for measuring film speed doesn't map directly to digital sensors. It would be more proper to speak in terms of exposure value, but since you didn't ask about that, I will avoid going off on that digression.)

All digital imaging sensors have a sweet spot where decreasing the sensitivity further either stops reducing visible noise or possibly even begins increasing it. You can count on this exposure mode not to go below that limit. Exposure challenges rarely come down to "Alas, I am burdened by an overabundance of light!" Because of that, there is rarely a situation where this practical sensitivity floor is a problem, and you'd wish the camera's fancy exposure mode would lower the sensor sensitivity below its design limits.

The opposite problem is far more often the case: not enough light to meet the meter's exposure guidance under the P mode rules lined out above. When the camera's imaging processor is faced with an exposure violating the 1/f rule even with the aperture wide open, this A+ mode gives it the freedom to raise the sensor's sensitivity to gain a faster exposure, at a cost of increased noise.

Once again we have a situation where the designer has the freedom to let "taste" rule. The camera will not wait to take this option as a final resort, when the needs of a proper exposure demands that it open the aperture as wide as it will go and run the shutter right at the 1/f rule's limit. Instead, it will ease into the condition under some set of heuristics set by the designer. If one possible solution to the exposure problem is "ISO" 200 at ƒ/1.4 and a shutter of exactly 1/f, it might well decide that opening the aperture to its max and running the shutter as slow as it can tolerate for the lens' focal length is silly. Instead, it could easily decide that the noise from ISO 800 isn't all that bad, giving it +2 Ev to distribute between the shutter and the aperture. It might therefore decide on 1/2f for the former and ƒ/2 for the latter.

(Or, yes, any of an infinite number of other possible solutions!)

I cannot speak to your R7's Fv mode from experience, but I do believe the above discussion will lead you toward correct conclusions of how it must operate. After all, a camera that is always surprising its operator with its design choices will be unhelpful to professionals who must develop a mental model of what the camera is "thinking" in order to pursue their craft successfully. Such products become unpopular, thus market failures. A company does not become as long-lived as Canon by pursuing such design philosophies.