Suppose you have a super camera that had no ISO limit - it could give you a noiseless image even at 1 billion ISO. With such a camera, would you be able to use bounce flash anywhere?
Not necessarily. When using bounce flash, the aperture and ISO combination must indeed allow enough light from the flash to reach the sensor, and of course unlimited ISO would help. However, the other limit is that the flash must be at least as bright as 1/8 of the ambient exposure (i.e. 3 stops below ambient). Otherwise, the flash exposure would not be noticeable.
In the hypothetical example of our super camera, even having infinite ISO does not mean you can bounce anywhere you want. If for example it's mid-afternoon on a bright sunny-16 day, you cannot bounce a flash from a wall that's 20 feet away no matter what ISO (or aperture) you choose because the flash would then be negligible compared to the ambient light.
What's the relevance of this to everyday shooting? What this means is that when we're bouncing while there is ample ambient light, we should start by selecting the sync speed to minimize the ratio of ambient light relative to the flash. We then choose a reasonably "bright" ISO and aperture combination to allow the bounce flash to show up in the image, *so long as the shutter doesn't go above sync speed*. If we choose an aperture so wide and/or an ISO so high that shutter is higher than sync speed, then our flash will be forced to use high speed sync, which causes the loss of around 2 stops of power. This is why we shouldn't automatically choose a wide aperture and high ISO when bouncing (unless ambient is dim).
What if ambient is so bright that even at sync speed, the aperture is narrow and/or ISO is low? That means it's quite possibly beyond the power of our flash to bounce under the circumstances. Increasing ISO or widening the aperture under such circumstances does not help the image - it only leads to overexposure of ambient. The aperture and ISO combo at sync speed for ambient (with at least slight underexposure) is a non-negotiable limit. If my flash can't bounce at those settings, then no amount of adjusting the aperture and ISO can fix the problem.
That's the mistake I made with the shot below. I selected ISO 800 as a typical minimum ISO for bouncing, and selected a wide aperture (f/2.8). Although I selected sync speed, the point I missed at that time, is that having a high ISO and wide aperture were not helping in those circumstances.
When I saw the overexposure on the LCD screen, I switched to aperture priority as a quick way of getting a ballpark exposure (I knew the kids wouldn't stay put). What I didn't notice was that because I was using f/2.8 and ISO 200, the camera was giving me a shutter speed of 1/2000, well into high speed sync territory. With high speed sync plus bounce flash, I couldn't hope to get enough light on the kids:
The best I could do was to try to salvage the image in post, which is never as good as getting it right in the first place.
Post-processing is not a panacea. For example, this one was had unrecoverable details from completely blown highlights (even though I was using raw).
I should have just realized I was asking too much from the flash and either used an umbrella instead or doubled up with the YN-560 plus SB-800 (which would let me gain only 1 stop more light). Fortunately some shots did turn out OK:
For future reference, I tried to test what was the "darkest" aperture and ISO combo setting I could use with an 8 foot white ceiling, sb800 on full zoom 105mm aimed straight up, and at full power. I got around f/14 @ ISO 200 (i.e., equivalent to f/10 @ ISO 100) with the flash bright enough to act as key light, with the subject around 8 feet away.