Urgent bounce flash question 760D owners

Thank you for posting these shots, it's actually very helpful. You don't mention if you are using a modifier (soft box, sto-fen, etc) on the flash, and if you are in HSS mode or not. I also would've loved to have seen Evaluative metering on the E-TTL as opposed to Average... but from some other shots on this site posted years ago, I'm convinced that if you had used FEL before the gray card shot, it would've turned out fine.

I would also say that, in the prime lens shots, you are at a much wider aperture, and therefore the ambient light is at a very different value than in your 300mm shots. For those you also have the framing the same on the 6D and 70D, so the 70D was significantly further away from the subject. Light power falls off rapidly with distance. This may somewhat explain the difference between those two cameras in this test.

Regardless, I don't think this has anything to do with the lenses, per se. More the aperture at the time of exposure. And in the case of bounce, distance information is not used anyway.

I see why you are expecting a different result, and only a Canon engineer can explain exactly what the E-TTL algorithm is doing here, but obviously when there is a low ambient light exposure setting on the camera, the flash metering is being fooled by the lack of a subject in your photo. There is no "foreground", and no change in any one area of the frame for the camera to determine the subject, so it may be unrealistic to expect a perfect exposure here - although it seems obvious to you that a gray card is an easy subject to get right, perhaps not for the flash algo under these conditions.

But really, you are just the latest in a very long line of people complaining about how E-TTL works, and there is nothing about your photos to indicate that this is anything unique to the 70D.

You would think Evaluative metering would be smart enough to recognize a snow scene and not turn it all gray - but it doesn't. I still have to add +1 or +2 EC. Is that a "firmware bug"? Is Canon "refusing to acknowledge" the well-known Snow Scene Problem? I'm not being fatuous here, I'm suggesting you just need to adapt, learn when to use FEC and FEL, and when the metering will be fooled like this. It's the same as any other automated functions on the camera. They work great 80% of the time. The other 20% we have to tweak them.