In the first shot, the flash TTL prefire was able to hit the wall behind the women, thus allowing it to give a partial meter such as 1/2 or 1/4 or even less because the TTL system was satisfied with less light, and it therefore appears that there is a good mix between ambient/flash. This shot may have been slightly underexposed (with regards to subject).

The second shot has a lot more depth to the background, thus the TTL saw blackness and fired at max power. This shot may have been slightly overexposed (with regards to subject).

When bringing the shots back to normal exposure (and making them look similar), one appears to have more ambient and one appears to have less because the initial subject exposures were incorrect.

You said these were edited, and you didn’t say the exposure was unmodified, so that’s my new guess.

The first photo simply had more light because you bounced the light from your flash directly on the flat ceiling (or maybe a wall), hence the light spread more evenly.

I do wish to show you some of my work from an event and be honored to have some critique!

Thanks for the brain teaser!

Thanks for helping us learn how to shoot better…

Looks to me like there are two things changed in the second shot:

1) zoom – the far background is further away so you’r closer to the subject
2) angle relative to the subject, you’re looking down the room for the second shot

The second point results in a different amount of light being emitted by the chandeliers most likely affecting the TTL balance for the flash as they’re more prominent in the second shot.

I also know that you like bouncing behind you and usually to one side, judging by the shadows slightly to the right in these shots but difficult to say. So if there’s a wall behind you in the second shot you’re going to get more bounce flash direct on the subject.

So I think in the second shot, you’re getting more direct bounce from a wall behind you and the TTL is over compensating for the additional lighting in the room resulting in a more exposed subject and less exposed background, that’s also slightly exaggerated by being slightly closer to the subject.

TTL meters using a combination of factors in addition to ambient light, one of which can be distance to the subject and focal length. In the first image you’re further away with a longer focal length leading to an increase in flash output. In the second image you’re closer with a shorter focal length. This leads the metering system to decrease flash output to correctly expose the subject.

The subjects are correctly exposed, but what about that ambient?

Flash output decreases exponentially over distance, so a light source that correctly exposes something 1′ away would mean something 2′ away is getting 1/4 the light, or a two stop underexposure. Let’s throw some numbers at your examples and see how it plays out. (Note: I haven’t built a model to measure distance based on guesstimated sizes of objects related to focal length, these are just really rough estimates). Assume you’re 20′ away from the subject in your first photo. The TTL system kicks in and fires the flash for a correct exposure 20′ away. That means that something 40′ away is two stops under. Not bad! The ambient is still there. Now step way in and take the second picture from 8′ way. Suddenly that wall that’s 40′ in the background is getting a -6 stop exposure. Much darker than the earlier image.

More ambient is gained by a being further from the subject (either you or the light source) and a higher flash power to decrease the light falloff. Of course you could also just drag your shutter down to 1/20th or 1/10th if you’re not too worried about the background or shooting in a situation that’s vastly underlit (think most clubs and bars).

Interesting brainteaser, I hope you do more.

I’d have to say after the first shot you reduced the distance significantly between the bounce surface and the subjects, likely by shooting behind yourself in the first shot, and vertically or slightly in front of yourself in the second shot.

As YOU changed your position with the subject, the TTL mode has changed.
As a result, the TTL flash exposure was adjusted accordingly via the camera and not YOU. Hence TTL.

Angelo
Australia