Jon are a couple images underexposed by no less than 2.5 stops. The darker image is closer to 4 stops down - and as everyone has pointed out - you have a shift in your white balance. Why did this happen? Because of the zoom. The raster of the image shifts because the information pool shifted with the amount of area realized by the sensor. Look at it this way: If you have an area 1/4 of the field that is white the sensor in auto balance will try and neutralize that data to that formula, change the magnification (zoom in or out) and either enlarge or reduce the amount of white area, the formula will readjust neutralization. In the first image you have more of her blouse showing, but you also have more of the hay, her body, and the ambiance bouncing off all of the darker zones. In the second, you pushed the lens in and although we see less of the shirt, there is more of the area covered by that white blouse - thus more area for the sensor to try and neutralize.
In low light or tight light situations, try and fill the lens with a card, a gray card, and let the sensor see 100% of what it is trying to register. You take into consideration the value and the ambience, nothing more and nothing less, the two most important parts of what you are about to shoot. Save this custom white balance and shoot under that setting, no matter what magnification - if you are at f4 and your model stays against that hay, then whether you shoot at 80mm 50mm or 32mm, you keep the f stop at 4 with that custom white balance.
Here is a fixed version of your shot with the original histogram [left] and the compensating histogram [right]. Notice how all of your information is in the lower 60% of the value range, the bottom 6 f stops. I had to adjust this image by almost 5 stops to pull it back into value - 4 for the brightness and then another .5 to 1 stops in individual channels for color density. You then can see the spread of the data to the new histogram and see full exposure.
SUGGESTION: Do not use the LCD screen image for value, use it for composition and lighting, always look at your histogram to make sure you are getting the information. If you shoot a shot and look at the data graphic and it looks like the histogram on the left, you are sooooo underexposed, you need to open up because you are missing information.
Hope this helps.