Yes, that is possible. You can change the portion size either immediately after analysing the photo or later, and enter either half or three-quarter portions, assuming you have not finished your meal.
Here's how:
Tap in the field before the weight unit and enter the desired portion size (e.g. 0.5 for a half portion).
Note:
If entering the comma (0,5) does not work, try using a dot (0.5).