Optical zoom is not possible with an iPhone or iPad but Dartfish Express lets you magnify part of the recorded video image using the pinch gesture.

The pinch gesture is where finger and thumb are placed on the screen together and moved together or apart to zoom in or out. The same placement of finger and thumb, without pinching, then lets you move the image around on the screen to allow you to maintain the zoom and view other areas of the image.

Alternatively, a double tap on the screen gives a quick x2 magnification centred on the region tapped.

It is not currently possible to set a zoom on the recording as it is being made.