We get asked this question very frequently. The answer is one simple expression - 'Shot Timing'.
So, what is 'Shot Timing'? - It is the time taken for the hot pressurised water to pass through the coffee grounds to give you your espresso shot measuring around 30 ml.
In a Semi-automatic machine, the user has to control the timing by sight. Which means you flick the switch and start the pump, let the brew pour out, watch the brew fall into the cup, and when you have estimated visually, that the brew collected in the cup is around 30ml, you flip the switch and stop the pump. This is the procedure you need to follow in a semi-automatic machine.
In a fully automatic machine, the electronics inside have a built-in timer which is programmed to stop the pump automatically after a pre-determined time. So all you have to do is press one button to get the pump started. No visual inspection required and perfect results every time.
In worthy to mention here that the industry standard of 30ml for an espresso shot is not etched in stone.
You could, depending on your personal taste or on the basis of the coffee bean characteristics like roast or origin profile choose to have your espresso shot either less or more than the 30ml suggested shot size.
This variation in the shot quantity can be achieved in both machines.
In the semi-automatic, the running of the pump is in your control anyway.
In the fully automatic, it takes a simple programming procedure to customise your espresso shot.