Events

I am having a little trouble understanding when certian events are triggered.

I have a portal that has 3 fields ; fielda,fieldb,fieldc

If fielda = ‘value’ then clear fieldb and fieldc.

The code is as follows:

if (Orders_to_Lines.fielda != null)
{
fieldb = null;
fieldc = null;
}

First problem:
I attached this method to fielda OnFocusLost ~~ if I have no value in fielda the method evaluates as true

Second problem:
I attached this method to fielda OnDataChange ~~ although I did not change any data, just clicked on fielda, the method evaluates as true.

I must be missing something simple.

Thanks for any insight,
Erich