There is a point referred to as the "Normal Respiratory Pause", this pause is the best time to exacute your shot.
Not all can be physicaly fit all the time, yes it is best.
With every breath, with every exhale, there is a pause, this same pause is when most of us talk, we don't talk as we inhale, normaly, and with practice, this same "pause" can be extended to 10 seconds. It takes training/practice.
What is the "pause"? Its the time just after you exhale, and just prior to inhaling the next breath, THAT is the perfect time to pull the trigger, your heart is at its calmest, your pulse (all this can be perfected by position-another thread!).
Clarity of the target can be blamed to the lack of oxgen to your eyes...this can be remedied by "blinking", if you are looking at your target, and it starte to "blur", you need to blink....simple? In other words, don't stare too long!