With today’s cameras all sporting super fast and accurate auto focus, the art of manual focussing has, for the most part, been neglected by many photographers. Perhaps those only using it today are the avid amateur or professional landscape and architectural photographers. So why should we focus manually with all this new technology around us?
Well, if maximum depth of field (sharp, in-focus subjects from foreground to horizon) is your goal then manually focusing your lens is the most accurate way to achieve this.
Your goal when manually focussing for maximum depth of field is achieving hyper-focal distance (HFD). Every lens has a HFD but only certain lenses will have the marking necessary for setting HFD. Generally the wider prime lenses will be marked with ‘f’ numbers on the focus window of your lens (as shown in the image).

I highly recommend spending some time on HPD; it will improve your landscape and architectural photography overnight.