I just finished reading Bryan Peterson's Understanding Exposure and there is a phrase that he uses in a lot of his captions under the photos he has in the book. The phrase is (he is describing what he did to take the photo), "I used an aperture of BLANK and preset the focus via the distance setting on the front of the lens" That last part is what I'm having trouble understanding. What is the distance setting? and how/why do you preset the focus? Any help would be appreciated! TIA!