What kind of lenses have small maximum aperture, like f/4, f/4.5, f/5.6, or f/6.3?

Spencer's blog claims that national parks are the best vacation destinations. which evidence is best for developing his claim? a personal account from a park visitor who describes positive adventures at grand canyon and yellowstone parks an interview with a travel agent who advertises cruises, eco-tours, and whitewater rafting trips a chart that compares the number of national parks in the eastern united states to that in the western states a study that proves adults who take yearly vacations have lower blood pressure than those who do not