There are benefits to fast prime lenses and high end constant aperture zoom lenses, but there are also costs. We were always told you need a small aperture for landscape, so you can get everything in focus. Having an idea of the effects of focal length, aperture and subject distance is helpful, both to know when you really need to stop down, and to know if that ultra expensive F/1.2 lens is really going to help you, or just drain your bank account.
This set of charts was created using the formulas in Wikipedia and values are slightly different than those the DOF calculator provides, but they are close enough and faster to achieve than bouncing between a spreadsheet and calculator. The focus distance for all these charts is in feet, at half-foot increments, and the DOF that results is in inches. At 16 mm, DOF increases very quickly and values went off the chart after a few feet from camera to subject. At 85 mm, DOF increases slowly and even at 10 feet from the subject, there is not very much available.
This chart was calculated for a 16 mm lens. I was thinking of the Canon 16-35 F/2.8 L when I did the chart. The chart would look pretty similar for an 18 mm lens.
This chart is for 50 mm. Is it worth spending money on the F/1.2 lens, or is the slightly faster focusing F/1.4 a better deal.
This chart is for 85 mm. By now it is easy to see that the longer the lens, the less DOF you have at any distance, and again is the F/1.2 worth twice the price of an F/1.4?