The depth of field is so great at those very short focal lengths that just guessing the distance is more than good enough for shorter distances and any long distance setting works for long distances.
At 8mm and f4 with focus at 2.00 ft, everything from 1 ft to infinity is in focus. At f8, the range is 0.5 ft to infinity.
At 15mm with focus at 2 ft, this changes to 1.5 ft to 3 ft at f/4, and 1.25 ft to 5.5 ft at f/8. If you change your focus to just over 3 ft, then everything from 1.5 ft to infinity is in focus at f/8. With a focus of 6.5 ft, everything from 3 ft to infinity is in focus at f/8. This can provide some very interesting effects.
Here's a few images of the 2000+ images taken with the EF 8-15 on a T2i, 6D over the years. I use it quite a bit less now that I have the EF 11-24. It may get a bit more usage when I want to carry a lighter load. The key is to have some foreground interest in most cases to give it depth. I use it in tight indoor spaces, hiking in the mountains, events, astrophotography and more. The first two are set for infinity focus, the last two for very close focus (nearly touching the lens) so little depth of field and curvature becomes more noticeable.
550D, ISO 100, f 11, 1/125, 10mm. Diagonal fisheye for scenery on a crop camera.
6D, ISO 400, f 4, 30 sec, 15mm. Moon and stars diagonal fisheye - slight start trails from the 30sec exposure.
550D, ISO 100, f 4, 1/60, 8mm. Nearly touching a horizontal log (maybe an inch away), a bit of the fisheye circle showing. 10mm gives diagonal fisheye on crop cameras like the 550D.
550D, ISO 3200, f 4, 1/25 sec, 10mm. Opportunistic cat selfy nearly touching the lens, no chance to change the settings or look through the viewfinder - just held it near the floor and shot. Love the focus on the eyes - pure luck.
