Originally posted by cardinal43 OK, I'm going to show my ignorance for a moment. I understand why you stopped down to f/11. I'm assuming you knew/guessed/preferred to prefocus to 15m. Right? Did this basically come from "experience"? Am I making sense?
Dave's picture shows it. Here's how I work it. I'll use my favorite example, a Tokina 21/3.8. That's easy, it's in my hand, it's today's Lens Of The Day.
Suppose I want to make sure I have deep DOF extending to infinity. I set the aperture to f/11. If I was using a 135/FF camera like a Spotmatic, I would set the focus so the infinity mark is at one of the f/11 marks on the lens. The focus mark now points to 1.2m / 4ft. The other f/11 mark is at just over 0.6m / 2ft. So the prefocus aka hyperfocus is 1.2m, and DOF is from 62cm to infinity.
But DOF is different on my K20D and all other APS-C dSLRs, by about 1 f-stop. And I'm paranoid, so I like to leave an extra margin -- I adjust the focus by a bit over 1 f-stop. With the lens still set to f/11, I'll set the infinity mark *inside* the f/8 mark. The focus mark is at 2m / 7ft and the other f/8 mark is just below 1m, just over 3ft. So the prefocus / hyperfocus is 2m and DOF is 1m to infinity.
With Dave's picture, the aperture is f/11 and prefocus is at 8m / 25ft. On a Spotty, you'd read the DOF between the f/11 marks: 4m / 12ft to infinity. On a Kx or K7 or whatever, read the DOF from inside the f/8 marks: about 15ft to somewhere closer than infinity.
On the mountain-climbing drive I mentioned before, I used an old Zeiss 50mm lens. Aperture at f/11. Infinity mark just below f/8; that puts the prefocus to 8ft. The other f/8 mark is at 4ft. So I had clear DOF from 4ft to infinity. No visual focusing needed. So easy! Then I saw marmots by the road -- gotta get pictures! I switched to an old Enna 100mm. Aperture at f/16; f/11 mark at 15ft; other f/11 mark at 8ft; prefocus at 10ft. So my DOF zone was 8-15ft, just right for shooting portraits of the fat rodents.
MOUNTAINEERS' TIP: Keep your eyes peeled for piles of marmot scat. Marmots like to have scenic views when they dump. Marmot latrines often have the best vistas. This is true in the Sierra Nevadas and the Rockies, I can attest.