There is only actually 1 plane that is in sharpest focus. The rest within the DOF are what is called "acceptably sharp". Furthermore, this is different between formats, but let us assume 35mm format.
Check this calculator out :
http://johnhendry.com/gadget/dof.php
For your example, (metering is assumed to be correct here) DOF range is between 5.32mm to 16.1mm, and the subject at 10mm will be acceptably sharp.
However, using this calculator will really slow you down in the field, it's just a guide. Using the markings on your lens can only show you a limited range. But you'll get used to it.
If I was the one shooting, in your example, I have some options :
- Since subject is quite a distance away, I can do a quick focus check (or more like range estimate) looking through the viewfinder. This is because I am more accurate at estimating near distances like 2m to 7m rather than far.
- if I can't do the above for some reason, at 1/500, it means I still have room to play, right up to 1/30 (safest speed for me). I can step down further right up to f/16, giving me a speed of 1/125, but expanding the DOF range considerably. I will have the point just before the infinity marking above the f/16 marking (the f/16 marking on the right side).
- but if I can't do the first one and really want f/5.6 only, then I will have the point just before the infinity marking above the f/5.6 marking (the f/5.6 marking on the right side), that should cover a lot as well. But this last option will be the most error prone, for me at least without looking through the VF at least to get an estimated range, since I am more accurate at near distances. So I would probably see if I can get even closer.
For option 2 and 3 above, if one looks through the viewfinder, subject will not be in focus. But it will come out fine, not the sharpest but acceptably sharp. Of course, what is acceptably sharp may not be for others, then the only way is to really focus pinpoint.
Here is an example where I prefocused at f/8 or f/11 (can't remember exactly) with a 40mm, pointing at the building behind, using a $200 Yashica Electro GSN RF, good camera if one can get past the dim viewfinder. I don't remember exactly the settings, but I wanted to take the subjects nearest me, so my DOF scale must have been from 1.5m (where I am more used to near distances). Notice that the building was not exactly within the DOF range since the building name is not so clear anymore, same with the people near it. If I stepped down to f/16, it would be different. This way, subjects don't know I am taking their picture, they think I am taking the building (I can crop most of it out if needed), and it is fast too, raise and click (the Yashica has AE so I didn't have to worry about the speed).
Now there are other pre-focus methods, like Infinity Focusing by Merklinger, or Hyperfocal Distance focusing, you can read up on it if interested. I find the DOF scale method has the sharpest though.
"The day is fairly sunny, but the pathway is shady"
- now this can be another long topic, just read up on the Zone System, or metering ...
Ok, I've been struggling with this for some time now. Let's see if I got it right:
Say I have a subject along the path outside Wisma Atria that I estimate to be 8-10m in front of me. The day is fairly sunny, but the pathway is shady. I use ISO 200 film and a 50/2, shooting at f5.6. So I pre-focus at 8m and I see from my DOF scale that f5.6 coincides with about 4.8m to over 20m. Shutter speed would be something like 1/500. Does this mean that anything from 4.8m to 20m is going to be IN FOCUS and SHARP, including my subject, which is actually 10m away? :dunno: