Need help focusing in a scene? Critical focusing? Your advice? Technique?

[ki6mf]

Carry a small pocket lever, i prefer these over the bubble levers that came on my camera, to check that front standards are level. I measure two directions left/right and front/back to once the shot is framed. This can help make sure you do not have a bit of tilt on one of the standards. Also a pocket level makes you look like a master of the universe. Digital photographers are in more awe, women will love you, and fish will fear you.

[Brian Ellis]

I often see comments like this. I am sure it makes sense with appropriate qualifications, but it is obviously not right in all circumstances. For example, if you focus at the hyperfocal distance, the amount in focus behind the focus plane is infinite, which is certainly much greater than twice the amount of focus in front of that plane. Also, for close-ups, the near DOF and the far DOF are approximately equal.

It is not too hard to see that the one circumstance in which the far DOF is twice the near DOF is when you are focusing at one third the hyperfocal distance. So it may be true that the rule is close enough in the near middle distance to be of some use.

Perhaps, someone can explain just what this rule means in an operational sense. It is repeated so often, that it must be useful in some wy. I'd appreciate it greatly if someone would tell me how he/she uses this rule in practice.

The only benefit I know of is that the 1/3-2/3 "rule" lets someone know that with some lenses under some circumstances the circles of confusion diminish in size quicker in front of a subject than behind it (i.e. there's usually not a linear relationship between the relative size of the circles of confusion in front of and behind the point of focus).

But how one would actually use or apply this "rule" in practice beyond understanding the general point is beyond me. Among other problems, the relative size of the circles of confusion in front of and behind the focus point varies with the focal length of the lens being used and also with the distance from lens to focus point. And since most of us use more than one lens and stand more than one distance from the subject on which we're focusing the variables dwarf the "rule." And even if one thinks the 1/3-2/3 "rule" is a good one and should be used when focusing, there's the problem of figuring out where the 1/3-2/3 dividing line is when you're anywhere except in a studio or some other location where distances can be easily measured.

All in all it's a "rule" that has so many exceptions and is so difficult to actually use with any degree of precision that IMHO it should be ignored once one understands the general point it illustrates.