When is depth of field about one-third in front of the focus point and two-thirds behind?
Asked 8/16/2018
10 views
2 answers
0
Some photography books state that depth of field extends about one-third in front of the focus point and two-thirds behind it, but diagrams and calculators often show different ratios. Is this 1/3–2/3 rule actually correct, and under what focusing situations does it apply?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
5
The front/back DOF fraction varies with the focused distance.
Specifically, the rule of thumb about 33% DOF being in front of focus is not always true, but is very closely true when focused at 1/3 of hyperfocal distance. Hyperfocal distance is the closest focus distance where DOF still extends to infinity (for landscapes), computed from focal length and f/stop and sensor size. Hyperfocal is typically a fairly close distance for shorter lenses stopped well down. But the 1/3 rule is dead on true then, if focused at 1/3 of hyperfocal.
Focusing at closer than 1/3 of hyperfocal is more than 33% in front, up to 50% at closeup extremes. Like about 40% in front if at 1/5 of hyperfocal. For 1:1 macro, DOF is near zero, but what there is will be 50% in front.
Focusing at farther than 1/3 of hyperfocal will be less than 33% in front. Like about 25% in front when focused at 1/2 of hyperfocal.
Maximum DOF span occurs when focused at hyperfocal, and then we know DOF does extend from infinity back to half of hyperfocal. So it might be a surprise that "half of hyperfocal" computes as 0% in front, only because the infinity behind is so much larger. Math involving infinity is awkward. :) But 1/3 into the scene has no meaning if infinity is involved.
Still, regardless if 1/3 in front is always very accurate or not, (and since we don't measure any distances anyway), it is a generalization often halfway close, better than not knowing anything, and as much as 50% will only be in front if macro close. It's always a good thing when the photographer realizes that depth of field is a zone, and that often (regardless of exact details), it's a good plan to realize we can center that zone on the important scene area, instead of just always focusing on the first object in front. Focusing back into the scene a bit can often help, especially if a close scene.
A good DOF calculator will show these fractions, front and back (and hyperfocal too).
My site has one at https://www.scantips.com/lights/dof.html and the above data is from https://www.scantips.com/lights/dof.html#into
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
7y ago
0
Generated from our catalog & community — verify before relying on it.
The 1/3 in front, 2/3 behind guideline is only a rough rule of thumb, not a general law.
The front/rear distribution of depth of field changes with focus distance (and with the lens/aperture/circle of confusion assumptions used). It is exactly close to 1:2 only at a particular focus distance for a given setup.
A useful way to think about it:
- At very close focus distances, front and rear DOF are much more equal, approaching about 50/50 in macro situations.
- As you focus farther away, the rear DOF grows faster than the front DOF.
- The classic 1/3 front and 2/3 rear relationship is approximately true when focused around one-third of the hyperfocal distance.
- Beyond that, the rear DOF can become much larger, eventually extending to infinity at the hyperfocal distance.
So the book’s statement is oversimplified. It may be a decent approximation in some mid-to-far focus situations, but it does not hold at all distances and should not be treated as a universal rule.
Recommended products
UniqueBot
AI7y ago
Your Answer
Related Questions
Why does depth of field often extend farther behind the focus point than in front?
How do you balance a boom arm on a light stand safely?
Why is depth of field greater behind the focus point than in front?
How can I use the rule of thirds when photographing two people in a portrait?
Does subject distance affect focus when using a 35mm lens at f/1.8?