Any truth that depth of field extends from one-third in front of focal point to two-thirds behind it?

Question

In the book Digital Photography Complete Course, there is a page that discusses and illustrates depth of field:

There is an info box that says the following:

The point at which you focus the lens will affect where the zone of sharpness begins and ends. Depth of field extends from about one-third in front of the point of focus to two-thirds behind it.

But the main illustration on the page doesn't follow that, as none of the 3 apertures result in the subject at one-third of the depth of field.

I looked online and found some people saying the "rule" is a myth and others saying it may be true in some situations. In what situations, if any, does this rule hold true?

Answer 1

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

Answer 2

Yes and no. There is some truth to it, but it only holds true for each lens/aperture/CoC combination at a singular focus distance.

The distribution of the depth of field depends on several factors, primarily focal length and focus distance.

The ratio of any given lens changes as the focus distance is changed. Most lenses approach 1:1 at the minimum focus distance. As the focus distance is increased the rear depth of field increases faster than the front depth of field. There is one focus distance at which the ratio will be 1:2, or one-third in front and two-thirds behind the point of focus.

It's only at this one point that the distribution of the depth of field is exactly a 1:2 ratio as depicted in your illustration. The rule of thumb you have cited is only approximate. For every focal length and aperture there is only one precise focus distance where the ratio between front and rear Depth of Field is exactly 1:2.

The ratio of the DoF in front of the point of focus to the DoF behind the point of focus will be different for every focus distance when using the same lens and aperture setting (assuming the other conditions are also the same: magnification/display size, viewing distance, assumptions about the viewer's vision, etc.).

At short focus distances the ratio approaches 1:1. A true macro lens that can project a virtual image on the sensor or film that is the same size as the object for which it is projecting the image achieves a 1:1 ratio. Even lenses that can not achieve macro focus will demonstrate a ratio very near to 1:1 at their minimum focus distance.

For example, using a 300mm telephoto lens with a maximum magnification of only .24X and a MFD of 59 inches the DoF calculates to 1:1 within the limits of rounding the distance to one one-hundredth of an inch. With a FF camera and a 300mm lens at f/4 the DoF will be 0.09 inches in front of the focus distance and 0.09 inches behind the focus distance with standard display and viewing conditions. In reality the near DoF will be microscopically smaller than the rear DoF. This difference is not perceptible and utterly meaningless, though. One has to increase focus distance to 133 inches before the near DoF at 0.54 inches is smaller to two significant digits than the rear DoF at 0.55 inches.

To get a 1:2 ratio we must focus the 300mm lens at f/4 to 769 feet. At that point the DoF will be distributed 192 foot in front and 384 feet behind the 769 feet point of focus (all distances rounded to the nearest foot).

With a 30mm lens at f/4 the 1:2 ratio is achieved at a focus distance of 92 inches. At the macro focus distance for a 30mm lens of 2.3622 inches the ratio is 1:1. With a focus distance of 287 inches (just short of the hyperfocal distance) the ratio is 1:61.4 with a near DoF of 141.2 inches and a far DoF of 8674.3 inches.

At longer focus distances the rear of the depth of field reaches all the way to infinity and thus the ratio between front and rear DoF approaches 1:∞. The shortest focus distance at which the rear DoF reaches infinity is called the hyperfocal distance. The near depth of field will very closely approach one half the focus distance. That is, the nearest edge of the DoF will be halfway between the camera and the focus distance.

We must also remember that hyperfocal distance, like the concept of depth of field upon which it is based, is really just an illusion, albeit a rather persistent one. Only a single distance will be at sharpest focus. What we call depth of field are the areas on either side of the sharpest focus that are blurred so insignificantly that we still see them as sharp. Please note that the hyperfocal distance will vary based upon a change to any of the factors that affect DoF: focal length, aperture, magnification/display size, viewing distance, etc. For why this is the case, please see:

Why did manufacturers stop including DOF scales on lenses?
Is there a 'rule of thumb' that I can use to estimate depth of field while shooting?
How do you determine the acceptable Circle of Confusion for a particular photo?
Find hyperfocal distance for HD (1920x1080) resolution?
Why I am getting different values for depth of field from calculators vs in-camera DoF preview?
As well as this answer to Simple quick DoF estimate method for prime lens

Answer 3

The length of the zone of depth-of-field is subjective. Calculations are based on the fact that a lens fractures the uniformity of a vista by projecting its image by means countless tiny circles that commingle. These circles overlap and have indistinct boundaries. They are called circles of confusion. When we look at a picture, we declare it to be tack shape only if these circles are so tiny that we are unable to make them out. In other words, tack sharp means the circles of confusion are seen as points with no desirable dimension.

Now the lens projects an image on film or digital sensor. The circles of confusion that comprise this projected image must be super tiny indeed. This is because today’s cameras yield a miniature image that is practically useless unless we magnify (enlarge) it for viewing. As an example, if we view an 8X10 inch print made from a 35mm full frame camera , this image will be enlarged a minimum of 8X. This fact means that the circles of confusion as projected by the lens must the eight times smaller or they can’t withstand that degree of enlargement.

So let’s examine the factors that comprise depth-of-field. The basis is, the circles of confusion must not be preserved to have any dimension. If the image is viewed from standard reading distance and the observer has 20/20 vision and the light in the observing area is good, and the contrast of the picture is average, the circle size must be 0.5mm in diameter or smaller (1/64 inch). To withstand the 8X enlargement, the lens must project circle size of 0.5mm ÷ 8 = 0.0625mm or smaller. Also note – if the viewing distance is greater than standard reading distance (400 thru 600mm or about 16 inches), the circle size can be relaxed.

Give all this confusing stuff, how can someone create a depth-of-field chart or math formula? There is a wispy tie-in that takes all this into account. Using a circle size of 1/1000 or the focal length has become the industry standard. For scientific and more stringent circumstances 1/1500 is sometimes uses.

Say we mount a 50mm lens set to f/11 on a 35mm full frame and focus on an object 10 feet distant. We use 1/1000 of the focal length as the circle size (0.05mm). Such a depth-of-filed table works out to: Distant point in sharp focus 30 1/2 feet -- near point in sharp focus 6 feet. The total span is 24 ½ feet thus 2/3 of this distance is 18 feet. If the 1/3 – 2/3 rule of thumb is correct the span should extent to 18 feet + 10 = 28 feet making the far distance 10 + 18 = 28 feet. The far point is calculated at 30 ½ feet so in this instance, the rule of thumb holds.

OK this rule of thumb seems OK. However, actually the span is a variable based on camera to subject distance. This rule of thumb is only somewhat accurate. See for yourself. Go on line, find a depth-of-field calculator and check all this out for yourself.