# does depth of field travel with the focal plane?

for simplicity it can be assumed all of the following discussion is relative to a prime lens

i recently learned about the technique of hyperfocal distance focusing. that is, by using the method to determine the distance to the hyperfocal point, focusing on this unique point results in everything before the point being out of focus, and after after that point, all the way to infinity being in focus.

so, all of my photography has been relative to shallow dof. just the opposite of the above technique. but i started thinking about any similar method for achieving shallow depth of field photos and i discovered i had a question.

does the depth of field, the tolerance around the subject that remains acceptably in focus although not at the focal plane, travel with the focal plane itself? what i mean is, when shooting at f/2 for instance, i focus on my subject. now my subject, and a narrow tolerance in front of and behind my subject, say by +/- y units is also acceptably in focus. and my subject is x units of distance from my camera. (for shallow dof shots, |y| will be quite small relative to x.) ok, now my question is, if i rotate my focus ring and as such move the focal plane (either farther away or closer), such that the new distance to the focal plane is now (x+r) units of distance from the camera. now i place my subject at this exact point. what happens to my dof surrounding my subject? is the dof surrounding my subject still +/- y units from subject?

the study of hyperfocal distance focusing lead to this question, because from that it seemed that while the focal plane is determined by the focus ring, the dof in a shot is a function of aperture/fstop.
so considering this, in my above example after having moved the focal plane and moved my subject to the new focus point, does this imply that in order to maintain the same dof for this shot (+/- y units) that i would have to adjust my aperture as well? if i used a zoom lens, or a prime lens with a longer focal length, would this enable me to use the same aperture to maintain the same dof?

secondly, if this is in fact what this turns out to be, does this mean that there are some limitations to achieving shallow dof shots, namely the distance of the subject to camera?

• in short; with hyperfocal distance focusing there exists a unique focus point to achieve the desired dof. for shots not at the hyperfocal distance, for a fixed focal length/aperture, does there exist too a unique focus point for the desired dof? Sep 8, 2014 at 16:58
• The DOF is dependent of the distance to the subjecct, so moving the focal plane will alter how wide the depth of field gets. I suggest that you take a look at this question for a longer answer.
– Hugo
Sep 8, 2014 at 17:00
• possible duplicate of What exactly determines depth of field? Sep 8, 2014 at 18:14
• I think the answer to this is contained within What exactly determines depth of field?; specifically, that yes, subject distance is an important factor. Sep 8, 2014 at 18:15

The change in the Depth of Field (DoF) when you change the focus distance is proportional. As you focus closer the depth of field will be shallower for a given focal length and aperture combination (assuming the other variables remain the same: viewing size, viewing distance, viewer's visual acuity, etc.). As you focus further, the depth of field will become deeper. And the amount in front of the point of focus and the amount behind the point of focus change at different rates. So how much of the DoF is in front of the point of focus and how much is behind moves from close to equal at short focus distances to almost totally behind the focus distance at the hyperfocal distance. After all, it is hard for there to be an infinite DoF between the camera and a point of focus only a hundred feet or so away from the camera, but the space behind the point of focus has no such limitation.

Also keep in mind what Depth of Field (DoF) is and what it isn't. There is no magical barrier at which everything on one side is in perfect focus and everything outside of that line is blurred. Rather, as the distance from the true point-of-focus increases, so does the size of the blur circle and we gradually begin to perceive that objects are not absolutely sharp.

Regardless of the aperture of a lens, there will only be one distance that will be in focus. That is, there will only be one distance at which a point source of light will be focused to a single point on the recording medium. Point sources of light at other distances will be projected on the sensor (or film) plane as a blur circle, or circle of confusion (CoC). If this CoC is sufficiently small enough to be perceived as a point by human vision at a specific display size and distance, it is said to be within the DoF. The limits of DoF change based on aperture, focal length, and focus distance as well as the display size and viewing distance of the image. You can print two copies of the same image file and if one is displayed at twice the size of the other at the same viewing distance by a person with the same visual acuity the smaller print will appear to have more DoF than the larger one (assuming the resolution of the image file itself is not the limiting factor).

• "...as a point by human vision at a specific display size and distance, it is said to be within the DoF..." This is not correct. The display size is not included in the definition of DoF. Source : I am an optical engineer, on track for Ph.D. and have an MS in Optical Sciences. Also, see the definition used by actual lens designers in this field guide.. Sep 17, 2014 at 17:03
• Sure it is. If you define blur circles using degrees/minutes/seconds then that translates to a specific angle which can just as easily be defined using a combination of display size and viewing distance. To the average, non-specialist reader it is easier to visualize the difference between an 8x10 or 16x20 print of the same photo viewed from the same distance than it is to visualize the difference between 30 arc seconds and 15 arc seconds. Sep 18, 2014 at 2:35
• @daaxix In an academic setting optics defines DoF based on the ability of an optical system to detect differences. The systems in question can usually detect very minute differences. In photography, the limits of the ability of the human vision system to detect differences must be included in the system. Thus display size, viewing distance, etc. are applicable to the definition of DoF in photography as opposed to laboratory optics where things can be measured much finer than the limits of the human eye/brain system. Jan 8, 2018 at 18:15

No, what happens as you focus further away is that the accaptably-sharp zone in front of your subject grows but the acceptably-sharp zone behind your subject grows much faster, until it reaches infinity. When this happens you have focussed at the hyperfocal distance.

Only at very close distances is the amount in-focus in front and behind your subject approximately equal, usually there's much more in focus behind.