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?