A question on hypefocal distances.

[Helen Bach]

Just to add a little to what Lachlan said: A DoF calculator (and the usual formulae) gives the distance from the lens (specifically the front or first nodal point as Lachlan says) to the subject. If you are using a lens that has a focusing mount it is likely that the distances on the mount will refer to the film plane - subject distance, which is unambiguous because the film plane is usually clearly marked (most movie cameras even have a tape attachment point there to help with measurements). Maybe this is the reason for the different stories you are hearing.

Best,
Helen

[Leigh]

Just to add a little to what Lachlan said: A DoF calculator (and the usual formulae) gives the distance from the lens (specifically the front or first nodal point as Lachlan says) to the subject.

Sorry... wrong.

The DoF calculator gives the distance from the FILM PLANE to the subject.

It has no knowledge of what lens you're using, nor where the front nodal point (first principle plane) might be.
In many cases the front nodal point is not even inside the physical lens. It cam be moved by hundreds of mm.

- Leigh

[Helen Bach]

Sorry... wrong.

The DoF calculator gives the distance from the FILM PLANE to the subject.

It has no knowledge of what lens you're using, nor where the front nodal point (first principle plane) might be.
In many cases the front nodal point is not even inside the physical lens. It cam be moved by hundreds of mm.

- Leigh

Leigh, I'm fairly familiar with both the rigorous and the simplified versions of lens formulae, and with a number of analogue and digital DoF calculators, and I believe that you are wrong about this. Emmanuel has already given an excellent answer. I'll give a shorter one. The person making/writing the calculator does not know the internodal space, apart from anything else. If you look at the popular online calculators (at least the ones whose authors appear to understand the subject), they state that they are giving first nodal point - subject distances. You need fewer assumptions.

If the "front nodal point is not even inside the physical lens. It cam be moved by hundreds of mm" then whoever is designing or writing the DoF calculator doesn't stand a chance of getting the subject-film plane distance accurate: the internodal space will be quite large.

Here is what the popular dofmaster website has to say:

"Is the 'subject distance' measured from the front of the lens or from the film plane?

This question is relevant only for close-up and macro photography. For other photography, any error caused by measuring subject distance from the wrong location is neglible.

The depth of field equations are derived from the "thin lens" equation, which assumes a single lens element with no thickness. And, subject distance is measured from the thin lens. So, technically, subject distance is measured from the front of a lens.

However, that doesn't apply directly to a photographic lens. These lenses have many elements, and the front of a lens isn't necessarily the location that subject distance is measured from. The actual location is something called the "front nodal point" of the lens. The location of the front nodal point isn't documented by lens manufacturers, nor is it easy to find experimentally.

I measure subject distance from the front of the lens. I believe that the nodal point would actually be somewhere between the front and rear elements. But, by assuming it is at the front of the lens, I get a conservative estimate of the depth of field from the calculator. In other words, the depth of field calculation shows less depth of field than will actually be seen in the photo."

Of course we are lucky: many of the modern lenses we use do have the nodal points given by the manufacturers, not that it matters a whole lot most of the time. It's also quite easy to find if high accuracy is not required.

I only mentioned it as a possible explanation of the conflicting advice that Alex had heard. There are many reasons why it is an approximate science in addition to those given by ic-racer. (basis for max. acceptable CoC; simplifications in the formulae etc.)

Best,
Helen

PS It's a principal plane, not a principle plane, but I'm sure that you know that.