Rethinking 4/3 Depth Of Field

Started Apr 11, 2013 | Discussions thread
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Michael J Davis
Senior MemberPosts: 2,814Gear list
Re: I'm thinking of topping myself ...
In reply to Detail Man, Apr 13, 2013

Detail Man wrote:

Michael J Davis wrote:

...yet another DoF thread & more nonsense!

... I expect!

So true, Michael. All that exists is some perceptual Circle of Confusion from which extrapolations are made to and fro in object-space. In the end, likely nearly all just guess in the field (and/or believe preview-screens). Once again, the deeply complicated extrapolations (for all but close-up/macro):

Hyperfocal Distance (in units of Meters):

HFD = (L^2 / (FC) ) + L


L is the Focal Length (in units of milliMeters);

F is F-Number;

C is COC diameter (in units of Microns)


Depth of Field (in units of Meters):

DOF = (2D) / ( HFD/D - D/HFD )


D is Camera to Subject Distance (front nodal plane to plane of focus);

HFD is Hyperfocal Distance.


Approximate DOF (maximum 11% error on the low side at 1/3 of the Hyperfocal Distance):

DOF ~ (2FC) * (D/L)^2


F is F-Number;

C is COC diameter (in units of Microns);

D is Camera to Subject Distance (front nodal plane to plane of focus);

L is the Effective Focal Length (in units of milliMeters).


Relative Background Blur:

BP ~ ( (100) / ( (F) (H) ) ) * ( (L)^(2) ) * ( ( Df - D ) / ( (Df) (D) ) )


BP is the percentage of the image-frame diagonal that the blur-disk represents;

F is F-Number;

H is the diagonal dimension of the image-sensor's active-area;

L is the Focal Length (when focused at infinity);

D is the Camera (front nodal-plane) to Subject (plane-of focus) Distance;

Df is the Camera (front nodal-plane) to Background Subject Matter Distance.


Under Constant Framing (FOV) Conditions (when D is the same; when Df and D are the same):

DOF2 / DOF1 = ( F2 / L2) / ( F1 / L1 ) = A2 / A1

BP2 / BP1 = ( F1 / L1 ) / ( F2 / L2) = A1 / A2

where A is the virtual aperture (entrance pupil) diameter.


Under Constant DOF Conditions:

The virtual aperture (entrance pupil) diameter being the same, the Total Light is the same, and different sensor sizes perform the same - except Photon Shot Noise will vary by the square-root of the Quantum Efficiencies, and Read Noise will vary in ways determined more by the image-sensor's individual design than other factors (which makes comparisons largely camera test-data specific).


Now that the mathematics and sensor process variables prove to be either too formidable, or unknown without test data, we can go back to guessing and peering at tiny preview-screens.

DM ...

Indeed. (Though I still find Merklinger puts it better than you!   )

My boredom with the subject lies with the fact* that DoF 'limits' are dependent upon the resultant image size - i.e. the magniification of the 'subject' in the final view, and the aperture, as you point out. The rest is noise (verbal).

* fact - well close approximation.

The rest is closer to bokeh discussion - "I like oof"

To be sure GB & Anders have had an intelligent 'artistic' exchange here on limited dof shots, that's more interesting, but less arguable!


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Mike Davis
Photographing the public for over 50 years

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