Jumping ships - DoF

Started Sep 29, 2012 | Discussions thread
CarstenKostrzewa New Member • Posts: 18
Re: Background Blur is related to ratio of Distance / Hyperfocal Distance

Detail Man wrote:

Thinking it through, where (using Merklinger's model):

... the "Near Focus Distance" (Dn) and "Far Focus Distance" (Df) at which some given Spot Size (S) can be resolved, are as follows:

Dn = (D) x ( 1 - ( (S) (F) / (L) ) )

Df = (D) x ( 1 + ( (S) (F) / (L) ) )

Dn is the Near Focus Distance;
Df is the Far Focus Distance;
D is the camera to plane-of-focus distance;
S is the Spot Size as required to be specified by photographer;
F is the F-Number;
L is the actual Focal Length.

... solving for Spot Size ( S ) in the above formula for Far Focus Distance ( Df ), and taking the ratio:

S2 / S1 = ( L2 / L1 ) x ( F1 / F2 )

... which is identical to the case of taking the ratio of Hyperfocal Distances - which, in the case of equal-framing , have been normalized for any existing diferences in Sensor Size, thus here (in this equal-framing case) equating to the "format independence" of Merklinger's model and formulas.

Unfortunately I am not familiar with Merklinger's model. If you however take the formulas I have used in the previous posts and define the spot size you mentioned to be the normalized blur spot (i.e. blur spot size b divided by sensor width w2 ), then I can derive the same equation. This means that if you are using a sensor, which has a crop factor of 2, that also the f-Number will have to be two times smaller to reach the same relative blur size.

As might be expected, it is here also true that in the case of an (equal-framing) "equivalent" configuration of the camera-systems (where L2 / F2 = L1 / F1 ), the ratio = 1.0 (no difference).

The above results suggest that it is unecessary to determine either, the ratio of the Cam-Subject Distance to the Hyperfocal Distances or the ratio of the Far Focus Distances to the Cam-Subject Distance, in order to perform quantitatively meaningful comparison between camera-systems ...

As I said, I am not familiar with Merklinger's model, but I susepct that those equations only hold true for certain distance D, no?

... DPRreview's formula (for comparing the ratio of the resulting blur-diameters between two camera-systems for the same subject at the same camera-subject distance) reduces to:

... relates only to the qualitative nature of the visible extent of de-focus (blur) that would exist in the case that the Camera-Subject Distance of any of the camera-systems compared is less than the Far Focus Distance.

The idea was not to compare qualitative blur (which is not possible anyway as real lenses are by far more complicated), but the quantitative value, normalize to the sensor width, as we all compare photos in one size (e.g. on our computer screen) regardless of which sensor the camera used (so absolute figures for blur sizes are essentially meaningless)

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