A radiographic source is not a point. It has a finite focal spot of size F, so every edge in the object casts a penumbra rather than a sharp shadow. The width of that penumbra measured on the film is the geometric unsharpness, Ug = F x t / d, where t is the distance from the object feature to the film and d is the distance from the source to that feature. The relationship is pure similar triangles: the shadow spreads in the same proportion as the source size spreads over the gap behind the object.

Two consequences follow directly. First, unsharpness grows linearly with the air gap behind the object, which is why the film is pressed hard against the part and why a raised weld cap on the film side is a real penalty. Second, unsharpness falls as the source is backed off, so the cure for a blurred radiograph is almost always more standoff, paid for with a longer exposure that goes as the square of the distance.

ASME BPVC Section V, Article 2, T-274.2 caps Ug by nominal material thickness: 0.020 in (0.51 mm) under 2 in, 0.030 in (0.76 mm) from 2 to 3 in, 0.040 in (1.02 mm) from 3 to 4 in, and 0.070 in (1.78 mm) over 4 in. For double-wall shots the relevant t is the full outside diameter, not the wall, because the source-side wall sits an entire pipe diameter away from the film.

Geometric unsharpness is only one of the contributors. Total image unsharpness combines Ug with the inherent unsharpness of the film or detector, and on digital systems the detector term often dominates once the standoff is adequate.

Worked example

Focal spot / source size	3 mm
Object-to-film distance (OFD)	20 mm
Source-to-object distance (SOD)	600 mm
Nominal material thickness	25 mm
Geometric unsharpness	0.1 mm
ASME V allowable Ug	0.508 mm
Source-to-film distance	620 mm

Ug = 3 mm x 20 mm / 600 mm = 60/600 = 0.100 mm. At 25 mm nominal thickness (under 50.8 mm) ASME V T-274.2 allows 0.020 in = 0.508 mm, so the shot passes with 0.408 mm to spare. SFD = 600 + 20 = 620 mm.

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