In industrial radiography, geometric unsharpness Ug is governed by the focal-spot size f, the source-to-object distance a, and the object-to-film distance b, combined as Ug = f × b / a [S3][S5].
The commonly held belief that pushing the source as far away as possible always sharpens the image breaks down once the source-to-film distance is large enough that the effective Ug, scatter, exposure time and geometric magnification penalties all outweigh the marginal gain in geometric sharpness, and ASME V caps Ug at 0.07 in for material over 4 in thick [S4].
Why SFD alone does not control Ug
Geometric unsharpness is the penumbra, the grey half-shadow, cast by a focal spot of finite size onto the detector, and the formula Ug = f × b / a is symmetric in a and b only at first glance [S3]. Increasing source-to-object distance a (moving the tube back) reduces Ug, but the engineering quantity that actually enters the formula is b, the object-to-film gap, not the absolute source-to-film distance (SFD) [S5]. Two setups with the same SFD can produce very different Ug values: a thick part sitting 5 mm above the cassette is sharper than a thin part hanging 200 mm off the floor, even with the tube at the same stand-off [S4].
NDTCalc illustrates the point with a worked example: a 2 mm focal spot, 400 mm source-to-front, 20 mm front-to-film yields Ug = 0.10 mm; if the operator pulls the source back to 1000 mm but leaves the part floating 200 mm off the detector, Ug rises to 0.40 mm, a 4× degradation, because b grew faster than a [S5]. NASA NTRS reporting on radiographic technique likewise treats SFD as a constraint, not a free variable, when sizing exposures to keep Ug under 0.020 in (0.51 mm) on aerospace specimens [S1].
Acceptance thresholds and the eye-resolution limit
Human inspection is the practical ceiling: the unaided eye resolves geometric unsharpness only down to about 0.25 mm, and good radiographs are usually held to Ug < 1.0 mm (0.040 in), with the stricter rule of thumb that Ug must not exceed 1/100 of the inspected thickness [S2][S6]. That is why a 0.5 mm Ug is acceptable on a 50 mm weld but unacceptable on a 5 mm turbine blade, even though the absolute number is the same.
ASME Section V, Article 2 sets the codified ceiling for heavy-wall work at a maximum Ug of 0.07 in (about 1.78 mm) when the material thickness exceeds 4 in (102 mm), which is the same order of magnitude as the visual detection limit but tightened to account for the loss of small-flaw visibility in thick sections [S4]. Codes such as ISO 17636-1 and EN 12681 carry the same 1/100-of-thickness rule expressed in different units, so any change to SFD must be checked against the controlling standard before exposure, not after the film comes out.
Comparison of the three control levers
Of the three geometric inputs, the focal-spot size f has a direct, linear effect on Ug and is fixed by the source hardware, so an Iridium-192 source (typical effective f around 2.5–3.0 mm) and a 1.0 mm beryllium-window X-ray tube cannot be sharpened by geometry alone [S3][S5]. Source-to-object distance a acts inversely: doubling a halves Ug, but at the cost of roughly 4× the exposure because inverse-square intensity loss scales with the square of distance [S4].
Object-to-film distance b acts directly: a 0 mm contact setup (film taped to the back of the part) gives the theoretical best Ug of zero, while a 50 mm air gap degrades Ug linearly. The decision matrix below is the working reference most Level II and III radiographers use when an SFD increase is requested on the shop floor: choose the lever that costs the least exposure time, not the one that sounds most aggressive.
When the source is too close, not too far
The opposite failure mode is the one most field techs actually hit: a 1.5 mm focal spot at 300 mm SFD on a 25 mm plate with b = 25 mm already gives Ug = 0.125 mm, which is well inside the 1/100 rule, so adding more SFD buys nothing visible and burns tube life [S5]. A pipeline crawler crew running an Ir-192 source at 100 mm SFD on a 30 mm wall, by contrast, sees Ug = 0.75–0.90 mm and is forced to either swap to a smaller source, push SFD out, or accept code noncompliance on the 0.30 mm envelope required by 1/100 [S2][S4].
For gamma sources specifically, where f is essentially fixed by the isotope (typical Ir-192 effective f around 2.5 mm; Se-75 around 2.0 mm; Co-60 around 4.0 mm), the only geometric knobs are a and b, and that is where the on-the-job trade-off lives, as laid out in the pressure transmitter instrument family decision logic where every parameter has a code-tied ceiling. The point is that an excessive SFD relative to the part geometry is a real defect in the technique sheet, and a too-short SFD is just as much a defect, so the radiographer's job is to land in the engineering band, not at either extreme.
Film, screens, and the second-order penalties
Geometric unsharpness is only one of three unsharpness components in a radiograph; the other two are film unsharpness (from screen and emulsion) and motion unsharpness, and they add in quadrature rather than linearly [S3][S8]. A 0.30 mm geometric blur combined with 0.20 mm of screen blur gives a total of about 0.36 mm, so driving Ug well below the screen limit does not pay back in image quality, it just costs exposure time [S8].
Film-based systems carry typical film unsharpness of 0.10–0.20 mm for industrial ASTM E1815 Class I and STRUCTIX-type films, while CR phosphor plates sit at 0.08–0.15 mm and direct digital detectors (DDA) drop that to 0.05–0.10 mm, which is why moving to DR relaxes the geometric budget by 2–3× for the same code compliance [S2]. The flow meter spec pages on SourceBySpec make the same point: pushing precision in one variable while ignoring the others never moves the system accuracy number.
Practical technique-sheet rules
Three rules travel well from the literature to the field: (1) compute Ug before the exposure, not after, using the actual f from the source data sheet, the real b including part thickness, and the planned SFD; (2) hold the part against the cassette whenever access allows, since contact (b = part thickness only) is the free sharpness gain; (3) when SFD must grow to clear geometry, grow it with a confirmed inverse-square exposure budget and re-check Ug against the 1/100-of-thickness rule rather than the absolute millimeter ceiling [S2][S3][S4][S5].
For the operator, the takeaway is that the laser distance sensor family documentation treats stand-off as a constraint on accuracy, the same way SFD constrains Ug, and the same engineering discipline applies: pick the source class, set the part-to-film gap first, then back-calculate the SFD that lands Ug under the controlling code ceiling. Track the next signal: a tighter ASME V / ISO 17636-1 revision on small-focal-spot Ir-192 substitutes, and a continuing shift of field work from D7-class film to DDA panels, both of which change the cost of geometric unsharpness relative to detector unsharpness in the next 6–12 months.
For related coverage, see Reach truck aisle width: 1000x1200 pallet, lengthwise vs crosswise.