Minimum source-to-object distance

Geometric unsharpness is Ug = F x t / d. Rearranged for the standoff, the shortest distance that still satisfies a given unsharpness limit is d = F x t / Ug_max. This is the number a radiographer actually needs on site: the crawler, the collimator or the tripod has to be set at least this far back, and anything closer produces an unacceptably blurred image no matter how good the exposure is.

The penalty for standoff is exposure time, which rises as the square of the distance. Setting up at exactly the minimum distance is therefore normal practice, with a small margin for positioning error. The limiting term is usually t, the air gap behind the feature: on a flat plate with the film in contact, t is close to the material thickness, whereas on a double-wall pipe shot t is the whole outside diameter and the required standoff jumps by an order of magnitude.

ASME BPVC Section V, Article 2, T-274.2 sets Ug_max from the nominal single-wall material thickness. A larger focal spot forces proportionally more standoff, which is why panoramic crawler sources and small-pellet iridium projectors are specified with their source dimensions on the certificate.

d_min = F x t / Ug_max
SFD_min = d_min + t
Ug_max from ASME V T-274.2 by nominal thickness