Spatial correlations of photon pairs from spontaneous parametric down-conversion (SPDC) underpin quantum imaging and entanglement certification. We present the first systematic study of spectral filter bandwidth effects on transverse spatial correlations in bulk Type-I BBO for degenerate and nondegenerate configurations. In the far field, the degenerate conditional momentum width is pump-limited and filter-invariant, while nondegenerate configurations exhibit monotonic growth in both marginal and conditional momentum widths—with the walk-off axis approximately 100 times more sensitive than the non-walk-off axis. In the near field, we identify a previously unreported flat-dip-rise profile: the conditional position width narrows by approximately 10% at an optimal bandwidth ${\mathrm{\Delta }}_{\mathrm{dip}}\approx 1.35\mathrm{\Delta }{\lambda }_{\mathrm{SPDC}}$ before rising due to geometric displacement. When the filter is placed on the idler arm, the dip shifts by the exact factor ${({\lambda }_i/{\lambda }_s)}^2$. Both results are universal for any nondegenerate SPDC source, requiring only a finite crystal length, $d\theta /d\lambda \ne 0$, and incoherent spectral averaging. The Reid Einstein-Podolsky-Rosen uncertainty product is consistently smaller on the walk-off axis—a structural advantage of bulk birefringent geometry absent in quasiphase-matched sources. The optimal filter bandwidth ${\mathrm{\Delta }}_F={\mathrm{\Delta }}_{\mathrm{dip}}$ is determined entirely by the intrinsic phase-matching bandwidth of the crystal and is directly readable from the X-entanglement spectral width of the source.

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• Quantum correlations, foundations & formalism
• Quantum metrology