The areal density of high density hard disk drives is reaching 1 Tbit/inch², in which very short bit lengths are close to 15 nm, although they should be around 10 nm. Many challenges must be resolved to achieve such short bit length recording. Read head resolution is one major issue, because shield-to-shield spacing, or a shield gap, is not easy to narrow to 10 nm. This study revealed the required read resolution for bit patterned media recording. Reciprocity with sensitivity functions calculated with finite element method (FEM) was studied to investigate read-back pulse, roll-off performance, and cross-track interference. We found that a narrow shield gap length of less than 20 nm was required in order to attain 5 Tbit/inch².