= Nominal section feed
<Nominal section feed> is the intended advance of specimen relative to knife between successive cutting strokes of a <microtome>. It specifies a machine setting or commanded increment. It is distinct from the recovered <physical section thickness>, which can change through cutting and handling, and from the separation of sections selected for an <image stack>.
JEOL describes presenting the block to a <diamond knife> by a preset advance and then making the cutting stroke. It also notes that material hardness and environmental temperature can cause recovered sections to differ from the preset thickness.<Ultramicrotomy by JEOL>[Glossary, preset feed/cutting sequence and following paragraph on actual thickness]{ref} Consequently the feed record is useful acquisition metadata but does not independently verify thickness.
An original acquisition ledger illustrates a changing feed. Let the ideal block face before cutting be at $z_0=0$, and stipulate successive feed commands of $60,60,100,60\,\mathrm{nm}$. If motion follows each command exactly, successive cutting planes lie at
$$
z_j=z_0+\sum_{i=1}^{j}f_i,\qquad
(z_1,z_2,z_3,z_4)=(60,120,220,280)\,\mathrm{nm}.
$$
If images represent the newly exposed faces after cuts 1 and 4, their separation is $f_2+f_3+f_4=220\,\mathrm{nm}$, not three times the initial $60\,\mathrm{nm}$ setting. This statement concerns block-face positions. For images assigned to centres of collected slabs, the coordinate convention must also account for their thicknesses: in this ideal example, centres of slabs 1 and 4 are $30$ and $250\,\mathrm{nm}$, a separation of $220\,\mathrm{nm}$, while centres of slabs 2 and 3 are $90$ and $170\,\mathrm{nm}$, separated by $80\,\mathrm{nm}$ rather than their face spacing of $100\,\mathrm{nm}$.
Retain feed changes, cutting order, omitted images and the coordinate convention used by the <image stack axis calibration>. Independently measuring specimen advance tests the feed mechanism; measuring recovered <physical section thickness> tests a different outcome. <Backlash> or <thermal expansion> can affect the first, while <section compression> and other <deformation (mechanics)> can affect the second. The ideal ledger assumes neither kind of discrepancy.
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