Thermal images pixel size

[monviso]

Hi all,

just a quick question. I see from here https://agisoft.freshdesk.com/support/solutions/articles/31000158942-thermal-imagery-processing, I see that is suggested for thermal images specify pixel size and focal length (mm) of the camera and set as fixed parameters f (I guess its dimension is in pixels). I have two question:
I-what is the rationale of that?
II- To my knowledge pixel size is computed as 1/FocalPlaneXResolution and 1\FocalPlaneYResolution, again what is the rational of that? For my camera it gives 0.0919118 x 0.11489. But if I compute pixel size simply by FocalPlaneXResolution/Xresolution and FocalPlaneYResolution/Yresolution my pixel size would be 0.017x0.017. Does this make any significative difference?

Many thanks!