Why are images to be processed by fourier methods usually of a size 2^nx2^n

Acountershaft is supported by two tapered roller bearings using indirect mounting. the radial bearing loads are 1120 lb for the left-hand bearing and 2190 lb for the right-hand bearing. the shaft rotates at 400 rpm and is to have an l10 life of 40,000 hours and an application factor of 1.4. assume ka2k 15, and find the required radial rating of each bearing