mena if go down then a bigger distortion will show up, afther some focal lenght - strong distortion Pscan will not align photos. too big error.....
What type of camera you use ?
18mm but on what camera , if Fullframe like Canon 5D you have 1:1 the focal lenght, on the Canon APS-C - you get 1.6 the focal lenght of lense.... 18*1,6 = 29mm aproxx... that mena on Canon 5D you get to limits of waht possible, on APSC it should work....