I will reply to the people who posted replies together,
On the subject of what USB Microscope to use;-
I bought a reduced price M/Scope from Maplin (Electronic supplier chain here in the U.K.). The M/Scope comes with a disc with a "cross-hair" and "capture and compare" measurement mode. As to the soldering application, using a chemistry beaker stand and beaker clamp gives about 3 inches of clearance between the board and the bottom of the M/Scope(I would cover the end of the M/Scope with cling film to stop any damage to the Lens from spatter and vapour from the solder resin). As an added feature, the M/Scope comes with a LED illumination ring.
The M/Scope has auto focus, turning the "focus" ring increases the Magnification from 3x to 300x.
As to the astigmatism issue, as the M/Scope is displayed on a PC screen with your selected magnification, this should not cause any forseeable problems(no pun intended).
On the posted subject of the "Measuring Microscope" on UTube;-
The measurement option comes with the USB M/Scope, however as this option finds few applications with me.
My appilcation is as follows;-
Use a Lab stand and clamp on the upper layer or a wooden "box" type stand with a hole milled to allow the drill bit to protude about 4mm above the upper surface. The drill stand is mounted upside down inside the box.
To operate, the board is laid with copper clad side upwards. Center the hole to be drilled in the M/Scope cross-hairs, Lift the drill stand handle upwards gently to drill the hole. Move the board to the next hole etc.
Hope this helps, chester