Patent Number: 
Section: claims

1. In a secondary-electron mapping-projection apparatus, an electrostatic projection-lens system comprising a front-power group and a rear-power group each having a middle electrode in common, the middle electrode and at least one electrode of the front-power group forming a front power, and the middle electrode and at least one electrode of the rear-power group forming a rear power, each of the front and rear powers being independently variable. 2. The projection-lens system of  claim 1 , wherein: claim 1 each of the front and rear powers has a respective center position; and  the respective center positions of at least one of the front and rear powers are variable by varying a respective voltage applied to the respective electrode, except the middle electrode. 3. The projection-lens system of  claim 2 , wherein at least one electrode in the front-power group is configured as a multi-pole electrode including at least four poles. claim 2 4. The projection-lens system of  claim 1 , wherein at least one electrode in the front-power group is configured as a multi-pole electrode including at least four poles. claim 1 5. A secondary-electron mapping-projection apparatus, comprising a projection-lens system as recited in  claim 1 . claim 1 6. A secondary-electron mapping-projection apparatus, comprising a projection-lens system as recited in  claim 2 . claim 2 7. A secondary-electron mapping-projection apparatus, comprising a projection-lens system as recited in  claim 3 . claim 3 8. In a method for performing secondary-electron mapping-projection microscopy, a method for producing an image of a specimen surface at a pre-determined magnification, the method comprising: providing an electrostatic projection-lens system comprising a front-power group and a rear-power group each having a middle electrode in common, the middle electrode and at least one electrode of the front-power group forming a front power, and the middle electrode and at least one electrode of the rear-power group forming a rear power, each of the front and rear powers being independently variable;  irradiating a region of the specimen surface with a charged particle beam so as to cause the specimen surface to emit secondary electrons;  routing the secondary electrons into an end of the projection-lens system; and  adjusting at least one of the front power and rear power to obtain an electron-image of the irradiated region of the specimen surface at a desired magnification.