Field lens

In imaging optics, a field lens is a positive-powered lens or group of lenses that comes after the objective lens and before the image plane or the eyepiece, serving to change the size of the image[http://www.colorado.edu/physics/phys1230/phys1230_fa01/topic43.html Judah Levine, University of Colorado Physics 1230: Light and Color: The Field Lens. Fall 2001][http://spie.org/x34410.xml SPIE Optopedia: Field Lens] or to provide image-space telecentricity. It is used for the reduction of detector size and, in instances needing high optical gain factor, it can correct aberrations through its several elements.{{Cite book|title=Optical Design Fundamentals for Infrared Systems|last=Riedl|first=Max|publisher=SPIE Press|year=2001|isbn=0819440515|location=Bellingham, Washington|pages=34}} Optical systems that feature multiple image planes are at risk of a potential problem, which involves the inability on the part of succeeding relay lenses to capture a cone of light from the primary objective lens.{{Cite book|title=Rotating Mirror Streak and Framing Cameras|last=Igel|first=Eugene|last2=Kristiansen|first2=Magne|publisher=SPIE Optical Engineering Press|year=1997|isbn=0819424617|location=Bellingham, Washington|pages=1}} The field lens - by behaving as a variably angled lens - solves this problem by bending or refracting the cone of light back into the succeeding relay lens.

In X-Ray microscopy, the field lens is used to produce parallel and homogeneous illumination of the stencil.{{Cite book|title=X-Ray Microscopy: Proceedings of the International Symposium, Göttingen, Fed. Rep. of Germany, September 14–16, 1983|last=Schmahl|first=G.|last2=Rudolph|first2=D.|publisher=Springer-Verlag|year=1984|isbn=9783662135471|location=Berlin|pages=120}}