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===Contrast transfer function===
Due to the nature of image formation in the electron microscope, bright-field TEM images are obtained using significant [[Focus (optics)|underfocus]]. This, along with features inherent in the microscope's lens system, creates blurring of the collected images visible as a [[point spread function]]. The combined effects of the imaging conditions are known as the [[contrast transfer function]] (CTF), and can be approximated mathematically as a function in reciprocal space. Specialized image processing techniques such as phase flipping and amplitude correction / [[Wiener filter|wiener filtering]] can (at least partially<ref name="Downing">{{Cite journal|vauthors=Downing KH, Glaeser RM |title=Restoration of weak phase-contrast images recorded with a high degree of defocus: the "twin image" problem associated with CTF correction |journal=Ultramicroscopy |volume=108 |issue=9 |pages=921–8 |date=August 2008 |pmid=18508199 |pmc=2694513 |doi=10.1016/j.ultramic.2008.03.004}}</ref>) correct for the CTF, and allow high resolution reconstructions.
===Three-dimensional reconstruction===
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