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Figure 7
Defocus modulation recipe examples. (a) Defocus step: the movie starts with low defocus to preserve high-resolution information from the undamaged sample and after an initial exposure the defocus jumps to a higher value for the rest of the movie to improve contrast. This recipe will be easy to implement and process because there are only two defocus values and just one switch point. (b) Defocus ramp: the defocus gradually increases throughout the exposure, matching the damage level of the sample. This recipe will be more difficult to realize experimentally because it requires proportional voltage control. The data processing will also be more complicated because of the multiple defocus frame groups in each movie that have to be handled independently during CTF fitting and image demodulation. (c) Defocus oscillation: the defocus alternates between two values during the exposure. This recipe will be more suited for electron cryo-tomography with the higher defocus value intended to produce contrast and the lower to preserve high-resolution information. It will be similar in difficulty to the defocus step recipe because it uses only two defocus values.

Volume 7| Part 3| May 2020| Pages 566-574
ISSN: 2052-2525