[3dem] [TEM] CTF and SNR

[Philip Koeck]

Hi.

I've recently encountered a bit of a conundrum:

I'll assume most of the noise in an image of an ice-embedded bio-molecule is due to variations in the ice, so called structural noise.
The CTF describes the contrast transfer for both signal and noise in the same way.
So, what is the point of changing the CTF by defocusing and/or using a phase plate.
The SNR should be unchanged.

I can think of one explanation: If the signal spectrum is very different from the noise spectrum one could chose a CTF that enhances the
resolution bands where the difference is big.

Else: Is there some other factor that affects visibility of the molecule than SNR?

All the best,

Philip
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