The CTF is the quotient of Image contrast and object contrast. The first zero describes the resolution of the TEM. CTF =1 is simple contrast transfer, CTF=0 is no contrast, CTF=- 1 is reverse contrast. The CTF is among other things a function of the focus (Scherzer focus).

Dear Chetan kumar Meena,

an interesting question, especially for EM-ists working with the modern classes of EM’s , especially also with cryo-TEM-applications. Having not needed practically such terms, e.g. “What is..” or “for correction of CTF” it is also of interest to me.

So I am just adding perhaps other valuable information in addition to Volker Klemm’s post above.

A really good explanation of CTF, including an example (with corresponding EM-images) you’ll find perhaps @http://www.wadsworth.org/spider_doc/spider/docs/techs/ctf/ctf.html

A free pdf - only as a practical example - containing:

"Contrast transfer function correction applied to cryo-electron tomography and sub-tomogram averaging"

by Giulia Zanetti, James D. Riches, Stephen D. Fuller, and John A.G. Briggs

in J Struct Biol. 2009 November; 168(2-24): 305–312 is available @http://www.sciencedirect.com/science/article/pii/S1047847709002111, & its suppl. Information: @http://www.sciencedirect.com/science/article/pii/S1047847709002111#MMCvFirst

Naturally you’ll find also descriptions/explanations within Wikipedia (if you trust)

@http://en.wikipedia.org/wiki/Contrast_transfer_function and

@http://en.wikipedia.org/wiki/Optical_transfer_function

More information: google:

< CTF contrast transfer function electron microscopy > and you perhaps will be satisfied with some 16,000 results (at least on the first 3 result pages)

Good luck and best regards, Wolfgang

thanks a lot Both of you Mr.( Volker Klemm and Mr. Wolfgang Muss) for your valuable answers

Some very simple background. In most cryo-EM specimens, there is almost no amplitude contrast, because electron interact very weakly with the light atoms of your biological specimen. In such conditions, you can generate phase contrast in your images (allowing you to actually see something in the images your record!) by defocussing the microscope (by say 1-5 microns). This phase contrast is very useful, but it means that the microscope is transferring information in a very surprising way.

Put simply (i.e. not mathematically rigorous!) , the contrast transfer function describes how information is transferred as a function of spatial frequency. Or to put it yet another way, it describes how contrast generated by your microscope varies as a function of resolution in your images.

Essentially this is equivalent in real space to a point-spread function (PSF). So your microscope actually spreads information from each (and every!) position into adjacent parts of the image. This is one reason why cryo-EM images look a bit odd at first sight! Typically the CTF is described as being an oscillating function with decreasing amplitude and increasing frequency. So you get reversals of contrast that happen more and more often as you go to higher and higher and resolution, and all the while you get less overall signal as you go to higher resolution.

If you want to go to high resolution in 3D cryo-EM you have to computationally correct for these effects - effectively you are reversing the effects of the PSF - putting information back where its supposed to be!

Not sure if this descriptive explanation is helpful, but I hope so!

It is simply modification of image of sample by setting the resolution in EM