Update About

About.md

@@ -16,7 +16,7 @@ where $`\lambda`$ is electron wavelength, $`CL_{eff}`$ the effective detector ca
 
 ##### Maximal usable defocus
 
-is given by
+Electron probe grows in size with induced defocus accordingly to the probe semi-angle. For proper reconstruction, the probe has to fit into \textit{probe window.} Maximum defocus which fits into the window is given by
 
 ```math
 \Delta f_{max} = \frac{w_{semi}  - r_{0}}{\tan \alpha_{corr}} [to test]