The depth of field is the range of distance along the optical axis in which the specimen can move without detectable loss of sharpness.
By far - electron microscope. To find out about why/how you can google your question.
The depth of field is dependent on the Point Spread Function (PSF) of the objective which is applicable to optical microscopes. It runs along the Z axis. The higher the Numerical Aperture (NA) of the objective the better resolution on the X, Y and Z axis. The resolution and PSF also is dependent on the wavelength of the light used - which roughly is half the wavelength of the excitation light (for eg. 405 excitation light, the XY resolution will be ~200 nm and the Z resolution will be ~600 - 800 nm depending on the NA of the objective used). If you question is about resolution of the microscope, the EM can resolve in single digit nm range. There are super-resolution microscopy methods which is capable of achieving high resolution.
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