Hi @rsalencar
Thanks for your prompt reply and the extra information. A modern cell phone camera is fine, provided it has reasonable resolution to easily detect the kernel edge, which is important. Exact distance to corn should not be a problem (because the ratio between kernel surface and its edge should be invariant and a CNN with augmentation should be able to detect it and calibrate itself just during training) but if you add a reference object such as circle of always the same colour (say red), this should facilitate enourmously the processing.
The other details you have given are excellent news. Please get back in touch when you are ready. We will take the discussion from there. By curiosity and if you don’t mind, where is the corn growing 
?
Take care Rafael too and stay safe !
Ael