Writing the log file to a output file

Hi, I am using P 5.1 release. As part of my kettle transformation, i have a step which i want to capture the errors if any.
For example, in the step "Select/Rename Values", one of the field Type is set to "integer". I assume as long as the
field is "Integer" then there should not be any output to Write to log file. But, when I run the job , i see the error log
gets created. I have tried Log level to "Noting at all" and also "Error logging only". Shouldn't the only time we Write to log is
when that step fails. ???

thanks