The Logging is turned off by default unless developer mode is active.

All exceptions are logged in exceptions.log no matter if logging is enabled in configuration.

Logging can be enabled by logging into Magento Admin and proceeding to:

• System > Configuration (top bar)
• Developer (left menu)
• Log Settings section
• Select Yes from Enabled dropdown list.
• Save Configuration in the right top corner.

Message variable type

Even though documentation defines that message should be a string, if an array is passed there's a code block in that method to take care of that with print_r:

if (is_array($message) || is_object($message)) {
    $message = print_r($message, true);
}

Log level

If the level parameter is set to null then DEBUG level is taken.

$level = is_null($level) ? Zend_Log::DEBUG : $level; The levels are declared in file: lib\Zend\log.php

const EMERG   = 0;  // Emergency: system is unusable
const ALERT   = 1;  // Alert: action must be taken immediately
const CRIT    = 2;  // Critical: critical conditions
const ERR     = 3;  // Error: error conditions
const WARN    = 4;  // Warning: warning conditions
const NOTICE  = 5;  // Notice: normal but significant condition
const INFO    = 6;  // Informational: informational messages
const DEBUG   = 7;  // Debug: debug messages

Constants in form of Zend_Log::INFO or integer number in range specified above can be passed as log level parameter.