Apple Numbers '08 - Manual (Page 245)

Chapter 12 Dictionary of Functions 245
LOG
The LOG function calculates the logarithm of a number with a specified base.
LOG(number, [base])
 number: A positive number.
 base: Optional; a positive number not equal to 1. If omitted, 10 is used.
LOG10
The LOG10 function calculates the base-10 logarithm of a number.
LOG10(number)
 number: A positive number.
Notes
To find the logarithm for a base other than 10, use the LOG function.
LOOKUP
The LOOKUP function finds a match to your search value in one range, then retrieves
the value in the cell with the same relative position in a second range.
LOOKUP(search-value, search-range, [result-range])
 search-value: The text or numeric expression you want to search for in the search
range.
 search-range: The range of cells that you want to search for the search value.
 result-range: Optional; If omitted, the result is returned from the last row or column
of search-range, whichever is the longer dimension.
Examples
LOG(8, 2) returns 3.
LOG(100, 10) and LOG(100) both return 2.
LOG(5.0625, 1.5) returns 4.
Examples
LOG10(1) returns 0.
LOG10(10) returns 1.
LOG10(100) returns 2.
LOG10(1000) returns 3.