Yashica ELECTRO 35 Instruction Booklet page 28

- USE OF FILTERS - - - - - - - - - - - - - - - - - - - -
The use of a filter is advisable for clear
crisp pictures in blazing sunlight, snow
or beach scenes, etc.
When using a filter an increase in
exposure time is necessary. This is
indicated on most filters in the form of
a filter factor indicating by how much
(e.g. 2 times, 3 times) the exposure
must be increased with the filter