Red Filter - 43mm to 58mm

The Red filter is used in black and white film photography to isolate part of the spectrum and modify your image.

It is recommended for panchromatic films.

The Red filter has the same effects as the Orange filter, but amplified. It lets yellow, orange, and red light pass through, and blocks blue light. It is recommended for landscape photos with sky or vegetation, and architecture (roofs with red bricks). It significantly darkens blue skies and their shadows.

You can find out your lens's filter thread size by measuring the internal thread; it does not correspond to the focal length at all: for example, a 50mm f/1.8 lens can have a 49, 52, or 55mm thread...