Say the aperture diameter is untouched and the focal length doubled the projected image dims to 25% of its original brightness (2 f-stops). Conversely, if the focal length cut in half, the image brightens 2 f-stops. The Math: 50mm lens 12.5mm aperture = f/4 --- 100mm lens 12.5mm aperture = f/8 --- 25mm 12.5mm aperture = f/2
The Canon EF 12mm Extension Tube II is slightly smaller than half the size of the Canon EF 25mm Extension Tube II. The 25mm review is applicable to the Canon EF 12mm Extension Tube II as well. The Canon EF 12mm Extension Tube II is pictured above with the included Canon Pouch.
Irregular polarization with EF 35mm f/1.4L II + Drop-In Filter in Camera Accessories 05-02-2023; Mount Adapter EF-EOS R Compatibility with EF 70-200mm F2.8 in EF & RF Lenses 03-22-2023; Extension Tube EF 12 II Compatibility with EF Lenses and EF-EOS R Mount Adapter in Camera Accessories 12-30-2022 EvFJ.