First, K&F Concept ND2-ND400 lens filter allows changing shutter speed based on the effect you want with the best sensibility and the same aperture. Second, K&F Concept ND2-ND400 lens filter allows creating a blurred background with a large aperture while the shutter speed remains the same. Third, K&F Concept ND2-ND400 lens filter is designed to prevent overexposure and maintain image purity and quality at the same time. *Like a zoom lens, the variable nd filters are suitable for multiple shooting situations. If you're looking for perfect image quality, a fixed ND filter is recommended. 

K&F Concept ND2-ND400 lens filter controls the amount of the light that enters into camera lens with two sets of polarizing glassof high transparency. 

The way u tell is to look at the end of you lens. There will be a 0 with a slash / through it and a number behind it. Match that number to the 58 of this neutral density filter. 

we recommend that you should reduce the density of the filter until "X" cross or trailing smear disappears. 

Hello the filter size is 58mm, so as long as your lens size i s58mm, the filter can work with your Canon EOS 20D