These are the filters most people think of for camera lenses. They screw directly into the lens filter thread. Common round filters include black and white contrast filters, polarizing filters, soft focus and black mist filters, plus UV filters and plain protection filters for those who like to keep their lenses safe.
Round filters come in different sizes – you have to get the size that matches your lens’s filter size. This may mean you need to get more than one filter of the same type for different lenses. However, you can just get a larger size and use step up rings to fit the filter to smaller lenses.