This is the construction of a type of filter. The type of process used to create a filter can affect its price. Filters can be constructed out of regular glass that sandwiches a coloured gel in between or in high-end filters, raw elements are added to the molten optical glass so there is no risk of uneven colour or fading.
This is the size of a front lens cap. Standard sizes for snap-on lens caps include: 27mm, 28mm, 30mm, 34mm, 37mm, 39mm, 40.5mm, 43mm, 46mm, 49mm, 52mm, 55mm, 58mm, 62mm, 67mm, 72mm, 77mm, 82mm and 86mm.