THE MAIN TYPES OF FILTER
SCREW- IN Fitting directly on to the thread of your lens, screw- in filters are made of high- quality optical glass. Depending on how many lenses you own, however, and if they have different filter threads, you may need to invest in the same type of filter to fit different lenses, which can be very expensive. An alternative is to invest in a ‘ step up’ ring, which is an adapter that lets you fit a large filter to a lens with a smaller filter thread, reducing the number of filters you potentially need. These 'step up' rings, however, are notorious for getting stuck on lenses.
While screw- in filters are smaller, easier to store, faster to attach and can be used with a lens hood, you’ll potentially end up carrying a lot more filters or adapters. The screw- in system isn’t quite as future- proof as the slot- in system either as if you upgrade your lens, you may find your filter thread changes too, forcing you to invest again in new filters with different sizes of filter threads.
SLOT- IN Typically square or rectangular pieces of glass or optical resin, slot- in filters are secured via a filter holder that attaches to your lens via an adaptor ring. One of the biggest advantages of this filter system is that all your filters will work with all of your lenses thanks to the inexpensive adaptor rings. However, they're larger and much more fragile than screw- in filters. Most filter holders are also designed with multiple slots, making it quick and easy to combine filters without any vignetting, which is common when you attach two or more screw- in filters at the same time. Many holders also come with space to attach a circular polariser, which is often the preferred style for this type of filter.
Slot- in systems vary in size, the most popular being the 100mm systems by Cokin, Hoya and Lee Filters. The adaptor rings are available in various thread sizes and the holders are known for slotting on and off easily.