Which One to Choose?
There are essentially two types of filter system – circular screwthread types or slot-in holders for square or rectangular types – but plenty of choices within each and a myriad of brands. Both have advantages and disadvantages. The circular filter comprises the filter glass fitted into a metal ring which has a screwthread in a particular diameter to match that of the mounting thread on the front of the lens. Circular filters are easy to handle, but are obviously specific in size so you’re very likely to end up needing a number of copies to match the different filters fitting on all your lenses. Typically, sizes range from 49 mm diameter to 82 mm, the latter becoming a more common fitting for either ultra-fast or ultra-wide lenses. Certainly in the case of polarising filters, but also other types, the larger the diameter, the more expensive the filter. You could end up spending a lot of money on a set of, say, circular polarisers for all your lenses. An option is to buy the biggest filter that you’ll need and then use step-down rings to fit it to the lenses with a smaller diameter filter thread. Step-down rings are a whole lot cheaper than a filter, but there can be issues with cross-threading, and a solidly-jammed step-down ring is a challenging problem to solve without damaging either the ring or the filter (or both).
For this reason, many photographers opt for a holder system. The holder can be fitted to a variety of lenses via different diameter mounting rings which, like step-down rings, are inexpensive. The filters then slot into the holder, most of which will generally accept up to three. This is a far more convenient way of combining filters than stacking screwthread types, when cross-threading again becomes a risk. There are various sizes of holder and filters, but the 100 mm standard (i.e. 100 mm wide filters) adopted by the movie industry is becoming more popular with photographers. A 100 mm filter pretty well covers any diameter of photography lens whereas, if you adopt a smaller holder system (such as Cokin’s A or P series), you may have to start all over again if you then end up buying a lens with a wider filter thread. However, the bigger holders and filters are, not surprisingly, more expensive. Consequently you need to think carefully about your current and future requirements before committing to a particular filter holder system. In the long term, it may well be worth making a bigger initial investment. In some cases ‘starter’ kits are available which include the holder, a mounting ring and a couple of filters (perhaps a grad ND and an ND), and these generally represent better value for money than buying all the components separately.
The main disadvantages with filter holder systems are the size of the holder which may well cause vignetting with wider-angle lenses, varying drawbacks with using polarisers, and the need to more carefully handle the filters themselves to avoid finger prints and scratches. The pro-level systems offer a choice of either resin or glass filters, the latter generally optically superior, but requiring even more careful handling as they will smash if dropped.
Of course, if you’re planning to use graduated ND filters a lot then there really is no other option than a holder system which allows the position of the filter to be varied according to the scene or subject. Circular ND grads are available, but obviously the transition zone will be fixed in one position within the image frame.