Camera manufacturers no longer standardize on a single (threaded) filter size for their lenses and you can easily find yourself owning lenses requiring many different filter sizes.—Joe Farace
Modular filters overcome this non-standard obstacle by letting you to purchase one filter that fits all your different lenses. That means you only need to own one ND, gradient, or polarizer filter and it will fit all of your lenses. These filters fit into a filter holder and work with various-sized adapter rings allowing them to fit different lenses and even different format cameras. All you need is a single, inexpensive adapter ring for each of your different lenses thread sizes.
Caption: Using Cokin’s Gradual Tobacco T2 Soft filter changes the mood once again. Cokin claims that it’s supposed to look “stormy” but I like the dramatic look it adds to an image. Exposure in Shutter Priority mode was 1/60th of a second at F/11.
Modular filters are square or rectangular, although companies, such as Cokin, offer round filters as well. These filters are also more than likely to be made out of plastic. Don’t let that word “plastic” conjure up the optical quality found in toy cameras or even my favorite (now disconnected) Holga. Plastic filters are typically constructed from the same kind of optical resin material used in eyeglasses. The sizes of filters and the type of adapters used vary by modular filter manufacturers, but some companies filters are compatible with other systems.
Cokin’s original A filter system uses 75mm x 75mm filters. The larger P (84 x 84mm) filter system was designed for use on medium format cameras, but an A/P adapter lets you use the A filters in a P holder so it’s not necessary to buy P filters if you already own equivalent A versions.
Tip: When using A-sized filters on shorter focal length lenses some vignetting may occur so watch out for this problem on your LCD. Cokin offers the large (6.7 x 5.1) inches X-Pro size that uses a holder similar to the A and P series but with more rugged construction. Cokin then launched their grande 130mm wide Z-Pro series. A Z/P adaptor lets you use Z filters on a P Holder and the Z/P Wide Angle Adaptor lets you use Z filters on a P Wide-Angle Holder.