Filter Holders for Landscape Photography
Filter holders are placed on the front of camera lenses to accommodate light, color, and other filters for controlling the composition of photographs. Holders normally accept square or rectangular filter frames that resemble panes of glass, and the holders themselves mount to an adapter ring that screws into the threads on the existing outer lens.
When to Use Stacked Rectangular Filter Frames
Landscape photographers often use them in combination with frame holders to create different effects, essentially stacking them one in front of another. One adverse effect of stacking circular screw-in filters is "vignetting," which is a darkening of the corners of an image in relation to its center. Vignetting occurs when a lens draws light into the camera in a conical fashion, and the frame of any lens, simply by its design, blocks some of the angular light into the lens. Stacking circular lenses on top of one another, or using step-up rings to mount different size filters on lens of different diameters, creates an increasing amount of blockage.
Square or rectangular filter frames actually are frameless, so you can stack them in a row in holder slots without creating that same light blockage. In addition, many holders come with accessories such as lens hoods, barndoors, and French flags that minimize the dispersion of light between the filters, ensuring that light directs into the lens itself.
How Do Holder Systems Mount to Cameras?
The first step is screwing an adapter ring into the internal threads of the existing lens. Some filter holder systems come in kits with a filter holder adapter ring, the holder, and the filter frames. The holder fits over grooves or slots on the ring and is secured with either thumbscrews or friction-mounting spring pins. The holder consists of a platform with slotted brackets to house the filters, keeping them square and parallel in front of the lens to maintain optical clarity. The assembly turns independently of the focal ring, so you can rotate the holder assembly in either portrait or landscape orientation.
How to Determine Lens Thread Size
The critical piece is the lens adapter ring. You have to get the right size for the thread size of the lens. The millimeter diameter of the lens and thus its thread size are not the same as the focal length dimension of the lens, which is the millimeter distance from where conical light converges inside the lens to where it's re-projected onto the image sensor (or film) on the back of the camera.
The most common lens thread sizes are 49, 52, 58, 62, 72, and 77mm diameters. The best way to determine the exact thread size you need for an adapter ring is to look at the inside rim of the outer edge of your lens. Find the symbol for diameter, which is an "o" with a diagonal line through it (ø). It has a millimeter number after it. This is the size of the thread, and the designation of the adapter ring and holder with which you have to match it.