Center-Spot filters are a type of special effects filters; they are variations of Diffusion Filters. The center of a center spot filter is usually a hole (It can also be a flat and clear glass). The hole can be round or oval. The surrounding of the hole is a diffuse glass, sometimes colored. As a result, the image taken with this filter will be sharp and clear in the middle (where the hole is), and blured around the center. Its easy to figure out that a larger center spot filter has a larger central clear area.
Center Spot Filters can be found in 4 different types as well:
• diopters with a hole at the center
• soft spot filters with a clear center and soft-focus ring
• diffusion filters with a clear center
• densely arranged concentric circles and a clear center (ex: Cokin’s Radial Zoom)
As for the dimensions of these filters with holes, an opinion can be made from this information: Hoya’s and Tiffen’s 49mm center spot filters have a center spot of diameter about 15mm and Cokin’s Radial Zoom has a clear center of diameter 9mm.
Its good to know that larger aperture and longer focal length will result in a more discreet and delicate result of the use of center-spot filter, and that the best results can be obtained with the Cokin Radial Zoom. Second thing to note is the fact that Hoyas hole edge is very visible in the resulting image, which is very distracting and anaesthetic. Third thing: Tiffen’s effect is not very clear in a small image.
A similar result as the use of these filters can be obtained with a software effect called vignetting.
For Cokin examples, I recommend this site. It has a large variation of filters, each with picture example.
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