Special-Effects Filters: Center Spot Filters

Filters discussed on this page may have a thread size (i.e., diameter) larger than 28mm. In fact, they are either of 37mm, 39mm, 43mm, 46mm or even 49mm. Hence, appropriate step-up rings are required, and, as a result, effects created with these larger filters may not fit completely on the image. Moreover, since step-up rings may block the flash sensor, using these filters may prevent you from using internal/external flashes. Proper exposure compensation applied to the camera and/or flash may be required.

Center spot filters are variations of Diffusion Filters. The center of a center spot filter is either a hole or a flat and clear glass, while the outer "ring" is diffuse and sometimes colored. Therefore, the center produces a clear image and the outer ring blurs the detail. As a result, the emphasis is the center. Unfortunately, most manufacturers only produce center spot filters of 46mm or higher. A larger center spot filter has a larger central clear area, which could be as large as the lens opening of the on-camera lens (of 950, 990 and 995, of course). If this happens, center spot filters are no more center spot. They become another glass in front of the on-camera lens. This could reduce image quality. Moreover, when zooming in the on-camera lens, the center spot could also become too large and cover the whole image area.

Hoya Center Spot Another Diopter Type Tiffen Cokin Radial Zoom
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Although the outer ring is diffused, it is in general different from a diffusion filter. Some center spot filters are actually diopters with a hole at the center (e.g., Hoya's above), while for some others the center part is simply a clear glass (i.e., the second image above). For this type of center spot filters, the outer ring of the diopter just blurs the image. If you look at the larger image of the second filter, you should be able to see a clear spot while the outer ring blurs the background. There are soft spot filters with a clear center and soft-focus ring. The third image above shows a Tiffen's center spot filter. Hoya and other filter manufacturers have similar products. Thus, this type of center spot filters are diffusion filters with a clear center. One of the most interesting is Cokin's Radial Zoom which replaces the diffusion part by a set of densely arranged concentric circles. These circles spread the incoming light into a trace of light. Cokin's Radial Zoom is shown in the fourth image above.

Note that Hoya's and Tiffen's 49mm center spot filters have a center spot of diameter about 15mm, while Cokin's Radial Zoom has a clear center of diameter 9mm. Consequently, Cokin's will generate a more noticeable effect. Moreover, because the filter is mounted using one or two step-up rings which push the filter farther away from the on-camera lens, sometimes it is possible that the "ring" of the center spot is visible clearly, especially when facing light sources. In close-up, the on-camera lens may "focus" on the boundary of the clear spot and the soft ring!

In general, larger aperture and longer focal length will make the use of center spot filters a more discreet and delicate result. Unfortunately, while we can choose a larger aperture on a 950/990/995, it is difficult to use longer focal length because once we do it, as mentioned above, the center of the filter could cover the the lens opening completely. Hence, using this type of filters with care.

The Effects of Center Spot Filters

Let us take a look at the differences of these filters. This comparison includes two parts. In part 1, we take a look at the effects of non-uniform illumination. Part 2 will compare the effects under a reasonably uniform illumination. The following images were taken using a 990 under the aperture-priority mode with an aperture value F2.7, the largest available for this particular setting (i.e., focal length 10.7, 35mm equivalent 58mm). This focal length was used because the center spot has to be in the image.

No filter Hoya A diopter type Tiffen Cokin Radial
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One of the most obvious problem with the Hoya is the "ring" that is the image of the edge of the hole! The image is blurred outside of the hole; but, the "ring" is definitely very distracting. One can carefully position of the camera to eliminate this "ring", but it could be a time consuming and tedious task. The image taken with a diopter type center spot filter shares a similar effect. More precisely, the out-of-focus effect caused by the diopter's outer ring is apparent. Tiffen's effect is not very clear in a small image. In a large image (i.e., 800×600), we can see some blurred part around the border; but, because it is of soft-focus type, the effect is not very clear. The image taken with Cokin Radial Zoom obviously matches its name. Since the diameter of the clear spot of Cokin's is smaller than those of Hoya's and Tiffen's, we have more control over which part should have a zooming effect. Because the background has a lot of different elements, the trace effect is very noticeable.

The following images were taken with a uniform illumination. The "ring" is still visible in the image taken with Hoya's center spot filter (the right-most image on the first row); however, if lens shade is used to block some damaging incoming light, it disappears and the result is a good one. But, double images (or glowing) occurs. This also happens to the other diopter type center spot filter. The Tiffen has a not so strong, but more pleasant, result. Again, Cokin's Radial Zoom generates a very interesting image.

No filter Hoya with shade Hoya without shade
A diopter type Tiffen Cokin Radial
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Clear Center

As mentioned earlier, it is difficult to use the diopter type of center spot filters. Moreover, because step-up rings are used which push the filter farther away from the front of the on-camera lens, it is possible that the lens may focus on the "ring" when doing close-up work. On the other hand, if positioned carefully, the "glowing" effect can create interesting and pleasant images. The following shows a few examples taken with center spot filters. Please keep in mind that a larger aperture should be used.

Hoya A diopter type Tiffen
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The following are examples that show you the differences of these filters used on a 35mm SLR. They were taken using a Nikon F5 with a 60mm Micro lens under the aperture priority mode. Then, they were scanned into image files using a Nikon LS-2000 with all default settings. As you can see, these center spot filters have a much better effect with a 35mm SLR film than those with a 950/990/995.

Hoya A diopter type
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Radial Zoom

To use the radial zoom filter, please keep the following in mind: (1) Use a focal length in the range of 35mm and 50mm (35mm equivalent). That is, the first quarter of the on-camera zoom lens scale (on the LCD monitor). (2) Use an average or slightly larger aperture. (3) Make sure the background has multiple elements to get the "zooming" effect. Because this filter has a diameter at least 49mm with a center spot of 9mm, you have to carefully position your camera and set proper focal length to get better shots. So, use the LCD monitor for your final decision. The following are shots taken with this Radial Zoom filter. It is clear that the center part is sharp and the surrounding ring has a zooming effect. The right-most image shows a very interesting shot. The lens zoomed all the way out and the sun light came in from the upper-left corner. The result is that the concentric circles on the filters are clearly seen and flare is visible.

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When illumination and background are reasonably uniform, a Radial Zoom filter can create an effect that is similar to that of a diffusion filter. The following are examples. All were taken with a Radial Zoom filter; however the left one looks better than the right one because of a uniform background. Moreover, the "ring" effect is visible in the right image.

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