These filters reduce contrast and create a soft halo around the highlights. They are also ideal for creating a hint of softness or blurred contours. Their effect is based on concentric rings and can be enhanced or reduced by opening or closing the aperture.
Soft focus filters are popular lens attachments. Although there are a wide variety of applications, there is a tendency to use them to achieve a stronger or lower contrast level. However, images created with soft focus filters with lens shaped structures in its surface retain their sharpness while softening contours and causing a pleasing halo effect around the highlights. This effect remains unchanged from the lens aperture opening with these types of filters. Special-effect filters which are slightly colored or have concentric rings in the glass create a look that is similar to a slightly-out-of-focus effect. This increases with wider lens aperture openings and diminishes by stopping down.
Soft-focus and fog filters are not only useful for still-life and romantic portrait photography, they can produce a surrealistic atmosphere for everyday subjects too. Partial soft focus via a filter with a clear center can help concentrate the attention of the viewer to a specific picture subject. In this case, using larger lens openings to create a nice flow between the sharp and unsharp portions of the image is recommended. Black and white pictures taken with soft focus or fog filters appear slightly dull. Underexposing can help remedy this.