Diffusion, fog and soft focus filters: How do they differ?
Today’s cameras, lenses and sensors have gotten so good that images can now appear too sharp, almost too perfect. As a result, photographers have sought out ways to achieve softer, more evocative images.
This being the age of digital photography, there has been a shift to software solutions for softening effects, and there is no end of third-party programs and Photoshop techniques to break down an overly-sharp image. Nonetheless, there is an argument to be made for shooting digitally with lens filters from the get-go, because let’s face it, who wants to spend more time in front of a computer screen if you can get the same result, or an even better result, from a filter placed over the camera lens.
Now it can get confusing. The softening filter world has become almost too nuanced, with manufacturers selling so many subtle effects it’s hard to keep it all straight. But if you break it down into the three categories of diffusion, fog, and soft focus the whole panoply of filters will fall into place. Let’s take a look.
Diffusion filters
A diffusion filter is an equal-opportunity softening effect; it breaks down the image regardless of tonal differences. The typical filter has a glass surface whose clarity has been cut down by sandblasting or other mechanical abrasion or scoring. This has the optical effect of diverting incoming light rays to a greater or lesser degree, thus blending fine details. At tonal borders between light and dark, the image will halate, with the light portion bleeding over into the dark portion.
Another approach is to impose a random pattern of opaque pinpoint dots across an otherwise clear glass surface. This breaks up the incoming rays in roughly the same way, without causing halation. i.e., pure softening without the bleeding effect. Keep in mind that black dot diffusion filters cut down the light slightly and thus require a small exposure compensation.
Because the visual effect of diffusion depends on subject matter, these filters are often offered in graded sets, with the intensity of the effect varying in steps. The idea is to offer a filter that has almost no effect to a filter whose effect is extreme. In my own experience, the filters “in the middle” are going to be your “go-to” filters; for a graded set ranging from 1 to 5, you are most likely to use grades 2 and 3, for example. Inasmuch as the cost of a full set is not minor, this approach makes a lot of sense.
Knowing which grade to use on a particular shot is a knack learned from experience; it’s hard to make clearcut rules. When I shot food for commercial clients, a touch of diffusion with a lower grade was really not noticeable, and added a slight enhancement by cutting the sharp edges of food. For a portrait of an older woman, on the other hand, a stronger grade eliminated small wrinkles in the skin and saved retouching time. For an utmost painterly effect, what I call the “Hallmark card look”, you would choose the highest grades.
You can mimic commercial diffusion filters with any number of improvised home-brew techniques, and yes, smearing vaseline onto a UV filter will diffuse the image. So will wrapping the front of the lens with a plastic bag, held on by a rubber band. I’ve tried many of these things, and offer no critique as long as you get a pleasing result. However, it’s not the same as using a repeatable, tested lens filter from a reputable manufacturer, and gaining knowledge of how they work time and time again with various subjects.
I photographed Beatrice in the studio using a Harrison and Harrison #2 diffusing filter. It breaks up details just slightly, and never intrudes on the image or looks like a focussing error. Halation at the edges is barely visible.
Fog filters
A close cousin to diffusion filters are fog filters. A fog filter is a diffusion filter added to glass that is not clear but infused with a slight white cast. The slight cast eliminates solid blacks in the image and thereby creates the look of an enveloping fog, hence the name. It’s all about the halation rather than the blurring itself. Fog filters are also offered in graded sets of varying intensity but in general their effect starts where diffusion filters leave off. There is no mistaking their effect on the image, so caution is advised and there is no way to undo the effect in post production.
Soft focus filters
Soft focus filters are an attempt to mimic the optical performance of soft focus lenses that were commonplace in the days of film photography and large format cameras. The goal of soft focus filters, and lenses too, is to blend (soften) slight tone differences while not touching areas of the photograph with great tonal difference. In a portrait of a female, for example, a soft focus filter would blend skin tones, minimizing pores and irregularities, but maintain the sharpness of eyelashes and hair.
In landscape work, a soft focus filter would blend sky tones but leave foreground details sharp and well defined.
A true soft focus filter is a lot more involved than simple diffusion and involves proprietary manufacturing technology; the two are not directly comparable. I use the Zeiss Softar in grades 1 and 2 and the design has been licensed to other companies. Cost is higher than the typical diffusion filter because of the optical innovation.
Hear is Beatrice again, only this time photographed with a Zeiss Softer #2 filter. The difference between the diffusion filter and the soft focus filter are subtle in these studio portraits, and may not show up on your screen image.
