Focus on… Focus: How Lens Settings Affect Filter Effects by Ira Tiffen

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Reference: StudentFilmmakers Magazine, May 2007. Focus on… Focus: How Lens Settings Affect Filter Effects by Ira Tiffen. Pages 6 & 7.

The lens is conceptually the part of your camera that dates back the farthest in time. Before lenses, even pinholes, no one was capturing image-forming light on photosensitive surfaces in a box. Lenses being what they are, regardless of how new it is your lens has some basic characteristics that function consistently enough among all lenses to make some general practical assumptions.

I won’t get into the issue here of whether depth-of-field increases with decreasing focal length (it doesn’t necessarily, but we can go into this another time). Instead, let’s look at certain types of filters that use discrete pattern elements to create their effect and how lens opening and focal length enter into the picture.

Diffusion nets and star effect filters are the types most of concern here. Nets involve placing a mesh pattern over the lens to produce what I call selective diffusion. As with some other types of softening effects, fine details are diminished while retaining an overall in-focus appearance. Nets date back to the early days of cinematography where they were responsible for many a glamorous star’s pristine screen image. They are still popular for certain situations and are available as fabric or glass.

Star filters create bright ‘points’ of light, actually long thin lines of light emanating from highlights in the scene. The effect is created by thin lines hollowed out into the filter surface by chemical etching. Sets of parallel lines run in different directions; for each direction there will be two points, one line running straight through the light. A four point star will have line sets in two perpendicular directions.

Creating stronger effects for both types of filters is accomplished by making the patterns finer – smaller openings in the diffusion mesh and closer spacing between the lines on the star filter. In each case the stronger effect has more ‘pattern’ on it, making it more of concern as we shall soon see.

So what are we concerned about? Well, at some lens settings, it is possible to actually see the filter pattern in the image, if you are not careful. This may be very obvious, or be completely missed until later, when the image is more properly displayed. The pattern will be most apparent when against a background with little detail, and when the camera is panning. Lighting can also have its effect. The lines of the star filter light up when bright sunlight hits them. Shooting reflective points of light off water waves may have you peering through a ‘waffle-iron grid’ caused by this.

How to minimize this happening has to do with understanding what is causing it in the first place. Simply, the lens is being set so that it is coming closer to focusing on the filter than it should be. This is going to be more likely when using either a shorter focal length or a smaller lens opening, but especially, both.

Depth-of-field also increases with decreasing size of the recorded image for an equivalent angle of view. This is a matter of physics. It means that as you move from a larger recording format to a smaller, for the same angle of view you will have increasing depth-of-field. So, for the smaller chips used in video, focusing on a patterned filter will be more likely than on, say 16mm film; and on 16mm it will be more likely than on 35mm.

Larger-chip cameras like those from Arri and Dalsa will accordingly have less of an issue with filter patterns than other smaller-chip cameras. It isn’t whether it’s film or digital, just the size of the recorded image.

The general rule when using patterned filters would be to make focusing on the pattern as unlikely as you can by shooting with a wider opening and using longer focal lengths.

You may find that a neutral density filter, either on the lens along with the effect filter or behind the lens as with a filter wheel, will reduce incoming light to allow you to use a larger lens opening than you might otherwise. You can also use a polarizer, even if you don’t need a polarization effect, as a stand-in for the neutral density, as it typically has a 1-2/3 stop compensation requirement. Just be sure that the polarization doesn’t cause some other problem at the same time.

Another issue to remember is that you are also more likely to focus on the filter when the subject you are focusing on is closer to the camera.

Filter wheels and filters mounted on the rear of the lens, are other options for using patterned filters ‑ either makes it less likely, but still not impossible, to see the filter pattern appear unwanted in the image. These will be available only if your equipment provides for them.

There are a number of filters that may seem safe from this standpoint, but virtually any filter with a pattern may cause a problem, so, especially for critical events, test in advance and evaluate the result on a proper display. What you see in the viewfinder may not be enough to signal that there’s a problem while there is still time to correct it.
There is another focus issue that lens accessories can assist with. In particular, I refer to the “split-field” lens, a close-up lens that has been cut in half. This provides the ability to have part of the image focusing normally, usually on an object in the background, while retaining focus on another part of the image in the foreground, especially when the distance between the two and the proximity to the camera prevents normal depth-of-field from keeping both in focus.

Mount the split-field lens on the camera lens, and position the lens-half to cover the near-field. Focus on the background object normally, and then move the camera forward and back until you obtain simultaneous focus on the foreground. You can properly control the frame and focus of your shot by choosing both the lens focal length and the diopter strength of the split-field lens accordingly.

You will usually have an out-of-focus line between the two fields of focus – this is best handled by placing it in an area with little sharp detail to minimize drawing attention to it. This line will be larger and softer at larger lens openings and smaller and finer at smaller openings. That depth-of-field issue again.

There are many focus issues that you will master as you go along. Focusing on the subject and not on the filter will also keep your audience focused on what’s most important – your story.

In over 30 years of making optical filters, Ira Tiffen created the Pro-Mist, Soft/FX, Ultra Contrast, GlimmerGlass, and others, netting him both a Technical Achievement Award from the Academy of Motion Picture Arts and Sciences and a Prime-Time Emmy Award. Elected a Fellow of the SMPTE in 2002, he is also an Associate member of the ASC, and the author of the filter section of the American Cinematographer Manual.

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