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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.�

We are all familiar with rules of thumb. They are concepts which fit a variety of situations but are not expected to be suitable for all circumstances. The purpose for having rules of thumb is to allow decisions to be made using a readily memorable guide to estimate results.

We are also familiar with old wives� tales. They are concepts that supposedly derive from prior wisdom that are to be applied to assist decision-making in similar circumstances today.

What happens when we mix the two? How about as in, �when you double the focal length, you should halve the strength of the diffusion filter you are using.� I have heard this often over the years. I am sure it has worked frequently enough that it is still being circulated. However, there are some things to know when applying old wives� rules of thumb to every situation. And, there is more to know than is covered by this �rule.�

First off, it is hard to define �halving the diffusion� since filters are not generally graded in multiples. Two grade 2�s do not a grade 4 make, and so on. That�s because most filter types are graded to allow control over a certain range of effect strength. A choice is made by the manufacturer for the weakest and the strongest grades, and the rest are dispersed in-between. Filter choices are then generally made by the cameraman, who evaluates each filter grade for each situation, running tests if necessary, to determine the proper filter to use. So there has not been a need to set grades by multiples.

The differences between the perceived strength of one type of diffusion and another do not allow for ready correlation. The same is true between different manufacturers. So the ability to estimate filter strength adjustments by a set �rule� is hampered by the inability to determine how to apply it. There is, though, an element of truth to this concept. That is, you may often find it helpful to reduce diffusion as you zoom in, to maintain visual similarity. Here�s why. In the first image, �Black ProMist 3 Wide,� we see the effect as desired, at a short focal length with a relatively strong grade 3 filter. In the next image, �Black ProMist 3 Zoom,� the flare produced by the same filter seems considerably stronger than it does in the original. Note, though, that the relative proportion of the amount of flare to the size of the Batman symbol stays the same; it is just magnified. But in doing so, it takes up a larger portion of the overall image, and appears too strong.

In the final image, �Black ProMist 1 Zoom,� I have reduced the filter grade from 3 to 1. Comparing this image with the original shows a greater visual correlation, and is better suited for this application than keeping the grade 3 on when zooming in.

Remember that each situation is somewhat different. A change in the distance, the actual focal lengths used, the relative brightness of the light and the darkness of the background, and especially, the particular look desired by the cameraman, will all contribute to making different decisions. However, it helps to remember that you may want to reduce the diffusion filter strength as you increase focal length, and vice versa. Just don�t narrow the numbers to specifics and expect them to apply well to most, if not all situations.

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Check out this article in the May 2008 print edition of StudentFilmmakers magazine, page 6.

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