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Stephen Sanchez

Lens Filter Strengths vs Focal Length

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Creative filters like Mists, Coral, and the like are never needed for the clean Direct Response work that I do. I witnessed a DP lighting for beauty using Promist and told he me that the strengths of diffusion filters like Promist or Low-Cons are intended to be stepped up as you choose a longer focal length to maintain consistency. So for instance, a 35mm with 1/4 Ultracon, switch to 85mm with 1/2 Ultracon and so forth. It makes sense if the tighter focal length is seeing a smaller area of the filter and thus requires more density to deliver the same effect. But I have no need to rent a set of promist to test this.

Is that really true? And do you simply double the filter strength with the lens? Even more so, what about a typical set of 18, 25, 35, 50, 75, 100... how do you choose which filter strength goes to which focal length? I mean I don't think there are enough in-between Promists steps. One strength would have to double for two lenses, which would then be inconsistent. How does that work?

I own a Tiffen Low-Con #1 and #4 that I got long ago when I was learning because I liked the look of The Terminator's first act so well. What focal length would you imagine those go best with? 

 

 

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I've never heard that logic...

There are two somewhat conflicting pieces of traditional advice -- one is that the longer the focal, the LIGHTER the diffusion filter, not the heavier, because the same filter will look heavier on longer lenses.

But the second is that the tighter the shot becomes, the heavier the diffusion because the viewer wants to see more detail in wider shots, and wider shots tend to be shot on wider-angle lenses, closer shots on longer lenses. Audiences want to see as much detail in a landscape but they don't need to see every pore in a close-up of a face.

So in some ways, the net result might be that if you want consistency but are using the longer lenses to get tighter on faces, just use the same filter for everything!

It basically comes down to what feels right, you have to judge the individual shot in terms of whether your filter is too heavy or too light. There is no formula to match filters to focal lengths because shot content and lighting contrast affects your perception of sharpness and what level of detail your eye wants. Not to mention what f-stop you are shooting at might have an effect too.

The simplest thing is to use the same filter for everything until you get a sense of when you need to decrease or increase the effect.

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Oh interesting! Well that is a funny bit of conflicting logic. I suspected that was a personal taste anecdote. Thank you David for clearing up my confusion.

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The conflict comes from one reason being a technical reason and the other being an aesthetic / perceptual reason. In the end, perceptions are what matter the most.

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You can stack two but it might not add up like you think in terms of heaviness, so keep an eye on it. For example, each strength of Hollywood Black Magic has a 1/8 Black Frost base, so stacking a 1/4 HBM with a 1/2 HBM doesn't get you a 3/4 HBM because you've doubled the 1/8 Black Frost.  Keep in mind that all these jumps in strengths were sort of eyeballed by the designer, there isn't really a scientific way of saying the a 1/4 ProMist has twice the affect of a 1/8 ProMist.

Also, the general problem with stacking glass filters is double reflections.

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