Good post on diffusion filters over on Hurlbut's blog:
He talks about how you have to change the diffusion strength when you change the focal length:
"Typically, the rule of thumb is to use heavier diffusion when using a wider lens and a lighter diffusion when going to a tighter one, pretty much like the net being stretched looser or tighter. So for a 24mm, I would use a number 2 SFX. Then on the 50-85, I would use a number 1 SFX. For anything above a 100mm, I would use the number 1/2 SFX."
@ChrisPeters: which focal length (and as a matter of fact which lens) were these tests done with? These are quite important parameters when testing structural filters...
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