I suspect this is design tradeoff. First, hepa filters tend to block flow of air A LOT. Lens designers strive to get lenses to focus as quickly as possible. An

I suspect this is design tradeoff. First, hepa filters tend to block flow of air A LOT. Lens designers strive to get lenses to focus as quickly as possible. Any blockage of air would necessarily make it very difficult for the lens to breathe quickly. Second, dust particles in the lens do not affect the image nearly as much as you would think they do. Just look at this pic: https://www.lensrentals.com/blog/2008/10/front-element-scratches/ https://www.lensrentals.com/blog/2008/10/front-element-scrat...