I think that would be a niche product, at best. Most users would want to use the screen of their smartphone as a viewfinder. That either means using both hands

I think that would be a niche product, at best. Most users would want to use the screen of their smartphone as a viewfinder. That either means using both hands or requiring a way to attach the thing to the smartphone. Also, to do the image processing, that cylinder would need to have most of the processing power of the iPhone and, thus, a fairly large battery.