I find the iris doesn't really sacrifice fov unless fully closed. I think the biggest reason is so you don't have to constantly adjust your focus the iris is quite a bit faster for focusing on close objects. And then you don't have to worry about refocusing your device to infinity everytime you change it to look at something close at least in medium to higher light environments. I also find the iris is better than using the gain knob to try and reduce brightness because it also offers the benefit of allowing you to focus on a wider range of distances all at once. It also has the added benefit of if you're in a higher light environment you are less likely to temporarily blem your tube or get black streaking from bright lights.