Why, when I use a wide angle lens I always get picture edges blurred and out of focus?”
Simply put, it is the quality of the lens. One of the biggest challenges faced by a lens designer is creating a flat focal plane for any lens with few abberations (optical distortions). There are many ways to do this and all cost money. The answer could be more lens elements, better glass or using complex (and expensive to make) curves on the lens elements. The designers all know this and usually make some decision on how good they will make the lens in relation to how much it will cost to make. The classic engineering trade-off. One thing you can try is not to open up the lens all the way, but keep it stopped down by one or two stops. If the lens is f/3 use f/4 or f/5.6 instead, this often avoids the problem at the cost of making your exposures a little longer. You can do this with the aperture priority mode in most cameras. The only other solution is to buy a better lens and read the reviews first. A good reviewer will cover that exact issue.
