This turns my 400mm lens into an 800mm lens. You lose a lot of aperture though - I use f/11 a lot, but it works well in bright light and it's difficult to see birds while cataloging. Unless you're willing to shell out $10,000 for a pro 800mm lens, this is a good alternative. I photographed a hummingbird perched on a branch in the shade across the creek. I have a newer mirrorless camera that can run at fairly high ISOs without much noise. The attached image was f/11, 1/160s and ISO 3200. If you can't get higher ISO shots you may have trouble getting a faster shutter speed. I also shot this without a tripod as most birds don't wait for me to set up a tripod.