My wife gave me a viewscreen magnifier for father's day and I finally got around to trying it. When using my Sigma 150-600 lens I must use manual focus when using a 2X teleconverter. When looking at the image on the viewscreen I had a difficult time achieving critical sharp focus as the screen is so small. The magnifier made a big difference with the Sigma lens and I found it very useful in making videos with a normal lens when focusing. I have had the teleconverter for about two years and hardly used it because of the focus issue. The photo of the rabbit was taken with the lens at about 950 mm. What I really like about the magnifier is that it attaches with rubber straps and will not mark the camera.