There is a little metal shim that is supposed to stay in place, but mine fell out (the brass looking part above the old focus screen).
The instructions deal with putting it back in and I found no difficulty in doing so. Again this basically just drops in with no force applied.
Once that was back in I went to install the screen itself. Same deal here, just drop in in the right spot. No force. But I put it in upside down. The correct direction is for the raised prism part to be facing the eyepiece and the smooth side to face the mirror. Once in place I then put the retainer spring back on. Make sure it is seated in the original position. If you don't snap it back in place fully, the screen will be slightly out of focus. So make sure it is fully seated as to the instructions.
On a difficulty scale of 1 to 10, I give it a 3 so long as you read and understand the instructions before beginning. You are dealing with small-ish and fragile parts, so you need to be gentle and have a semi-steady hand. There is also another guide out which I did not use, but it might help someone by bringing a second view to the procedure, PDF here.
And now, the results...WOW. Not only does it make it easier to manual focus, or in my case, make it possible at all (I could never manual focus accurately before). But what surprised me more was that even my auto-focus is better, not because the auto-focus actually works any better, but now I can see when it is not quite spot on. It is much easier to see when the image is not critically sharp now. So I expect my consistency in achieving focus will go way up. For me, this gem of a tool is going to help take the guess work out of my focusing.
I don't use the camera metering, so can't really speak to any changes the screen may or may not make in that regard.
Now I also had the 8x10 crop marks installed. And they don't appear to be critically accurate. If you scale an 8x10 to 16x20, the crop marks on the screen would be closer to 16x21, not terrible, but I am going to have to mentally allow a tad bit of room on the inside so details won't be cut out when the image is cropped to 8x10, but the nice thing is that I now have a definite reference to go by. Again, taking the guesswork out of the composition. If anything could be improved in the focus screen it would be the accuracy of the crop marks, I would rather have it too conservative a crop rather than to liberal and risk loosing image details. I don't know about the accuracy of the other types they offer, I only got the 8x10. But I am still glad I have them there. When I order a second screen, I will again get the 8x10 marks installed.
The cameras focus points still light up and are visible just like normal, that is controlled separately from the focus screen.
I don't find the new screen details distracting, and when I was first testing it, I forgot I had the 8x10 crop marks and had to go back and try again to specifically notice and use them. The split prism area extends past the center focus point, so you can get a clear area if you need one. The micro-prism ring area scintillates when out of focus, and I find it a wonderful combo with the split prism. I am very happy with this screen. Alongside my light meter, and my pocket wizards, this was one of the best purchases I have made for the camera.
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