I decided to upgrade my Nikon D700 from full spectrum to 720nm. One problem I had was blur. If you put a filter back in, the plane of focus shifts because of the extra layer of glass and the different wavelength passing through it. What can you do?
You can calibrate your camera.
If you have Liveview, it’s not such a problem because you can focus that way, which means the camera focuses based on contrast and digitally (don’t ask me about the details!) and the photo always comes out sharp. But what to do if you don’t want to or can’t do that because the camera doesn’t have that feature?
You have to calibrate the camera manually. And you can’t do that by taking off the lens and fiddling around in the box with the mirror. Hands off that area. The only way to do that is with two small screws that are at the bottom of the camera. Somewhat hidden. See picture. I don’t think you can do it that way with all cameras, but at least with the Nikon D700 and also the Nikon D600 it works. So take off the bottom – you can do that by loosening 4-5 screws – and look for the two holes.
Find the two screws and a suitable L-shaped screwdriver. Then you have to try it out. I do it by attaching a wide-angle lens and setting the aperture to the minimum. I used a 35mm lens and f/2.
Then I tightened both screws all the way. That was my starting point. After that, I loosened a little on the one, took a picture, and so on. Until the image came out sharp. It is always best to focus on something far away when taking a test picture. And at some point it fits.
In general, if you have backfocus (which is usually the case with IR), that is, if you are focusing on something in the foreground but the background is sharp in the image, you need to turn the screws to the right (the lens or the mount is facing away from you.), maybe 1-2 turns per screw, depending on the lens.
This is how you calibrate a camera after IR conversion. Now I can take wonderful photos with the aperture wide open, and they always come out sharp.