Back Focus Revised - New Info...

Started Dec 29, 2005 | Discussions thread
OP PhotoLeon1935 Senior Member • Posts: 2,305
Re: No need to change the order

I pretty much agree with Wally but for a slightly different reason. Doing the manual focus first is iffy because the subjective nature of manual focus. I had no confidence that I could discern a sharp focused image by eye on that misserably faint matt surface in the viewfinder. I was aware of the virtual image effect where the matt surface is "lost" and the eye becomes an extension of the lens, and the eyepiece lens and lastly your own adjustable lens at the front of your eyeball. That is the reason a seasoned photographer is so paranoid about the diopter adjustment of the eyepiece.

The inexperienced user does not know what to look for in avoiding the virtual image effect. If you misjudge manual focus badly then you will wind up basing your auto focus on the fulcrum of a mirror in the wrong place. As Sergei warned us originally, the design is based on a perfect 90 degree relationship between the two mirrors and a 45 degree relationship to the image sensor.

If you feel that you can do a perfect manual focus then number two would be a good place to start. True you would never have to go back and do Number 2 again after doing #1. Having had three eye surgeries in the last year I realize we all see differently and that the norm is defective vision.

I am now working on a much easier work flow to do adjustments that does not require processing the image while adjusting. It just so happens that the maximum zoom of quality image review is adequate to judge image sharpness and direction of adjustment by looking at the screen on the back of the camera. I am working on a new focus pattern that would allow analysis and correction for three or four lenses in about 10 minutes if you don't stop and hold your breath. Yes, and one of those lenses was the Sigma 12-24 lens and the back viewer enven reported moire fathfully as it was viewed a 100% crop on my computer screen. This will save a lot of time when I get the procedure published.

I met a guy on the other forum that had a focus problem with the D200 and the fix is similar but more difficult because of the screw placements.

By the way, for manual adjustment you can use any bright light and I even used the popup flash to get the image. Remember who does the focus and when the flash goes off. It works.

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