D90 : get focus indicator from viewfinder

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D90 : get focus indicator from viewfinder

Postby renArD » Thu Feb 14, 2019 6:02 am

I was wondering how to know when my D90 has focus with a manual lens, without putting my eye in the viewfinder.
I didn't find a way to do so : no information on the top LCD, no information on the back LCD, no information through HDMI, no information from softwares like DigiCamControl.
So I decided to "hack" the viewfinder. I started to look for the pins related to the focus point but LCDs aren't driven with simple voltages. So I realized I may understand how the driver chip is controlled. It's through SPI. Reading driver's documentation and testing made me guess the function of each bit, including the focus indicator. I then made a simple Arduino program to read SPI and warn when there is focus.

PS : I dared to update the Viewfinder's article.
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