GX10 manual focussing problem - ideas please...

On my GX10 I had used the debug menus to adjust the autofocus with good results. However, today I used it with the Samyang 85mm (manual focus) for the first time. The resulting images were focussed past the intended point.

Here are a couple of test shots. The point of focus is very clear in the viewfinder - it is the edge of the decking. This was done very carefully, and with the viewfinder dioptre adjusted correctly. There is no doubt about when the edge of the decking is in focus in the viewfinder, the fine detail of the end grain visibly snaps in and out.

First the whole frame:




and then a 100% crop:




The total distance to the subject is 5 metres, the backfocus is about 0.5 metres (taking the actual focus to be where the shadow line crosses the gap between boards).

So clearly the factory setting of the plane of the focussing screen is incorrect, and I think the lens/screen distance needs to be reduced. This could be done by adding some shim material between the existing focussing screen shim and the screen. The existing shim is about 0.3mm I believe.

Anyone care to try to work out how much shim thickness is required, given that the backfocus is about 10% of the subject distance at 5m ?

Because I've adjusted it based on test shots the autofocus confirmation hexagon is a bit more accurate than the visual focussing, but I do prefer to see accurate focus in the viewfinder.

(Alternatively, rather than try to fix this I could just get the camera converted to IR, which entails various larger changes anyway...)

I've made some modifications and this is the result (note that the lighting is completely different):




The above was manually focussed as before on the end grain of the decking. This time the focussing screen has had three layers of Post-It note added to the edges (as very thin strips). This moves the screen closer to the lens by a fraction of a millimetre, and appears to almost correct the discrepancy. Maybe one more layer is needed, but I think I'll check that when the lighting is better and I can do a wider variety of tests.

I understand you can buy the shims to adjust the position of the focussing screen.

Sure I've seen reference to this here in the Forum.

Yes, the original shim looks like a frame that the screen pops up against (not the frame that holds the screen when it is dropped). However, the shims themselves come in many thicknesses, and the P&P is extortionate. By using this solution and further real-world testing I can establish exactly what thickness is required (by measurement of the eventual Post-It thickness plus the original shim) and then order one to suit.