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09-13-2017, 10:11 AM   #1
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AB-82 split image focusing screen for 645z

I just acquired one of these apparently somewhat uncommon screens at what looked like a fair price. It's my understanding that this 645N screen is a direct replacement for use in the 645z. Has anyone any personal experience with installing this screen in the 645z?

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09-13-2017, 12:35 PM   #2
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Not that exact screen but I got the DK-80 Matte version with the golden ratio grid.
I had a slight problem installing mine, I guess it wasn't properly seated by first attempt and I noticed my auto-focus would hunt back and forth which is strange in good light. I switched back to the original screen and all was fine. Then I re-inserted the DK-80 and was a bit more careful making sure it was seated properly and the frame was fastened perfectly. Autofocus was fine the second time around. All of this occurred over around 5 minutes so overall it is a pretty easy mod.

When you take out your original screen, you will be able to tell if your new screen is going to fit. Make sure the two side tabs are in the same locations and you should be good to go.
09-13-2017, 02:17 PM   #3
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Good to know about having to try twice to get the screen seated properly. Thanx.
09-15-2017, 12:50 PM   #4
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Installed the AB-82 in the 645Z. Perfect fit. Replaced the screen without incident. The metal tweezers supplied with the screen are not confidence inspiring. I had some plastic ones about which I felt better working so close to the mirror. Nonetheless, no problems with replacement.

Tested with a 75mm and a 55 - 110, both FA versions. Bright sun, hand held, f8. Mis of vertical lines on my old fence and centers of flowers with no vertical lines at all. Did the AF shots using continuous autofocus, so that could have been a contributor, as I did not use a tripod. Need to try that.

Seems as bright as my gridded screen. May not notice the loss of the grid lines outdoors, as they are so fine.

The split prism is visible when using autofocus, but I did not notice that it interfered in any way.

Using autofocus, the correspondence between the autofocus results and the observed position of the split prisim appeared to be exact.

Autofocus is unaffected. I'm under the impression that the focus screen is not part of the autofocus chain. Is that correct?

While I had never noticed any tendency to miss focus with my Leica's, I did find my hit rate lower with the 654z than I would have anticipated. Smaller rangefinder base I would think. Using the vertical lines targets I was more accurate than using the flowers with no verticals. No surprise, probably need a micro prism finder for those.

I also seem to notice that the manual miss focusing is more prevalent with the 55 - 110 than the 75. I don't see this happening in auto focus. Perhaps due to the f2.8 vs f5.6 max aperture ?

EDIT .........
So, repeating my tests on a tripod with a 45 - 85 zoom, my manual hit rate dropped to essentially ZERO. No idea why this would make things worse instead of better. As it stands and based strictly on this last set of tests, I'd have to conclude that, with me driving at least, manual focus with this screen is unusable.

Which makes no sense to me, but I've run out of time and will have to explore further at a later date.

Last edited by postnobills; 09-15-2017 at 02:21 PM.
09-15-2017, 02:46 PM   #5
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QuoteOriginally posted by postnobills Quote
Autofocus is unaffected. I'm under the impression that the focus screen is not part of the autofocus chain. Is that correct?
That would typical for an SLR with phase detect AF. The mirror is transmissive (usually around 45%) and allows the 'scene' to pass through it and is then deflected downwards (by a second mirror) to the AF sensors. The other 65% is reflected up to the focusing screen and onwards into the penta prism and metering sensors.

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