Here is an email that I received from Katz Eye Optics on a Sunday less that 12hours after writing to them. In my opinion, Katz Eye Optics is a first class operation!
You are correct in that you have reached a physical limitation of the
design. The microprisms are only functional up to f5.6. With your 500mm f4
and a 1.4x TC, you're at exactly f5.6, so the fact that you can use them
only with difficulty makes perfect sense. With the 2x TC, you'd be at f8
and the microprisms won't be effective at that aperture, as you have found.
The reason is that the microprisms are too small to apply the principle used
the split prism which increases the useful range down to smaller apertures.
With the TCs on, the split prism will still work well in those situations
where you can use it. But in faster action situations, your only options
would be to pre-focus using the split and wait for the target or to focus
during the action using the matte surface of the screen. Since the Katz Eye
screen also has a better matte than the OEM screen, you should be able to
focus more effectively even under those conditions.
If you do finally decide the screen won't work for you, then by all means
send the camera in and we'll reinstall the original screen. I will
authorize our technicians to perform the reinstall at no cost and the cost
of the Katz Eye screen will be refunded. We will have to charge you for the
return shipping of the camera, but that can be deducted from the screen
refund. Should you decide to go that route, please let me know and we can
make appropriate arrangements.
In answer to your other question, there are no plans to redesign the
microprism area at this time. But your suggestion is well taken and if an
opportunity to rebuild the molds presents itself, we will certainly consider
your feedback. If there is anything else I can do for you, please let me
Katz Eye Optics