Edgar in ATL wrote in post #16464403
KatzEye web site states for the 70D: "The KatzEye™ focusing screen for the Canon 70D has no effect on existing viewfinder information. All active viewfinder information, including the focusing points and on-demand grid will still function normally."
If you search their web site, one page offers,
"Why Does Brand X Focusing Screen Not Affect the Meter?
Any split prism screen will produce changes to the light metering - this has been the case since the split prism screen was first introduced, in every camera where they have been used. One need only reference the manual for any camera that has a factory option for a split prism screen (like the Canon 1Ds) to see the manufacturer’s notes about exposure changes. Any screen manufacturer that claims there is no metering effect is simply not telling the whole story. At KatzEye™ Optics, we would rather be upfront and present all the detailed results we have available, so the customer can make an informed decsion. "
On another page...
"Whenever one introduces a prism into the light path of the camera, there is always the possibility the camera’s light meter will be affected to some degree. We have done our best to summarize the effects on the page for each camera model, but if you need clarification or more information, please feel free to contact us. "
And on another page...
"How Can I Minimize Light Metering Errors?
"This varies quite a bit in different camera models, so please refer to the page for your camera model for specific answers. In general however, metering modes that use the entire frame, such as centerweighted average metering and full frame evaluative metering have the most accurate results. Metering modes that weight the center portion of the scene heavily, such as partial metering, spot metering, and some manufacturers’ evaluative metering will be most affected. Fortunately, with the availability of on-demand histograms, it is generally a simple matter to take a test shot, check the histogram, and adjust the exposure compensation accordingly. The exposure shift is generally constant for a particular lens (when using auto-aperture lenses), so once the necessary compensation is known, it can be set with each lens change. Again, for more specific answers, please refer to the product description for your camera model."
And this old post of mine from 2008 mentions what Katz Eye used to state, as well as what Haoda used to state. As I reported, POTN members have verified up to 2EV error (I cannot recall which manufacturer's focus screens)