Hi Edmond,
Thanks for your detailed reply!
> The important > thing is that the final result (by comparing with my second ARIA at > the same time) indicated that there is no noticeable change on EV > measurements
Wasn't one of the reasons for you changing the screen was that it was purportedly brighter? Your complaint was the viewfinder was dim. Did you replace the screen believing the T-90 screen was brighter? If it is a brighter screen, then how can your metering results be correct?
Also, did you check it in the various modes with different lenses? I believe the metering pattern of the Aria, knowing how the internals are designed as I do, is quite dependant on the screen.
> However, I cannot explain why the fresnel lens from T-90 > would not induce a focus shift.
It would only not induce a focus shift if the plane of focus were placed in exactly the same plane as the Aria screen would be...more on that below...
Do you believe the Aria screen is not a fresnel BTW?
> (c) Alignment accuracy of the screen is determined by the position of > the resting frame inside the body which was set at factory. Once a > focusing screen is placed on the resting frame, an accurate > positioning will be achieved.
Not necessarily. Two things that can effect that. Screen being too thin...that would rely on gravity keeping the position resting on the frame (easy fix I believe). Second is screen being too thick. It could push down on the retaining frame, and therefore not actually be in the correct position, and possibly skewed (not an easy fix). There, of course, is the chance that the T-90 screen is within a few thousandths of an inch of the original Aria screen, and once made to fit in the frame, won't have the focus shift problem. I have the measurements from the T-90 screen, and will measure the Aria screen this afternoon and let you know the results. I also have a link that discusses the tolerance/significance of the focusing plane with respect to focus changes I'll provide.
> Position accuracy will not be affected > by using different brand screen as the LaserMatte side would rest on > the same position as per ARIA screen.
Well, that is assuming that the screen matches the original thickness, if it's held in with the same type of mechanism the Aria uses. The Canon F series is very different, as it has a rigid frame around the screen. I've also seen screens that have a routed outer mating edge...which would make the plane of focus on these different than ones that didn't have this. I do agree in general, the the bottom of the screen for 35mm SLRs is going to be the plane of focus though, so providing there aren't other mechanical issues (size, thickness etc.), it stands a good chance of working.
> (d) Direct comparison of the brightness of viewfinders from different > body to determine the brightness of LaserMatte screen is not a > conclusive testing since different body mirrors will have different > partially reflective factors and also different prism sizes, thus > affect the final brightness of the viewfinder.
Very true.
Your experiment is a very good one, and if you are happy with the results, that's great. I still have my original concerns about metering accuracy and focus accuracy though. I didn't check, but do you know if the top of the Aria screen is pressed down by foam (or something of that like)? If so, then certainly the screen could be slightly thicker and would be accommodated correctly. A thinner screen could be accommodated by adding some material above it. Of course, the goal is to get the new screen coplanar with the plane of focus of the original screen.
Regards,
Austin
