B&H Photo has Zacuto Z-Finder.
Andrew P writes:
I wanted to let you know that my Zacuto 3x viewfinder arrived today and I am amazed by the result. I wondered why I would want to enlarge the low res image from my live view panel, but now I see that it looked "low res" because my eyes weren't able to focus on it. With the Zacuto diopter, the image on the LCD is clearer at the default 3x zoom than it is at even 10x zoom without the viewfinder.
I had been tempted to get the Nikon OVF 2x magnifier because it would magnify whatever was in front of the lens, thus avoiding pixellization, but now I'm not sure that would have worked. It would have been a much smaller addition to the camera though. With the Zacuto on one side and a Zeiss Otus on the other, my D800 is starting to resemble my old Sony DSR 500WS video camera.
DIGLLOYD: I’ve been recommending a good focusing and viewing loupe for years, and the Zacuto Z-Finder is the best out there as far as I know (I’ve tried several, most are good but all fall short of the Z-Finder). Such a loupe not only magnifies the image (eliminating presbyopia issues), but also cuts off all glare.
In truth the rear LCD is low-res—one need only glance at an iPhone Retina display to see just how crummy camera LCDs are. Why don’t camera vendors “get” this? It would not only enhance focusing ability (with a loupe), but look fantastic as well.
I’ve lost track of all the Zacuto products, but as near as I can tell this particular model is the current model closest to the one I’m using.
Regarding optical magnification, an optical viewfinder is a complete failure for high-res digital: the light path to the optical viewfinder is separate from the optical path to the sensor. These rarely match, and are often at odds by 40 micron or so—far too much for critical focusing, particularly with high-res digital with a high-grade lens (40 microns is bitingly sharp vs visibly fuzzy). With a world class lens like the Zeiss 55mm f/1.4 APO-Distagon, a minor 'miss' is painfully obvious, because the Otus at ƒ/1.4 is better than many lenses at ƒ/5.6.
I use the Z-Finder as shown—no bracket—because the bracket interferes with my preferred L-bracket on the camera. I simply hold it against the rear LCD on any and all of my cameras. It is why you’ll see me carrying it around my neck whenever I am shooting in the field.