Tasha, thank you; I'm trying to answer your questions fully. Obviously, in a public forum such as this one there will be a wide variety of different viewpoints expressed, some of them in direct contradiction to mine. We all have our own preferences. That's why seeing and trying out the gear yourself, in person, is valuable.
I've not seen the CFV 100c myself, so can't compare the two screens. Could the visibility of the image on the CFV 100c really be fundamentally inferior to the visibility on the IQ4? As for the reviews you've read, well, different people bring different expectations. Are these reviewers using it as a digital back on a technical camera? I suspect not. Most reviewers come from a DSLR background and are only accustomed to using a normal camera with an enclosed viewfinder, and anything less is a fail to them. The CFV 100c serves an altogether different purpose. There are a few forum members who are already using the CFV 100C as a digital back, so perhaps you can seek their comments. One forum friend who has just bought it to use as a digital back texted me a few days ago saying that he's finding it a joy to use.
Ok, I just found the Cambo CS-30 viewing loupe that I bought from Specular a few years ago. It has a high-quality Schneider magnifier lens that is almost 40mm in diameter. It's huge. At the other end, the inside of the open rectangular housing measures 68mm x 52mm. That end is placed over the LCD screen. The kit supplied contains a separate mounting frame that can be stuck onto the digital back, but it's not required; I would just hold it against the screen protector whilst focusing. I would cover the edge with some soft plastic tubing to protect the screen protector. This loupe was pricey, but it's high-quality.
Here is the product page on the Cambo website:
CS-30 Loupe system - Cambo
Absolutely fine up to 210mm. How long do you want to go?
My current digital lenses are the SK 28XL, RDK HR 32, SK 35XL, SK 43XL, SK 60XL, SK 100 and SK 120 Aspheric.
The SK Apo-Digitar 100 and Apo-Digitar 120 Aspheric can be mounted, depending on your preference, with either a long front barrel or with a short front barrel and a Cambo 48mm rear spacer. The rear spacer, being at the rear and much wider than the front barrels, enables maximum use of the lens's image circle. Which is exactly what I want for shooting architecture. This is where some others differ; they shoot landscape and don't require so much shift and don't want to have to put the rear spacer on and take it off. That's why Cambo offers a choice.
I love my Cambo 48mm rear spacer because it gives me the rise/fall and lateral shift I want from my lenses, whilst also keeping the lens itself small, which makes packing the lens into a camera backpack with other lenses easier. It's very rigid and on the kitchen scale weighs just 202g.
I've used the SK Apo-Digitar 100 and Apo-Digitar 120 Aspheric on only a few occasions each since mounting them recently but have not experienced any problems with vibration. The images were all sharp. I try to not have to photograph architecture with longer focal lengths in windy weather. If there are vibration issues due to wind, keep in mind that the problem would be a lot worse with a traditional view camera with a bellows that becomes a sail in the wind. At least with a rigid pancake camera and rear spacer one can drape something over it to dampen vibration without collapsing a bellows. The long focal length lenses themselves are tiny and are still compact when attached to the WDS mounting plate and focus helicoid.
In the near future I will be mounting and testing a 150mm lens and 200mm lens using spare mounts and focus helicoids. Those lenses require two 48mm spacers. To avoid mounting the IQ4 on the end of two 48mm rear spacers I have bought a 48mm front spacer for the lens side, which will distribute the weight more evenly.
Yes, a camera employing a rail and bellows can mount a long lens more easily using a standard lens board. I still have my Arca-Swiss F-Metric 4x5 with Orbix geared front tilt for occasional use with really long lenses such as my SK Apo-Tele-Xenar 5.6/400mm. But I can't walk around carrying two cameras. My WRS is far more compact, far more robust, and is far faster and easier to use, and handles lenses to 210mm. And I get the accuracy and guaranteed parallelism of lens and sensor that allows wide angle to normal to longer focal length lenses to be used quickly and with reliably sharp results across the image.
Rod