Couple of basic principles to consider:
- a brightline finder will typically be brighter than a straight-through finder;
- not all brightlines are created equal, and some have framelines that will disappear in the wrong light;
- multi-focal-length finders are useful, but turrets tend to be large, and cropping finders (a la VIOOH) result in tiny boxes to look through at the longest end;
- using viewfinders of one brand on another brand's camera typically causes framing issues (or won't fit correctly);
- you'll want some way to store, carry, and protect them. Most of them have lost their original cases now - little leather or cloth drawstring bags will do the trick.
I was going to write up a description of every finder I own, but realised that would go on a bit - 17 at current count! - but those general rules pretty much cover it. I will say I've never seen much point to using the SBOOI on a Leica that has a 50mm finder included, but it is very nice. However, one of the huge benefits of a Barnack setup is you can choose exactly the right viewfinder for you; all the combined RF/VF setups feel like a compromise compared to some of the external VFs I've got lying around.
Also, you mention the IVSb; I
hate the viewfinder in those early Canons. Whenever I use either of mine (a IIIa or a II-F, which is very similar to the IVSb), I leave the viewfinder in the camera set to 1.5x and put a VIOOH on top. It's a much nicer experience over dealing with the poor eye relief and fuzzy edges of the built-in Canon finders.
And on the storage note: a company here in the UK called Futon make
these great trinket shelves, which are perfect for displaying your favourite finders and keeping them safe. The VIDOM sticks out a bit too far, but other than that: perfect.
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