However, unless - as J. Patric said - they are to be used with film that's only available in 35 mm, i too don't think these thingies make much sense.
That same nice portrait could be had by cropping the 6x6 the Rollei normally produces. No need to have to expose a full roll of 35 mm just because for one shot cropping to a smaller format would create a nice angle of view for a portrait.
Even the 35 mm adapters that make use of the full width of the camera's film gate don't make much sense to me.
Still, nice that they made it.
Better to have an option and not need it than not have the option at all. That thought may sound silly, but it has other virtues. It's that thingy we call "luxury", and somehow we all appreciate it.
+1 Back in the day it may have been a good idea to use all the films that were available, but that is no the case with the prolific choices we have. Move forward not back. If you want to shoot 35mm use a 35mm camera. It might be nice to have it for a novelty but not for practicality. I see all the time on eBay for the conversion kits to use the whole film of 35mm in a camera meant for another format, If I am going to modify, I will modify my old bellows six-16 camera to use 4x5 sheet film not the other way around.
+1 Back in the day it may have been a good idea to use all the films that were available, but that is no the case with the prolific choices we have. Move forward not back. If you want to shoot 35mm use a 35mm camera.
There was a market for the Rolleikin when it was made, and some Rolleis had the 35mm counter film knob already installed when new, but that was dropped with the F-series. I've used the Rolleikin on a Rolleicord once, just to try it out. Nothing wrong with it if you want to shoot 35mm with a Rollei TLR. If you want to, you want to, and it may be fun. I have no other reason to use my 1930's Rolleis, when I have the post-war models, than for fun and the "feeling", but hey! I like them!
If you have a subject that calls for both 35mm film and a waist-level finder, I can see the advantage of the Rolleikin.
It is a little bit like the 645 backs I recently acquired for my RB67.
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
I have used the Rolleikin with my 3.5F and transparency film. Results are superb and pin sharp as only the centre of the image area is used. Portrait orientation of course, unless the prism finder is used.
I'm into painting with light - NOT painting by numbers!
I picked up a Rolleikin for my 2.8C because it was only $15 and the camera was all set up for it. What the heck! I ran a roll through it and was amazed. The sweet spot of that 2.8C was really sweet - SHARP and contrasty. It is not a rinky dink conversion but a very well engineered one. Who knows, someday we made need something to shoot up the last of the 35mm film in.
It is possible that I don't know what the heck I am talking about, that's what my wife tells me. I had not considered the 35mm conversion kit for my Rollie's due to the fact I have several 35mm cameras with a large selection of glass that give excellent picture resolution. I may have to rethink that but the reason I shoot MF is for the larger negative and clarity when enlarged beyond standard print sizes. If I am missing out on something please enlighten me that's why I'm here, to learn.