I just wanted to talk about my experiences with the format. I don't have any Eureka moment, I think I stumbled across the reference to the format while reading about Zeiss folders and realized the half frame of 6x9 was the 645. Nearly 3 x as large as 35mm. this is a 4:3 aspect ratio. A 6x7 negative is only 25% larger than 645. About the same time or thereabouts I saw this small folder, I beilieve the Zeiss number scheme was 520 for the first generation with letters ( A B C D) or later numbers ..15 16 18 etc denoting the film sizes. . it gets confusing. As it was for me, I came into possesion of a first generation 520 Ikonta. It was in realtively nice condition given the age (pre-war) and price was probably more than I would now pay for such an old camera, but seduced , I took the plunge. Another luring factor on this example was the Tessar lens. Part of the learning curve and of course a benefit of the format is 16 frames on a roll, There are two ruby windows on the back . The proper way is to count frame one first on the left, and then on the right,. Sounds simple enough. I'm too scatter-brained and it took some learning to adapt or adopt. A sub theme of this post is light leaks and my sloppy handling are likely suspect too. I still am haunted with this and it can be be blamed on both the design (the angled back), the ruby windws and slow speed film vs the higher sensitivity films of the present era. I used knitting yarn, additional masking with black backing paper and covering the the exterior windows as well, viewing only when winding and I still have problems on the later frames. Another tricky thing; and this is specific to this model being a folder, is portrait vs landscape. given the fold-out; the portrait stance is actually the landscape and vice-versa. Need to go slow and think! My next foray into this format comes many years later when I came into a Voigtlaender Bessa 6x9. An adventure in and of itself. Imagine my surprise when I opened the back and discovered "the 645 mask" This was probably an Eureka moment. Lucky me!! I had other hurdles to overcome, ie opening the camera -no clip to pull, shutter jammed-timer set, no advance key and then dogged for three rolls of film at least, with a mysterious light leak that wasn't ruby windows related. In all the trials I struggled again with the frame counting, not to mention the jury-rigged advance key but mostly the poor viewfinder framing. In my ignorance I didn't immediately understand the the masking or rather un masking of the ruby windows. It is actually ingeniously taken care of, by masking the one window when shooting 6x9 and uncovering the second window when the 645 mask was used. DUHHH!! In my humbl(ing) experience, the added advantage of the 16 frames is not so well done mainly due to rudimentary viewfinder framing. [ I believe the 645 mask will fit the Bessa 1 with some slight modification , which has a much improved finder- a future post ] The light leak was finally conquered with tape on the tripod mount. Although, in a recent trial, a mild leak on the edge has reared its ugly self.. Hard-pressed to find any acceptable images in 645 after fixing the leak. 6x9 suits better My next encounter is far more pleasant with less drama. I had the extreme good fortune to receive from a kind neighbor, a Rolleiflex T . Lucky me!! Included in the package was what might be a Rolleikin set. After shooting at least two rolls in 6x6 I realized this kit had an adapter mask for 645. It is still in the camera. There is also a mask for the finder. It was a flimsy brittle orange plastic piece that was already broken when I unpacked it. I managed to place both pieces under the frame of the finder. I don't have any bothersome light leaks, only the dilema of landscape vs potrait as you have to turn the camera 90° . I remember I had shot a roll with the Ikonta folder and one with the Rollei. While hanging to dry it was quite obvious which was which. Missed frames, unequal spacing, poor contrast etc. In fact, I haven't used either of the previous models since. Meaning I don't use the Rolleiflex for 6x6 nor the Bessa for 645. I did/do use the Bessa in 6x9 though. Of the four cameras mentioned here three of them them have been obtained through the generosity of others. A professional model Mamiya 645 Super, with two lens no less. has been given to me from a fellow with a guilty conscience who felt he owed me something. This is quite modern compared to the others. It is also demonstrative of the important place the format alone has now garnered. In fact 645 as a format is almost always now considered a professional format. Many top manufacturer's models support outright the format. As with some of the other featured models, I initially had light leaks with this model too. I had a little accident ie "teething problem " when loading for the first time. I fumbled a bit and the roll dropped on the desk and partially unrolled. The second roll was less spectacular and there were not light leaks.. The Mamiya is extremely well constructed. It feels good and solid. It was well thought out. All the controls being foreign to me and not having other cameras in this class to compare to, I can only comment that its size is daunting and I have no strap either so I am frightened to death I will drop it. The viewfinder, while ugly IMHO, is quite functional. Light years above the two folders and supports the 645 format better than the adapted Rolleiflex square view. Pricing a WLF is a humble experience though, when I think of "SLR" this WLF is the look I want. Aesthetically suitable. The prism looks like a periscope Albeit a very functional one! At the end of the day, hands down the Mamiya is the definitive photographic tool, which brings me to ask, why only 15 frames? Reminds me of the Zeiss 532/16 Super Ikonta. ; 11 frames on 120 film. I believe Henry Scherer tried to answer this.