Considering...

A year ago I returned to my film roots in photography by acquiring a Hasselblad 500CM and a couple of lenses. I've very much enjoyed composing and image making with the 6x6 format. Along the way, I've noticed some wonderful work being done by folks using pinhole cameras. I especially enjoy the 6x12 aspect ratio as it seems like a natural pano-evolution of my square format endeavors. Accordingly, I have been getting serious about dipping my toes into the pinhole waters. I've read some comments that wide aspect ratio pinhole cameras need to be kept level and the horizon bisecting the middle of the frame to avoid distortion on the edges. If i've understood this correctly, it reminds me of similar considerations I have to give my Nikkor 16mm Ai-s f/2.8 lens. All that being said, I've seen numerous images made with 6x9, 6x12 and 6x17 pinhole cameras where the horizon appears to be above or below the centerline without any noticeable distortion.

Have I misunderstood what I've been reading, or is there a few tricks/techniques that can be employed to avoid serious distortion on the edges?

Could this be a function of the pinhole size being used?

Your comments and suggestions are appreciated. BTW, I've got my eye on a Zero 612F camera to get this new photography addiction started properly

The big fun with pinhole is to build your own.
I like the 6X9 or 6X12 cm and as with my wooden box 4X5 inch format. I like where the distance is the same on the edges as it is from the centre i.e. a curved interior.
I like too using paper negs. See this link with the preceeding 2 images along with 2 images further along called Pin 2 taken with (my self made wooden box pinhole camera): http://www.ipernity.com/doc/987971/37687396
Good luck with the project.

A pinhole panorama capture on curved film usually looks best of the horizon bisects the image. Otherwise the horizon will be curved. If the film plane is flat, a straight horizon will remain flat.

Examine your extant skills as a photographer with traditional imaging machinery, namely the Hasselblad, then consider if some or all of the images shot on the Hassie would translate well, better or not at all in the pinhole format. I speak from long experience using the Zero Image 6x9 multiformat and found personally that what I excelled at with 35mm and 6x7 didn't necessarily translate the same in the pinhole format with its technical limitations and requirement for critical analysis and weighting of a scene. One terrible thing I am seeing more of is people buying a pinhole and demanding the image be sharp (!). If that's not achieved, damn it, they wield the USM trinket in PS to make it sharp. This is not in the spirit of traditional pinhole photography at all, where the camera will feature a good old fashioned soft focus effect but excellent definition. That's a bit different to the Hassie's trademark look of sharpness, hmm?

I have never seen any distortion in my pinhole photography, but then again I have never tilted the camera up or down other than to align it horizontally or provide some margin for framing error. Of course the wide angle of view will cater well to sweeping landscapes, but you have to remember the extreme depth of field can bring you undone with objects that "appear" at a respectable distance but which result in the end being too far away and lacking the planned significance. After a while of use though you do get the hang of managing the extreme depth of field.

I think you should talk to a photographer using a Zero Image 6x12. I think though your major problem will be progressive light fall off with the very wide angle, more pronounced with the 6x17.
If going the DIY route, you can introduce some interesting effects if you "roll your own", but if you want a camera with a technically refined pinhole (0.07" I think?), ZI delivers. You can also go the Zone plate way; indeed, some ZI cameras allow both pinhole and Zone plate work.

I agree with Poisson that sharpness is not usually the goal in pinhole, and some people even like the curved horizons and distortions from a curved plane camera. What Jim said is right, if the film/paper plane is curved then the horizon will be flat only if the camera is level. You can, however move the horizon up or down ( include more ground or sky ) by building in "rise" or "fall". By moving the pinhole itself up, the horizon will be lowered ( more sky ). One neat feature of a pinhole camera with the pinhole above center is that you can just turn it upside down and then you get "fall". The camera will still need to be level to avoid curved horizons. Have fun with it!

I apologize if my initial post seemed to indicate I was looking to produce pinhole images with the same sharpness as my Hasselblad - I am not. I not only accept, but embrace the "soft" look that is inherent in pinhole photography. It is one of the reasons why I find pinhole images so wonderful. If I wanted razor sharpness, I would not consider going lensless. I simply wanted to better understand how distortion is introduced into pinhole images and how that relates to keeping the camera level. I appreciate the comments thus far and hope others chime in.

I'm with those who say light fall-off is the most notable feature of ultra wide pinholes. At least on a flat film plane, as you move away from the central axis, the pinhole to film distance is increasing, an effective shrinking of the aperture. On top of that, as you move off-axis, the projection of the pinhole becomes an ellipse, shrinking to closed at 90º (even sooner if the plate containing the hole has any significant thickness).

As to distortion, a pinhole is essentially ray tracing, simple proportionality between subject and image. The distortion comes about when the film is not in a flat plane. There's some delightfully crazy stuff shows up when film/paper is rolled up in a tin can and used with a pinhole. That's all about conic projections and such. It's still strictly proportional ray tracing, but the image plane is wandering around and affects the numbers.

Anyway, it's all grand fun. I mostly get active with pinholes for Worldwide Pinhole Photography Day, the last Sunday in April each year. But in 2014 I did do a couple of extra projects over the summer, playing with a home built 8x10 and X-ray film. Contact prints from some of that wound up in a juried art show, and two are still in a museum exhibit on the history of a canal in this area, so warm fuzzies abound.

Some people make pinhole cameras that are beautiful works of art in themselves. The camera itself becomes part of the "art" in the whole process! Some people buy pinhole cameras made with precision and "optimal" pinholes.

Some people ( me! ) like to make pinhole cameras as simply as possible from common everyday items... for me it is not about "look what I can do without spending money" but it's about utter simplicity. There's something almost primal about it -- the light goes through the pinhole and lands directly on the paper. Sometimes I feel like a kid being astonished by a photo coming out of an oatmeal tin. There's something else about it too.... it seems "not so serious", and that makes it easier to play around and take chances and try different things just to see what will happen. It's a 25 cent piece of photo paper in a coffee can, and that makes it more like play and less like "serious photography". There's no disappointment ( usually ) when it's not what you expected, and there are lots of surprises and serendipity. And fun, of course!

Pinhole photography is a different world. It usually is not sharp. Not that that is bad, just different. Enjoy it. Try it in color too.

In the last 5 days I have shot more on my ZI 6x9 pinhole (in the 6x6 format, which should please the Hassie-hauling Mr. Sirius...) than on 6x7! In 2010 I bushwalked for 4 days carrying nothing more than the pinhole camera, tripod and Sekonic meter; the resulting images are indisputably benign — places seen before long ago with an ordinary camera, but all were printed and framed, and are memorable and cherished for the very different, often curious visual oeuvre that is endemic to the pinhole genre.

In regards to keeping the camera level — a subject that does worry prospective users of ZI cameras, there is no visual cue other than a bubble level on the top (two, in the case of the pano cameras). A tripod head with bidirectional levels is very handy too. You do get the hang of "sighting" the scene and levelling the camera visually using the top plate; this works for me as the tripod head I used for the ZI 6x9 does not have bubble levels. Of distortion, this link to Zernike Au's own images made on the 6x9 camera (handheld!) show you how profound the distortion is deviating from pependicular shots. This distortion is definitely not something I chase in pinhole (or orthodox 6x7 work)! This is what I meant by saying earlier that I have never seen any distortion ... not in my images!

There was a post here on APUG a few days ago featuring the work of a pinhole photographer with a number of quite atmospheric and well-thought out BW scenes. I can't recall what format he was shooting in -- possibly pano.

The OP should note that some recent Zero Image pano cameras (and LF?) now have a filter adaptor on the front, which could be useful fitting a centre-spot filter to counter the effect of light fall-off, though it will still be pronounced at 6x12 and 6x17.