Diy pentamirror blueprint

I'm making a large format camera for novelty use as a camera obscura and I want to flip the image so it isn't upside down. It's ok if left/right is backward so I think a simple parallel set of mirrors should work.

I need dimensions and angles and to size it for my lens. I guess I could just blow up a diagram from wikipedia but not certain that's accurate.

The plan is to put the mirrors in front of the lens and there won't be a viewfinder lens. That might mean I could eliminate the mirror that flips in an SLR because presumably the viewfinder lens also flips the image.

In the hundred plus years of large format cameras with ground glass and a black cloth over your head, I can't find any example of this. People always tolerated an upside down focusing screen. Hopefully that's just because they didn't want extra mirrors degrading the image and there aren't any technical reasons this won't work.

Baynatives wrote:

The plan is to put the mirrors in front of the lens and there won't be a viewfinder lens. That might mean I could eliminate the mirror that flips in an SLR because presumably the viewfinder lens also flips the image.

In the hundred plus years of large format cameras with ground glass and a black cloth over your head, I can't find any example of this. People always tolerated an upside down focusing screen. Hopefully that's just because they didn't want extra mirrors degrading the image and there aren't any technical reasons this won't work.

I decided this is physically impossible, or at least impractical and difficult to experiment. The lack of a viewfinder lens means one more image flip is removed. If I went from 3 mirrors to two, you would have to stand in front of the lens and block the view. Maybe 4 lenses would work but it's so convoluted.

Another issue is the viewfinder lens is much smaller than the capture lens, so each mirror is smaller but I want a full size image, so the ordinary pentaprism layout may not be valid. Each mirror would have to get larger and it would either not be geometrically possible or would be ridiculously bulky. I'm not certain but this definitely muddys the waters.

My solution is to make a mini camera obscura to view the image on a round table with the lens on the roof and a single mirror angling out to the landscape. But instead of walking into the camera, I cut a peep hole on the side of the roof so you can look down on the projected image. It's not as impressive but it works and you can put your cell phone camera on the peep hole to capture a cool shallow depth-of-field image and play with tilt/shift. Plus, the image is projected onto a nice solid white surface which looks a whole lot better than a ground glass translucent focusing screen. The ground glass always has a hot spot in the center when captured by a camera.

Here's my previous large format play camera where you can see the hot spot: https://www.flickr.com/photos/edgehill/albums/72157626735509238

Here's a camera obscura at our store that a friend built, if you aren't familiar: https://www.instagram.com/p/BvHtk-0hrJc/

One more thought: maybe the viewfinder lens doesn't flip anything. Eyeglasses don't flip the view. The peephole is much less impressive. Maybe a viewfinder lens in the peephole would make it more compelling? Definitely projecting onto a solid screen is more impressive compared to the translucent ground glass focusing screen but also having a big 16-inch image in front of your eyes is more impressive than peering through a peep hole.

I used to have an old super-8 movie projection screen from the 1950's which had millions of tiny glass beads glued to the surface of the canvas roll-up screen. That helped to brighten the image. Brightness is a big deal. That's why I think this is the right approach. OTOH, if I could get the pentaprism idea working, that would be awesome. I always like re-inventing the wheel!

OK, so I built it without the penta-mirror to ground glass, but rather: projecting on a solid white screen tabletop. One 45 degree mirror points the vertically oriented lens at the scene and a peephole on top lets you view the projection screen tabletop just like a walk-in camera obscura room.

The solid white screen makes a much brighter image than translucent ground glass without the need for covering yourself in black cloth and the peep hole lets you at least partially correct the upside down look, which was very disorienting. The peep hole is not very satisfying compared to a large ground glass but the best use is to place a cell phone camera on the peep hole. It's also not very satisfying to watch the phone's screen but the photos you can capture are very captivating shallow depth of field "tilt-shift" / lensbaby / lomography images.

The technique can be gimmicky lensbaby shots but can also be used in a serious way for selective focus as in a shallow depth of field portrait lens with soft dreamy background. An angled plane of focus can be goofy lensbaby kitch but can also be used to place emphasis on select parts of a scene in pretty extreme ways. And I think the image quality can be pretty nice, using oddball old lenses. This build uses a cheap little old broken telescope lens with an 18" image circle but I've got another huge old movie projector lens which makes about a 6-inch image circle of very fine quality. I hope to build another camera with that which works with cell phone viewing and will be much more portable. The current build sits on a rolling cart, though the basic 18" black box cube can be toted around.

I believe I can get much sharper and more contrasty image than this early test. Note the extreme twisting of the plane of focus.

The basic 18x22" black box camera sitting on a rolling cart. 45 degree mirror on the top, with the vertically oriented lens projecting onto the tabletop below. A peep hole near the lens lets your cell phone see the image.

The white screen inside is hanging by 4 ropes which go through drilled holes and are tethered off on screws outside. A better solution would have fishing pole reels to adjust each rather than tethering. An even better setup would be a potentially complicated pulley system that brought all the fishing reels together in one place on the top and had a master reel for adjusting overall focus distance using all 4 ropes together.

Focusing a traditional large format or tilt/shift camera like this is complicated, slow and counter-intuitive. Focusing involves turning numerous knobs and dials in sequence, then going back through the sequence to fine tune. The truly ideal setup would be to bring all those ropes and pulleys to an airplane pilot's steering wheel to make those 3 dimensional adjustments intuitive.

Get a good lens and hook up a DSLR and this could take some extraordinary images. Unless you are doing huge prints this setup should be able to take some extraordinary images on a cell phone by utilizing inexpensive old odd-ball lenses. I've experimented for years with how to use these old lenses with relay lenses and whatnot and I think this is the most useful system yet. The image quality projected onto a good screen should be great for digital capture, making full use of the lens' original capability.

Another couple technical details: The peep hole is located in front and left of the lens. This means the phone is pointing at the screen at an odd angle which could mess with the focus alignment and geometric trueness. Not a problem if it's easy to fine tune the focus with a pilot's steering wheel.

The way it works is you stand on the left side facing away from the subject (because the image is inverted) and use your right eye (or right hand to operate the phone).

I've talked about tilt focus but not shift. Shift is about correcting parallel perspective for architectural photography. The 18" image circle for this camera is pretty generous, to allow for panning across the projected image till columns are plumb by panning over to the side of the image. This is a matter of how wide the angle of view is on your phone and how tall or low you can place the lens. We are getting into obscurities here: #cameraobscura

The original purpose of tilt/shift was to maximize depth of focus, but this exercise is potentially more about retro gimmicky effects.

The classic reason for tilt-shift was a field of wildflowers with a mountain range in the background. Tilt the plane of focus so that the foreground flower closeup is in focus and the distant mountains in the top part of the frame. Other objects outside of that plane will be out of focus but it doesn't matter.

The original utility was to achieve deep depth of field through cheating by tilting the plane of focus. These lenses have shallow depth of field but when you stop down the aperture to increase the depth of field (making it darker with a smaller opening), then you get a nice sharp image foreground and background in focus in spite of the lens' natural shallow depth of field.

The gimmicky approach takes the opposite approach by emphasizing the weaknesses of the lens.

Thanks, I see now how that reflex viewer idea works with one more mirror and a viewfinder. I've actually seen it on a medium format camera once briefly but didn't put the pieces together. i was thinking about putting the extra mirror in front of the lens, which didn't work but now I see the mirror goes behind the focusing screen with an added viewfinder lens. The viewfinder lens is another flip to correct for upside down which was left out in the configuration with the added mirror in front, between the lens and subject. I will be setting up something like that in the future. That's great how it eliminates the need for a black cloth over your head.

Without that revelation, I went ahead with the opaque white 'movie' projection screen option instead of ground glass. Viewing by eye is awkward but set a cell phone on the peep hole and it works great. The tilt/shift/marionette focusing mechanism also works flawlessly and intuitively: https://photos.google.com/photo/AF1QipN0eIik_m1aGV1iBaMMEOtVcaxgPwnJvcpAl2cU

All that remains is to run a cable around the top of the box with a clamp lever arm to lock in the 3-D marionette focusing system ropes. Tightening the cable will push down spring loaded brakes to set all 4 rope locations - simple solution.

Note in the video that one of the ropes would have been right in the middle of the lens view so it had to be re-routed with pulleys on a plywood U-shaped frame.