Pentax HippoCam: Parts is Parts

Here is a sneak peak at the parts I'm using to build my (version 1) Pentax HippoCam, which is based on the Fotodiox RhinoCam™.

The main components are:
• Front plate (bears the Pentax 6x7 lens)

• Rear plate (bears the ground glass for composition and the PK mount for camera attachment

• A mechanism that allows for the sliding of the rear plate for positioning either the ground glass or the camera behind the image circle
• A material that prevents light leaks into the optical path
• The lens receptor mount taken from the Pentax 6x7 body

• The ground glass from the Pentax 6x7 body

• The prism from the Pentax 6x7 body
• A PK mount that will stand off the rear plate to hold the camera sensor at the proper flange to sensor distance.

• (not shown) A way to hold the ground glass and prism that will also stand off the rear plate to facilitate image composition

• (not shown) A way to mount the whole enchilada on a tripod

Wandering around my local Menards (home improvement store) with an eye to finding components, I spied these cool little bamboo breadboards. They are approximately 1/4" thick and strong because they are made up of bamboo strips that are tongue and grooved on the ends. In addition to strength to weight ratio, the bamboo is smooth and slick, and low friction is one of the things I'm looking for, since the back plate must slide back as the overlapping images are taken (and also to position the ground glass over the lens opening).

I decided that I would have the fewest alignment issues if I made the front plate and the back plate be in contact with each other (or nearly in contact).

The back plate will slide in the aluminum trim channel that was made for 1/4" plywood (which is dimensionally  a bit under 1/4"). This will be cut into two equal sections that will be mounted on the backside of the front plate. The rear plate will then slide into these channels from the side and be able to move horizontally.

I considered making a rabbet or dado in the backside of the front plate to accept the aluminum trim channel, but decided instead to simply bolt it on with no rabbet or dado. This means that there will be a gap between the front plate and the back plate that is equal to the with of the sidewall of the aluminum trim channel (approximately 2mm).

To fill that gap, I will be using a piece of black foam sheet that will be sandwiched between the front and rear plates. It will probably be affixed to the backside of the front plate (the non-moving part) with spray adhesive. These 12' x 18" foam sheets (called "Silly Winks Foam Sheets") were found at my local Hobby Lobby store and cost under $1 each. I bought two sheets, one 2mm thick and the other 3mm thick. The 3mm will need to be compressed a bit, but will be used if I can still get the backside plate to slide easily, since it will provide better sealing.

In the photo above, I've simply set the 6x7 ground glass and a m42/PK adapter in the approximate positions of where the real components will go. The PK mount will probably be made from a old PK 2x teleconverter. It will need to stand off of the back plate approximately 12mm to allow me to twist on the Pentax K-5 II (with its protruding prism and grip). The total flange to sensor distance must equal that of the original camera, which was 80mm.

The ground glass holder is a bit up in the air. I originally planned to use the original ground glass holder from the original camera, but this will require some serious sawing and machining of the piece. I may still do that (I work for a machine shop, although I'm not a machinist) or I may decide to design a simpler box that accomplishes the same purpose. The original Pentax 6x7 prism will be held in the holder by mechanical means, perhaps as low tech as a bit of elastic affixed to the back plane. The eyepiece will point upwards and composition can then be done without a dark cloth (as is necessary with the prism-less Fotodiox design.

Shown in the photo is my Super-Multi-Coated Takumar/6x7 200mm f4, however I plan to use this mostly with a Pentax 6x7 55mm f4, which should be arriving tomorrow.