Editorial: Parallax Correction

A couple of comments on the Kickstarter comments page asked about parallax correction, which is an issue characteristic of all TLR cameras. There are a number of ways to correct for parallax error - moving the entire viewfinder image, Rolleiflex-style, a moving needle to note the top of the frame, a la Mamiya, and finally fixed correction markings in the viewfinder. To minimize complexity, Duo will use the last option. 

But the problem still remains of how to calculate the error itself. We could of course measure it empirically by shifting the camera up and down by the lens separation of 75mm, or by comparing the VF image to the one projected onto ground glass in a roll film back. Either would work, but why not use some math to characterize the parallax error throughout the entire focusing range?

Editorial: Cameralogical Baggage

Although PetaPixel is the third-largest referrer to Duo's Kickstarter page, the comments section is a pit of derision (welcome to the internet). That said, this blurb of soaring praise raised a question that I couldn't help but ponder the answer to.

Selecting a Roll Film Back

Duo's roll film back is an optional extra that allows you to mount a 2x3 Graflex roll film back on the camera. This greatly expands the capabilities of the camera by allowing you to compose images on 6x4.5 up to 6x9 frames.

Graflex RH12, courtesy of Dr. Rawhead

Compatibility: 

Any roll film back designed to fit Graflex type backs will fit the adapter. Backs that have been confirmed to fit are as follows:

• Graflex 
• Lever advance models: RH8, RH10, RH12, etc
• Knob advance models: "Graphic" 22, "Graphic" 23
• Horseman (e.g. Horseman 6x9 and 6x7 holders, in 120/220 variants)
• Mamiya RB67
• 6x7 and 6x4.5 lever advance
• Motor advance models not yet tested

I'll continue to add to this as I try new backs - this list is by no means exhaustive, and there is a good chance that a back not listed here will work.

NOTE! The Graflex and Horseman backs sometimes have a big rectangular adapter that will allow you to mount the back on a 4x5 camera. These will not work. We're looking for backs meant to fit 2x3 cameras, and generally the side facing the lens will look like this:

Horseman 6x9 Back (no longer in my possession)

Lastly, a video on how the adapter works:

After-the-break bonus: first photos on Duo using a roll film back.

Selecting a Polaroid Back (Updated Jan. 25)

UPDATED: January 25th (Polaroid 600SE backs are CONFIRMED to work.)
UPDATED: January 20th (More photos added of compatible Polaroid backs.)
UPDATED: January 16th (Polaroid 600SE backs SHOULD work too - currently testing)
UPDATED: January 15th (Mistake corrected - Mamiya M80 backs don't work.)

The Duo kit also requires a Polaroid back (provided that you want to shoot on instant film!), NOT included in the kit.

Compatibility:

Duo is designed to use a Mamiya Press Polaroid Back Type 1, 2, or 3 (Model No. CB100, CB101, CB103). These backs were originally used on the Mamiya Press/Universal cameras. They were also often sold for use on the Mamiya RB67 with a "P Adapter." Some useful search terms for finding these backs on the used market are:

• Mamiya Press Polaroid Back
• Mamiya Universal Polaroid Back
• RB67 Polaroid Back

Duo will also work with Polaroid backs from the Polaroid 600SE camera, which was a nearly identical clone of the Mamiya Universal. You'll want to look for a "Polaroid 600SE Back".

IMPORTANT things to be aware of:

1. Third-party (e.g. Proll, NPC) Polaroid backs are unlikely to work on Duo without significant modification.
2. Polaroid backs made specifically for the RB67 Pro-SD will NOT work. They have an integrated "P Adapter" that cannot be removed. See photos in "Additonal Notes, Part 4", below.
3. There are some backs with wire closures (Types 1 and 2), and others with plastic closures (Type 3). Both will work.
4. It was originally stated the Mamiya M80 backs would work. This was a mistake, the wrong model number was used. They rely on extinct film Type 80 film that no longer exists!

Again, slight modifications are required to attach the Polaroid back to the TLR. All you need are some screwdrivers, and you'll be done in no time (~10-15 minutes).
   

Additional notes after the break.

Selecting a Lens

The Duo kit requires donor lenses that are NOT supplied in the kit. Luckily, suitable lenses are easy to find on the used marketplace. After reading this entry, I hope you'll have a better idea of the lens you should buy to complete the camera.

First off, you need a Mamiya-Sekor 105mm f/3.5 lens pair. These lenses were originally used for the Mamiya C-series TLR cameras (e.g. C220, C330s, etc).

Some disassembly required.

There are four (4) variations of this lens! It is important to understand that there are some subtle differences between each lens, but all will work on Duo, and each is easy to identify:

• "Chrome" - The lens and shutter are chrome. 
• "Black" - The lens and shutter are black, and have no red engravings on the lens ring.
• "D" - The lens and shutter are black, and has a red "D" engraving on the lens ring.
• "DS" - The lens and shutter are black, and has a red "DS" engraving on the lens ring.

The biggest difference between the lenses are that the "Chrome" and "Black" lenses are a Tessar optical design (4 elements, 3 groups), and the "D" and "DS" lenses are a Heliar optical design (5 elements, 3 groups). The following are two photos taken using a "Chrome" and a "D" lens:

"Chrome" lens

"D" lens

Some conclusions drawn from these and other photos:

1. The Tessar lenses cover the whole Polaroid frame at infinity focus, with minimal vignetting.
2. The Heliar lenses are subject to vignetting of the corners at infinity focus (less of an issue for closer subjects). 
3. The Heliar lenses seem to have better color rendition and contrast.

And some totally subjective conclusions:

• Due to the different optical designs, the "D" and "DS" lenses require an additional spacer when building the camera (a trivial addition).
• The "DS" lenses have nice features like a DOF calculator/preview, and a self-timer on the shutter.

In short, all of the lenses discussed above will work. The Tessar "Chrome" and "Black" lenses provide better frame coverage at the expense of contrast, and vice versa for the Heliar "D" and "DS" lenses. That said, there should be no vignetting problems on the Heliar lenses when using roll film.

Now what do you do once you get your lens? You get to take it apart, of course! This procedure is easily completed in ~20 minutes with basic hand tools, and is completely reversible.

Editorial: The things I've seen through these. . . lenses

Duo was never intended to be a trophy-case camera. It was specifically designed so that it could be used in the field, without a tripod or a whole slew or support equipment. A few features make it particularly useful for field work:

• It's light! It weighs in at just under 3 pounds with a loaded Polaroid back. 
• It's quick! Relatively fast lenses (f/3.5) make it easy to shoot handheld in most situations.
• It's robust! Throw it in a bag or sling it over a shoulder - there isn't much to break.

Alright, enough of sounding like an infomercial. Click on through for additional photos.

Welcome!

Think of this as the "overflow" page for Kickstarter - the repository for information that would otherwise be unwieldy to post on that page or over on Flickr. Informationals, tutorials, FYI's, etc - this will be your source.

In the off chance that you've stumbled across this page without knowing whatever it is that I'm talking about - I'm talking about Duo, the twin-lens-reflex camera designed for Polaroid film. Enjoy!

Self-Portrait of Duo Prototypes: Mk 1 right, Mk 2 left.