Converting a 4×5 analogue Technical Camera to digital


Converting a 4×5 analogue Technical Camera to digital.

Intro

I love to shoot architectural structures; a Technical Camera (TC) is a necessity-enabling raise, fall and shift functions for correct registration.

cambo1Technical Camera’s

Many of those TC’s are really for studio work and rather bulky, my preferred TC would be an Arc Swiss or Cambo.  A new TC is not really an option, architectural photography is not my full time occupation, hence the return on investment could be years if ever. I opted for a solid second hand analogue TC.   There are plenty of TC’s on the market for reasonable prices.  Finally I found a Cambo DS Wide from 2006 for as little as 2700 euro.   With a  Sneider super angulon 5.6/58mm and an 4×5 analogue back with glass plate, view finder and documentation. A good buy for a camera in such an excellent state.

 

A cunning plan, a super-duper  virtual  digital sensor

The intend is to convert the Cambo, more specific replacing the glass plate and mound a Phase One IQ-140 back on it (mid format digital back).  The digital sensor of the IQ-140 is  44 mm by 33 mm. The Cambo with Lens provides an image circle that easily covers a  100 mm by 120mm surface.   The idea is to use the raise/fall and shift left/right capabilities of the TC  to expose the digital back in multiple locations. The Cambo allows for a 20mm movement in each direction from the center place.  By taking a shot in each of the 9 positions, I can acquire a real large digital picture of 84mm by 73mm or virtually double the mid format digital sensor format with an abundance of pixels. Lens cast and luminance drop off will certainly be an issue, however that can be compensated with an LCC adjustment.  By taking a shot of the white LCC Plexiglas plate for each new position of the back.  Lens Cast Correction will be executed in post processing (Capture One 7 Pro) prior to the stich operations of all 9 shots.

shifts

Mounting a Phase One Digital back.

adaptorI contacted Cambo and explained what I wanted to do,  so far there was no problem and a special adaptor plate WD614 was available for the PO Digital Back. Not cheap, 870 Euro for a piece of fine machined aluminum.   Two weeks later I received the adaptor. I removed the glass plate fixture and mounted the adaptor.  It all fitted perfectly.   Finally I mounted IQ-140. Ready for the first test.   What a disappointment, it was impossible to acquire sharpness. No matter how much I turned in and out the lens.

Back to the drawing table

What could be the cause of the lack of focus, there isn’t that much that can go wrong. It’s a matter of simple physics as long as the IQ-140 digital back is in the same exact place as the glass plate.  I compared both distances “glass versus adaptor plate”.  It turned out that the adaptor plate was 10mm short, no wonder that it didn’t work.

glasbackI contacted Cambo and explained the situation; very quickly they confirmed that the wrong adaptor was delivered. The WDS614  is matched for “Digitar lenses” . Since I used an analogue Lens the focal plane is much further then for a “Digitar Lens”, as what I expected.  I got two options, change the lens plane and by a new “Digitar lens” while keeping the adaptor or return the WDS614 and obtain the WDS508 matched for analogue lenses.

It’s not just a matter of money.

A new Digitar Lens will cost 3500 euro, were as the new adaptor plate for the current analogue lens is 330 euro cheaper then the current adaptor plate.  However the Digitar Lens will not give the freedom to move around the back as much as with the analogue lens. Since the area of coverage is tuned towards the sensor format.

Lens

Some will say that the resolution factor of the analogue lens will not be good enough for a digital sensor as it is limited in the amount of lines it can resolve (film 100x120mm versus a CCD at 44x33mm) , it would require a resolution power which is 3x bigger, expressed in lines per millimeter.

cop7_iconChromatic Aberration is another issue. Yet my plan is to use the analogue lens and a virtual large CCD (IQ140) by moving the IQ140 into nine different positions. That should hopefully compensate the resolution factor issue. The aberration and light fall off will be compensated through the LCC adjustments in Capture One Pro 7.  The new adaptor plate is on order, can’t wait to see if my theory will work.

Waking up the Digital Back.

IQ140

The next challenge is how to get he IQ140 Digital Back awake.  Phase One backs use a dormant architecture, only when a picture is imminent the back is powered on by a Wake-Up signal.  Normally that is an electronic and automatic communication between the digital back and the Camera Body.  However in this case we are talking about a dumb piece of precise aluminum, the Cambo.  There are no electronics.  The only available signal is the Exposure / Capture pulse from the Copal shutter in the lens.  The Copal shutter provides a DC closure during the shutter “open” state e.g. 1/125 sec. (Shutter is in the lens).

Flagging the Digital Back when the capture/exposure starts and stops.

The IQ140 digital back requires a second signal called the  “Capture Signal” .  Identifying when the shutter is open and when it closes, or the end of the capture. The Copal shutter in the analogue lens does exactly that. It places an electrical short for the duration of the shutter time e.g. 1/60sec.

Combing “Wake-up” and “Capture” signals for the Digital back.

The Digital Back needs both, first a wake-up pulse to activate the Digital back and followed within 5 seconds the Capture signal for the duration of the shutter time.  Two states with one control point “ the Copal shutter contact”.  The solution is a “two-step cord”, connecting the Copal shutter contact to the Digital Back with a parallel manual Wake-up button.

cord1

The method is simple:

  •  Measure the light and adjust the camera settings (diaphragm and shutter speed). The Cambo cannot measure; it is done with a handheld light meter.
  • Adjust the ISO on the digital back if needed and set the wake-up speed to normal (on the IQ series).
  • Arm the shutter on the lens
  • Depress the “Wake-up” button on the two step cable (this makes an electrical short and the back wakes up.
  • Release the shutter within 5 seconds after the wake-up action.
  • The Copal shutter creates a second electrical short circuit for the duration of the shutter “open” time and is used as the capture signal on the Digital Back.

Lots of fun isn’t it.  That is what real photography is all about.  I hope to be able to show some captures real soon (once I have the correct adaptor plate).

The future will tell if I had it right or wrong, although I am convinced that it will work.

Thanks for reading,

Steve

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