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Author’s Note: This is not the most well-written thing in the world.  I have been pretty tired.  I can’t seem to articulate what I am trying to say, so if you need any clarification, just send me a message!

I have a Bronica S2A that has a problem with the ground glass.  The foam between the retaining frame and the glass, meant to press the glass flush against the camera body, had rotted away into this powdery mess.  Having removed all the foam, I was left with a camera with an unknown focal plane.  Basically, in the S2A, the ground glass position ought to coincide with the film plane when the mirror slides down (not flip up, because this is an odd camera).  If the ground glass position is not exact, what you see will not be what you get on-film.

I prepared a few pieces of plastic (InstaMorph, again!) of varying thickness to test the proper focal plane.  First, I took a picture using a digital camera, focused at infinity.  I display it on my computer monitor and looked through my camera waist-level finder.  As I turned the focus knob, I realized how dumb I was – the image on the screen is an out-of-focus image on a flat screen, not an out of focus, three-dimensional scene.  The directionality of the light has been completely removed in the image taking process, that maps the scene onto a two-dimensional sensor.

So what next?  Well, I know a laser is a collimated light source, and it turns out I had a laser pointer in the apartment.  So I shone the pointer into my infinity-focused lens.  At this point, I noticed that the spot size is not changing much (at least not to my eye), and the intense laser spot was just about to blind me.  I suppose the combination of high power light source and the low resolution eyes made it impossible to judge proper focus (which would ideally present as a diffraction-limited spot).

So what else is there to do?  Well, I recall from high school physics that if you place two lenses along the same optical axis, an object at the focal distance of one will be in focus at the focal distance of the other.  I also know that a camera lens always focuses at the flange-to-film distance (FFD) on the image side.  Therefore, if I focus a camera lens at infinity, and place an object at the FFD, I should have a collimated image on the other end!  A quick try with a lens shows that that is correct.  So I gathered some tools to begin checking the focus.  Here’s what you’ll need:

1. A flashlight;

2. Film camera that can be held in bulb mode (we’ll call this the CC, or calibrating camera).  I am using a Mamiya RB-67 with a Polaroid back, but any camera will do;

3. A camera that you know has accurate infinity focus (AC, or accurate camera).  I will be using my Yashica MAT 124 TLR;

4. Small slide or negative; and

5. Camera to be calibrated (the Bronica S2A).

I am using all waist level cameras to better photograph the focusing.

Step One: Stick slide somewhere near the film plane of the CC and put it at infinity focus

Developed negative stuck to the Polaroid back.

Infinity focus achieved when lens is closest to film plane

Step Two: Set CC to bulb or time mode and keep shutter open
If the camera is put in bulb mode, it helps to use a shutter release cable with a locking knob.

Step Three: Check CC lens position using AC
Since the plane in which the film/slide is stuck will not be exactly the focal plane of the CC, it is necessary to adjust the film/slide-to-lens distance.  Set the AC to infinity focus, and aim it at the CC.  Shine a flashlight through the back of the CC to help visualization.

AC and CC lined up to calibrate the actual infinity focus point with respect to the film plane on CC. Note the flashlight on the right-most edge of the image

After the position of the CC lens has been adjusted, we know that the image coming out of the front of said lens is collimated.  Now switch the camera to the Bronica, and adjust the ground glass position until a sharp image is seen.  Move the camera around to make sure that the focus is accurate over the whole field.

Adjusting the ground glass to attain perfect focus

Now you have a fully serviceable Bronica S2A!

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