JAN Column: Minox GT-E Film Plane Architecture

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What is the characteristics of GT-E film plane ?

-- martin tai (martin.tai@capcanada.com), January 01, 1999



Minox GT-E has a unique film plane unlike other 35mm cameras.

All 35mm cameras have film pressure plate, in most cases, the film pressure plate does not actually presses the film to focal plane as one would think, instead it presses on a pair of outer guard rails; there are another pair of narrower focal plane rails, which are also shallower than the outer guard rails by 200 microns. The pressure plate together with the pair of out guard rails, and a pair of focal plane rails form a 'film channel' of 200 micron deep.

                   pressue plate
                 ||       film           ||

                  Film channel

Since film thickness is about 100-120 microns, the film sits between focal plane rails, guarded on top and bottom by two outer guard rails from moving sideways, but the film many wobbles to the back or to the front in the channel; if it moves to the front, it is on the focal plane (assuming the lens is in focus ), if it moves to the back, it deviates from the focal plane.

Typical 35mm camera film plane channel dimensions:

  1. Distance between two outer guard rails: 35 mm
  2. Width of 35mm film : 35mm. The 35mm film sits inside the two outer guard rails.
  3. Distance between two Focal plane rails : 28 m
  4. Film pressure plate width : 37 mm, covering outer guard rails but taller then the focal plan rails by 200 micron.
  5. Channel depth: 200 microns

The 200 micron channel depth is more or less standard in today's 35mm cameras be it SLR or P&S. Some camera has more precise film channel than others, which means that the two rails are highly parallel, and the channel depth deviates very little from 200 micron.

I found that Minox GTE's film plane architecture differs from any other 35mm camera and immediately recognized that GT-E had take a design theme from Minox ultraminiature cameras-- high precision leewayless film plane.

Minox GTE has four focal plane rails of equal height its pressure plate presses directly on the film to four focal plane rails.

                     pressure plate
                 ||    film               ||

              GT-E:  four equal height focal plane rails


GTE has no "film channel" The channel-less film plane rails contribute to the exceptional sharpness of Minox GT-E (and Minox 8x11) cameras.

GT-E focal plane rail dimensions:

  1. Distances between two outer focal plane rails: 33 mm;
  2. 35mm film width=35mm.The film sits on top of outer rails, not inside them.
  3. Distances between two inner focal plane rails: 27 mm;
  4. Pressure plate width 37 mm;
Conclusion : Minox GT-E pressure plate presses on film, which in turn directly presses the four focal plane rails -- hence the focus error cause by 200 micron film channel so common in many cameras is eliminated

Let us take one example. Suppose we use Agfa HDC 100 film which has a film thickness of about 125 micron; in ordinary camera the film plane error = 200 -125 = 75 microns.

The effect of this film plane error is reduction of resolution of a lens. If a lens has resolution of 100 lpmm at F8, due to this 75 micron uncertainty, the resulting resolution may be any where between 50 lpmm or 100 lpmm, depending upon where the film stays in the film channel, if it wanders up front and in close contact with two film plane rails, then it may resolve 100 lpmm; if it wanderss back, deviates the focal plane by 75 micron, then its resolution will drop to 50 lpmm.


The deterioration of sharpness due to film plane error is even greater at f2.8, and 100 lpmm may drop to 30lpmm.

Hence with common 35mm cameras, the potential of lens cannot be fully realized.

Since Minox GT-E has no built in film plane error, the full potential of it excellent Minoxar lens will be delivered in full (Of course, you still have to focus accurately )

Minox GT-E and Minox 8x11 cameras share the same characteristics of precision film plane and excellent sharpness.

-- martin tai (martin.tai@capcanada.com), January 01, 1999.

The only 35mm cmaera with a flat film plane is Contax RTS III, which though as conventional film channel, but has a built in vacuum film back, which pull the film tightly against pressure plate, and release it while film advances.

-- martin tai (martin.tai@capcanada.com), January 09, 1999.

As for the question of why earlier Minox did not use the same type of film plane as GT-E, the answer is: they followed the norm-- film channel system. Why picture coming back from lab were not all sharp ?

There may be many reasons, but film plane is not one of them. Because zero channle film plain makes camera sharper then film channle cameras, but without it the sharpness is still well above the acceptable limits ; the most probably causes of lack of sharpness, from my experience may due to sloppy 1 hr labs-- they don't always take the time to align their system regularly-- some times I had to send the prints back for rework.

-- martin tai (martin.tai@capcanada.com), May 09, 1999.

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