Doug Kerr
Well-known member
Often in connection with discussions of alternative camera configurations, we hear mention of the pellicle mirror, sadly often under an erroneous conception of what that means.
Pellicle means, essentially, "film" (from the Latin for "skin"). A pellicle mirror has a reflective substance deposited on a thin transparent film, in contrast to the more substantial glass or plastic substrate we more often see in mirrors.
A well-known use of a pellicle mirror in cameras is in the Canon Pellix SLR (introduced in 1965), which took its name from the unique type of reflex mirror it used. The significant design factor, though, is that the reflex mirror doesn't move. The image is viewed by reflection from the mirror (as in conventional SLR cameras), but the image is taken through the mirror (much as the autofocus sensors observe the image in Canon EOS cameras). I'll tell you why a pellicle mirror was used in a moment.
In any case, because of this context, many people assume that "pellicle" refers to a reflex mirror that doesn't move. But it only means that it is on a film substrate.
Why a pellicle mirror on a "fixed-mirror" SLR? If we have a conventional glass mirror, as the rays that will form the image pass through the oblique "glass plate", they are displaced slightly to the side (down, in the arrangement we would usually have in an SLR). The amount of displacement depends on the thickness of the glass, the angle (45°, no doubt), and the index of refraction.
But the index varies with wavelength, and thus image components of different wavelength are displaced by different amounts, leading to lateral chromatic aberration.
By making the thickness of the mirror very small, the magnitude of this is reduced to an inconsequential amount.
A pellicle mirror can be attractive in a more conventional SLR as well. Typically, a pellicle mirror will be much lighter than a conventional mirror. Thus it is easier to move it quickly in SLR operation.
Aren't you glad you asked?
Next week: "The greater trochanter and its management when replacing the coxal articulation."
Pellicle means, essentially, "film" (from the Latin for "skin"). A pellicle mirror has a reflective substance deposited on a thin transparent film, in contrast to the more substantial glass or plastic substrate we more often see in mirrors.
A well-known use of a pellicle mirror in cameras is in the Canon Pellix SLR (introduced in 1965), which took its name from the unique type of reflex mirror it used. The significant design factor, though, is that the reflex mirror doesn't move. The image is viewed by reflection from the mirror (as in conventional SLR cameras), but the image is taken through the mirror (much as the autofocus sensors observe the image in Canon EOS cameras). I'll tell you why a pellicle mirror was used in a moment.
In any case, because of this context, many people assume that "pellicle" refers to a reflex mirror that doesn't move. But it only means that it is on a film substrate.
Why a pellicle mirror on a "fixed-mirror" SLR? If we have a conventional glass mirror, as the rays that will form the image pass through the oblique "glass plate", they are displaced slightly to the side (down, in the arrangement we would usually have in an SLR). The amount of displacement depends on the thickness of the glass, the angle (45°, no doubt), and the index of refraction.
But the index varies with wavelength, and thus image components of different wavelength are displaced by different amounts, leading to lateral chromatic aberration.
By making the thickness of the mirror very small, the magnitude of this is reduced to an inconsequential amount.
A pellicle mirror can be attractive in a more conventional SLR as well. Typically, a pellicle mirror will be much lighter than a conventional mirror. Thus it is easier to move it quickly in SLR operation.
Aren't you glad you asked?
Next week: "The greater trochanter and its management when replacing the coxal articulation."