When the users look inside the eyepieces, at a glance they are able to obtain a good perception of what is observed, quickly evaluating the sample.
When the image is projected to a monitor/screen, the situation is different: people is often expecting to have the exact cor¬respondence between what is observed into the eyepieces and what is projected on the screen, but the rule “What You See Is What You Get” is not always applicable in digital microscopy.
The reason is pretty simple: we are comparing a round image observed in the eyepieces and a rectangular image projected on the monitor/screen...
Cameras’ sensors are rectangular and the image is a part of what is visible through the eyepieces.
Phisically, it is not possible to carry out the circle squaring, therefore it is absolutely impossible to frame the whole field ob¬served through the eyepieces, but we have to consider the diagonal of the sensor which varies according to its size.

It is easy to understand that the rectangle and the circle cannot frame the same portion of the image. Between microscope and camera it is necessary to put a system, commonly known as “adapter”, that can project on the sensor the image coming from the microscope, making it available to focus and to monitor projection.
There are several adapters (with various magnification factors) that can enlarge or reduce the microscopic image and make it available for the camera.
An optimal situation could be to frame all the possible field using the full diagonal of the sensor: in this case, the center of what is observed thorugh the eyepieces will be visible on the monitor/screen, whilst the side parts of the image are lost.
An alternative solution (but not recommended for the final effect on the monitor) is to enlarge the image until the long side of the sensor matches the field of the eyepieces.
In this case, the image is “vignetting”, generating black corners.

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