Camera Adaptor and Digital Microscope-3

3. Ways to pick up the microscope image

              Depending on the set-up, it can be with intermediate optics (Reduction Lens or Camera Adaptor) between the camera and the microscope’s objective, or without it.

3.1 Directly pick up method

              The image produced by the microscope objective can be directly picked up by the sensor of a camera, without an eyepiece or other intermediate optics. Here objective of a microscope produces a real image directly on the camera’s sensor. The objective produces a relatively large image (for 18mm ocular tube, it is 18mm, for 20 or 22mm ocular tube, it is 20 or 22mm), compared to the small sensor of many cameras. Unless the sensor is large, there may be quite much empty magnification and low bright image (the larger the field, the lower the light).

Directly pickup method

3.2 Photo project ocular method

             The image produced by the microscope objective is passed through a photo projection ocular (photo eyepiece), which then projects a real image on the sensor of an SLR camera. There are no camera objectives involved. The projection eyepiece corrects optical errors which are produced by the microscope objective. These photo projection eyepieces are compensating optical elements. This means that they are designed to correct various lens errors that the objectives produce, including field curvature and chromatic aberration.            

            These projection oculars are therefore manufacturer dependent and must correspond to the objectives of the manufacturer. Besides image quality, another advantage is, that parfocality is maintained between the camera and the eyepieces (i.e. both images are in focus at the same, and there is no focus deviation).

Photo project ocular method

3.3 Eyepiece first, then adaptor lens

            The image produced by the microscope objective is first passed through a regular eyepiece. A virtual image is produced this way, which cannot be used to directly make a picture. A camera (with its own objective) then picks up the virtual image and projects it on the sensor. The camera works like the eye, which converts a virtual image to a real image. This system is used in a focal photography, in which a regular compact camera (with its own objective and all) is attached in front of the eyepiece.

CMOS sensor after the eyepiece method

            From the above discussions, one can find that this is with bad quality because of the eyepiece is a large aberration system for only human eye. Human eye has the ability to compensate the image shift in the focal plan so that an amount of accommodation is allowed for the eyepiece. For the low price eyepiece, the image quality is even worse. So that high quality eyepiece with high MTF value is needed. Here we show the moderate eyepiece and it’s MTF for your reference.

Kellner eyepiece and its MTF

3.4 Pick up through intermediate reduction lenses

            The image produced by the microscope objective can also pass through a reduction lens (or camera adaptor or eyepiece adaptor) before reaching the camera sensor. In this way the image produced by the microscope objective is reduced in a more precisely size to match the small sensor size of the digital camera.

           The reduction lens produces a real image on the camera sensor. Without the reduction lens the image would be magnified too much. The reduction lens also results in a brighter image. This is an improvement to the first point from above. Eyepiece cameras for microscopes use this system. The Reduction Lenses is not a compensating photo eyepiece and therefore does not correct lens errors produced by the objectives.

Pick up through intermediate reduction lenses

           From figure shown above, it can be found that a field lens is often insert before or after the intermediate image to refract the light toward the CMOS sensor

Camera Adaptor and Digital Microscope-4