Good Combinations for Scanning

Introduction:

While many combinations of objective lens magnification, microscope coupler magnification and camera are possible, not all of these combinations are efficient.

The goal is to select a combination, that can just resolve the desired details but avoid the so called empty magnification (Structures appear larger, but do not resolve additional details).

Attempt to explain "empty magnification":

Imagine you have a picture 1000x1000 pixels. You open the picture in an image processing software and resize it to 5000x5000 pixels.

The bigger picture does not contain more information obviously because it is only a resized version of the original image. But it is 25 times the (file-)size of the original image.

For optical reasons, a typical 40x/0.65N.A. objective lens sends an image of 1200x1200 "pixels"(size of the smallest resolvable" detail) to the camera. That means that a camera with a resolution of 1200x1200 pixels can resolve every detail present in the image.

Acquiring the image using a higher resolving camera does not add real information. It just creates an image of bigger file size and looks blurry. This is called empty magnification.

More information can be found here.

Disadvantages of empty magnification for manual slide scanning:

• 1. the more an image is magnified, the smaller the field of view of the camera gets (e.g. empty magnification of 2 results in 4 times the scan time)

  2. empty magnification demands more light from the light source (2x empty magnification => 4x light necessary)

  3. resulting size of the scans (2x empty magnification => 4x file size)

Matrix of objective lens, camera and sensor combinations and the resulting performance:

quality relative to the optimum megapixels of a scan

not resolved = < 70%

loss of detail = 70 - 90%

excellent = 90 - 125%

very good = 125 - 175%

good = 175 - 250%

slow = 250 - 400%

very slow = > 400%

critical => the image sensor is too large for the camera adapter. this combination will not work for slide scanning because the corners of the camera image will be dark. However, the image of the sensor can be cropped to remove the dark corners.