The formula to calculate the total relay magnification with the afocal method is:
(eyepiece magnification) X (camera lens focal lenght) / 250
If you use a 10X eyepiece with a 50mm lens you get
10x50/250 = 2X, working but a bit too cropped for APSC
To get 1.6X a good fit would be with a 8X adequate eyepiece (not easy to find) or with a 40mm camera lens. A 40mm
pancake will be ideal, but a short zoom 18-55 at 40 can work well enough, superzooms are not adequate
Because the microscope image field is round you always will need to crop or you will have corner vignette, to capture the whole image with small relay magnification it will be a circle inside the rectangular frame
At
www.krebsmicro.com you have links to excellent articles on the subject, although more oriented to other methods afocal is also covered
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