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We respect your privacy and when you create an account you don't grant us any rights to your data, except for the ones that need for the application functioning. We don't use your data for any commercial or non-commercial purposes and share it with nobody. You can learn more in the terms of service.
App Settings -> Manual white balance = Yes -> Tap Gray Card button or change with sliders.
It's OK if you have black corners (the application automatically crops 15% from each border). But if the adapter not ideally fits with the eyepiece, you can get additional shadows around the borders. This affects the scan quality and cell counts. To avoid the problem:
try to move the adapter
increase the crop level: App Settings -> Cropping