The brain is an excellent image processor and can ignore image defects in real time, but when viewing a captured image, the image stands alone and the brain then sees the defect. Performing a Köhler alignment will ensure that most common defects, such as uneven illumination, will not be present in the final image.
View the Kirsch Note here
Köhler alignment really does improve uniform illumination, especially when capturing high-resolution micrographs. It's interesting how digital processing can’t compensate for physical setup errors. Worth checking how similar alignment strategies impact contrast in image analysis tools like those featured on Status Online.