Less well prepared preparations can degrade image quality to degrees that range from barely perceptible to ruinous. Whether these degradations are considered problematic is a subjective matter. Some microscope users are simply insensitive to quality issues, whether by design or default. Cover glass and mounting medium thickness should be controlled within narrow tolerances to avoid unacceptable spherical aberration, which is a geometric optics consideration. Spherical aberrations means that light rays of different wavelengths arising from an object point and that pass through the same area of a lens are not brought together as a single image point.
Spherical aberration makes object images look washed out, hazy, cloudy, milky, and low in contrast.
It is the same look as, and is visually indistinguishable from, that produced by glare and flare. Glare and flare are defined as light that does not contribute to image formation, noise in other words, and produce eyestrain similar to that experienced looking through a dirty car windshield while driving into the sun. Glare arises in the object space, located before the objective, while flare arises within the image space, located inside the microscope. Glare and flare look the same, and are usually referred to as simply glare, as the two are inseparable. Where glare goes, flare follows in lens systems.