Bright field microscopy

[superwolf]

What gives raise to contrast in bright field microscopy, except shadows, pits and scratches?

[Woland]

Well bright field microscopy relies on light passing through the sample on its way to your eye. Different types of tissues would absorb the light differently, and this is what gives the technique its contrast. To enchance the contrast the sample can be stained, which would help absorb more light than by the tissue alone.

[Andy Resnick]

Generally, contrast in brightfield imaging is limited to absorption. However, contrast can also occur by scattering off inhomogeneities. The contrast from scattering is generally low but can be increased by decreasing the numerical aperture of the condenser or by using oblique illumination.

The lack of contrast is what drove the development of alternate imaging methods- darkfield, Rheinberg, phase contrast, DIC, polarization, fluorescence, etc.