A brightfield microscope has an aperture diaphragm to adjust the amount of light that is transmitted through the lenses and to the eye of the observer. An observer closes the diaphragm to achieve greater contrast, which may be necessary with specimens that don't have a lot of color. At a certain point, increasing the contrast may result in image distortion.
In general, live specimens cannot be observed using brightfield microscopy. This technique requires an intense light source, which can sometimes cause heat damage to the specimen. Also, staining is often necessary and may kill live specimens.
Because brightfield microscopy generally is not useful for observing colorless specimens, slides are often stained using bacterial staining procedures. In some cases, staining may introduce extraneous details that are not characteristic of the specimen.
A brightfield microscope achieves its greatest resolution with the 100x objective lens under oil immersion. This resolution however is not great enough to allow for the observation of intracellular structures.