The objective of a microscope is the most important and complex element of the light microscope. It is important because the objective is where most of the magnification provided by the microscope occurs. Its complexity lies in the fact that to fulfill its function you need different lenses of high quality and precision.
How are the objectives constituted?
The objectives are constituted by a complex set of multiple lenses that focus the light waves that originate in the sample and form an intermediate image that is later magnified by the eyepieces. The objectives are responsible for the formation of primary images and play a central role in establishing the quality of the images that the microscope is capable of producing. Furthermore, the magnification of a particular sample and the resolution under which the fine details of the sample are also highly dependent on the objectives of the microscope.
The most difficult component of an optical microscope to design and assemble, because it is the first element that light encounters as it passes from the sample to the image plane. Targets were named for the fact that they are, by proximity, the closest component to the object or specimen being analyzed.
What is the difference between an achromatic and a planarchromatic objective on the microscope?
The most commonly used objectives in laboratory microscopes are achromatic objectives. These targets are corrected for axial chromatic aberration at blue and red wavelengths, which are approximately 486 and 656 nanometers, respectively. Both are grouped into a single common focal point. Achromatic objectives are also corrected for spherical aberration in the color green (546 nanometers). Limited correction for achromatic lenses can result in images with a magenta halo if focus is chosen in the green region of the spectrum. The lack of correction for field flatness (or field curvature) presents an additional problem.
In a planochromatic microscope objective lens, this lens curvature has been corrected and results in a sharper and clearer focused image.
That is why we can say that the difference between an achromatic and planar microscope objective lens is the degree of flatness of the field. Because an achromatic microscope objective does not have a correction for lens curvature, the very outer edges of the circular image you see through the microscope will be slightly out of focus.
When looking at the two images, it is a bit difficult to recognize a big difference between achromatic and planarchromatic lenses. So perhaps using objectives with planochromatic lenses is not as necessary when the microscope is used for pleasure or education, but for work, especially if you need to see things in context (wide field for long periods of time) , a field that looks flat (not concave) and fully focused is preferable.