As well as numerical aperture (which I’ll cover in a future blog post), the other pieces of information found etched into the barrel of an objective lens include the magnification and the optical correction. There are many different types of corrections/objectives available, but I’ll explain the four most common ones you are likely to use and encounter. As with the eyepieces, objectives can look very simple- a metal tube with a lens at either end, whereas, in reality, these are made up from a (sometimes) complex series of lenses which are designed to complement each other and offer corrections due to the optical aberrations.
The ‘Achromatic’ objectives (abbreviated as ‘Achro’ or ‘Achromat’ on the body of the objective) are the most widely used in microscopy. The correction in these objectives is for an optical phenomenon known as ‘axial chromatic aberration’. This phenomenon occurs as white light passes through a convex lens. As it does so, this results in the splitting of the white light into the component wavelengths of green, blue and red. In optical terms, this split produces coloured fringes around the edge of the image and also causes the viewed image to be blurred. In the achromatic objectives the corrections bring the blue and red wavelengths to approximately the same focal point as the green wavelength.
The next stage of correction comes with the ‘Plan-Achromatic’ objectives (abbreviated as either ‘Achroplan’ or ‘Plan Achromat’ on the objective). In addition to the correction found in achromatic objectives, the plan-achromatic objectives are corrected for a natural phenomenon known as ‘field curvature’. This anomaly arises when light passes through a curved lens. If using an objective which is not corrected for field curvature, then you would only be able to focus clearly on the edges of the field of view, or the centre of the field of view, but not uniform focus across the whole sample. For routine microscopy, such as checking staining, this isn’t such a problem. Nevertheless, if you would like to capture images to be used for presentations and publications, then a plan-achromatic lens is needed.
Another anomaly which occurs when light is focussed through a curved lens is that of ‘spherical aberration’. This is a similar phenomenon to field curvature- due to the curving of the lens, the resultant parallel light waves which pass through are not focussed on a single point and are instead spread along the horizontal axis. This is where the ‘Semi-Apochromatic, or ‘Fluorite’ objectives come in (abbreviated as ‘Fluor’, ‘Fluar’ or ‘Fl’ on the objective barrel. The name ‘fluorite’ derives from a time when such lenses were made from the calcium fluoride mineral of the same name. This mineral is commercially known as ‘fluorspar’ and although still found in the manufacture of some semi-apochromatic lenses, many are now made from synthetic materials. Semi-apochromatic objectives are corrected for one or two component colours. You may also see the abbreviations ‘Plan FL’ or ‘Plan Fluor’ on the sides of objectives. As well as the correction for spherical aberration, these lenses are also corrected for field curvature.
The utmost level of correction (and indeed, expense) which you are likely to encounter and use comes in the ‘Apochromatic’ objectives (abbreviated as ‘Plan Apochromat’ or ‘Plan Apo’ on the barrel). The ‘plan’ part of the name means they are usually corrected for field curvature. They are also spherically corrected for up to three wavelengths. Furthermore, the apochromatic correction means they are chromatically corrected for the three component wavelengths (red, green and blue) and as a result, these three wavelengths will be focussed to the same focal point.
Author: Martin Wilson