As seen in the previous blog, chromatic aberration obscures edges, and it can decrease resolution. Historically, many optical engineers devoted themselves to devise a means to remove this optical defect. The development of lenses to remove these color fringes was heralded as a major advancement in improving image fidelity. In this blog post, I include a picture taken with an achromatic lens system, and you can see the improvement in edge definition by comparing this image to the one in the previous blog.
Many microscopists concern themselves with the quality of their objective lenses and how well they eliminate color errors. They devote themselves to finding the perfect lens that will produce the highest fidelity view. For them, it is critical to understand the terminology for color correction. The most basic degree of correction is achromatic and are designed to bring blue and red light to a common focus. In spite of its name, an achromatic lens does not remedy all color errors. When a lens delivers the two extreme wavelengths, blue and red, to the same focus, there are still colors with wavelengths longer than 490nm and shorter than 600nm that are not directed to the same point. A lens having this defect is described as displaying residual chromatic aberration. To correct this, more advanced objectives were developed. These are the semi-apochromats and the apochromats. Both of these designs require using more optical elements composed of an exotic material, such as fluorite. These lenses are significantly more expensive than achromats.
So which objectives should a photomicrographer select? The easy answer, if spending money is no object, is to buy apochromatic objectives. A significant penalty of this strategy is that these optics have a shorter working distance and one can run into the cover glass if a specimen lies deep in the slide mountant. When you are studying permanent slides, such as stained sections or diatom frustules, this is usually not a problem. It is more of a problem when one is studying protists or rotifers in a temporary aqueous mount. If saving money is essential, then one may consider buying a set of semi-apochromats. These objectives use exotic materials such as the mineral fluorite, and their optical correction is intermediate to that of an achromatic and apochromatic lens. However, their performance is closer to that of the former rather than the latter. So buying semi-apochromats can be an economical situation.
One might get the impression that good pictures cannot be done with achromats. This is not the case. Most failures in achieving good results are due to poor technique rather, not inferior optics. Unsharp and low contrast images are usually caused by a failure to focus precisely, vibration induced blur during exposure, and non-optimal lighting. When working with the highest power objectives, near the limit of resolution of light optics, unsharp pictures may be the result of exceeding the limits of resolution imposed by the design of the lens. The most important thing is to first work on fundamentals. In other words, if you are not getting sharp photographs with an achromatic lens, it is not a surety that your photography will be improved by getting a more expensive lens. In later blogs, we will discuss how to achieve critical focus, how to avoid vibration, and how to adjust the microscope lighting.