Field curvature is of critical importance in photomicrography because without flat-field correction, the resulting photographs will not have the entire specimen in focus. This tutorial explores how changing the microscope focus will affect which part of the specimen remains sharp and clear. To operate the tutorial, use the slider to adjust a simulated focus of the specimen as we would see it in the eyepieces.

The specimen appearing in the simulated eyepiece above is a histological thin section of an elephant toe. As the slider is adjusted toward the top and bottom of the rack, either the center or the periphery of the specimen is brought into focus. The center of the slider rack simulates flat-field correction.

Curvature of field in the image is an aberration that is familiar to most experienced microscopists. This artifact is the natural result of using lenses that have curved surfaces. When visible light is focused through a curved lens, the image plane produced by the lens will be curved. When the image is viewed in the eyepieces (oculars) of a microscope, it either appears sharp and crisp in the center or on the edges of the viewfield but not both. Normally, this is not a serious problem when the microscopist is routinely scanning samples to observe their various features. It is a simple matter to use the fine focus knob to correct small deficiencies in specimen focus. However, for photomicrography, field curvature can be a serious problem, especially when a portion of the photomicrograph is out of focus. Modern microscopes deal with field curvature by correcting this aberration using specially designed flat-field objectives. These specially corrected objectives have been named plan or plano and are the most common type of objective in use today. Plan objectives are also corrected for other optical artifacts such as spherical and chromatic aberrations. In the case of a plan objective that also has been mostly corrected for chromatic aberration, the objective is referred to as a plan achromat. This is also the case for fluorite and apochromatic objectives, which have the modified names: plan fluorite and plan apochromat. Adding field curvature lens corrections to an objective that has already been corrected for optical aberrations can often add a significant number of lens elements to the objective. For example, the typical achromat objective has two lens doublets and a hemispherical lens, making of total of five lens elements. In contrast, a comparable plan achromat objective has three doublets and three single lenses for a total of nine lens elements, making it considerably more difficult to fabricate. As we have seen, the number of lens elements increases as lenses are corrected for spherical errors as well as chromatic and field curvature aberrations. Unfortunately, as the number of lens elements increases so does the cost of the objective. Sophisticated plan apochromatic objectives that correct for spherical, chromatic, and field curvature aberrations can contain as many as eighteen to twenty separate lens elements, making these objectives the most expensive and difficult to manufacture. Plan apochromatic objectives can cost upward of $3,000 to $5,000 each for high-magnification units that also have a high numerical aperture. For most photomicrography applications, however, it is not absolutely necessary to have the best correction, although this is heavily dependent upon the purpose, the specimen, and the desired magnification range. When cost is important (when isn't it?), it is often wise to select more modestly priced plan fluorite objectives that have a high degree of correction, especially the more modern versions. These objectives provide crisp and sharp images with minimal field curvature, and will be sufficient for over 90 percent of photomicrography applications. Field curvature is very seldom totally eliminated, but it is often difficult to detect edge curvature with most plan-corrected objectives and it does not show up in photomicrographs (19, 23). This artifact is more severe at low magnifications and can be a real problem with stereomicroscopes. Manufacturers have struggled for years to eliminate field curvature in the large objectives found in stereomicroscopes. In the past ten years, companies like Leica, Nikon, Olympus, and Zeiss, have made great strides in the quality of optics used to build stereomicroscopes and, while the artifacts and aberrations have not been totally eliminated, high-end models are now capable of producing superb photomicrographs.

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