Microscope Activities, 12: Field Curvature
In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.
EXPERIMENT 12: Field Curvature
To become familiar with detecting field curvature, and its correction.
Using your 4X objective, focus on the scale of a stage micrometer. Move the scale to one side so that it reaches from the center of the field of view, all the way to one edge of the field of view. Focus the microscope very carefully so that the very center of the field of view is in the sharpest focus possible. Now look at the scale at the extreme edge of the field of view. Notice that the scale is not in sharp focus at the edge of the field of view. Can it be made to come into sharp focus at the edge of the field of view? Try touching up the focus to see if you can get the scale in sharp focus at the edge of the field of view. You will find that you can get the scale in sharp focus at the edge of the field of view, but now the scale at the center of the field of view is not in focus! In fact, you find that you must continuously adjust the focus knob as you look along the scale from center to edge, and back. This is because the image of the scale is curved; this aberration is known as field curvature (Figure 12-1). From your knowledge of which way to turn the focus knobs to make the stage go up or down (Experiment 3), state whether the image of the scale is curved upward in the center, or downward in the center.
Field curvature is a natural consequence of using simple lenses, because light is slowed down by different amounts in the very center of a lens, and at the periphery of the lens. Field curvature has been an inherent characteristic of lenses and of microscope objectives for hundreds of years. For visual use, field curvature is not a problem; the microscopist merely changes the fine focus to accommodate whatever part of the field is under investigation. The problem comes in recording fields of view; that is, when a photomicrograph is made, not all of the specimen will be in sharp focus—usually only the center. To circumvent this problem of field curvature, photomicrographic attachment cameras are usually designed to capture only the middle third or middle half of the field of view, and not record the outer out-of-focus areas.
In the 1930s, special negative eyepieces were designed specifically for photomicrography; they could not be used visually, only for projection, in which case they would curve the image in the opposite direction to flatten it for photography.
In the 1950s, optical ways were found to correct field curvature through the incorporation of additional lenses in the objective. This necessitated the lengthening of objectives from a working length (or parfocal length) of 37 mm to the present day 45 mm. Such objectives that have incorporated correction for field curvature are known as “Flat Field” objectives, or “Plano” objectives, and they will be marked on the objective with various engravings: Plan, Plano, PL, Flat Field, etc. These kinds of plano-objectives are more expensive, and of primary benefit in photomicrography. The objectives supplied with your Boreal/Motic microscope are not flat field objectives.
Perform the directions in the Procedure section, and record whether the field curvature of your objectives is higher or lower in the center of the field of view than at the edge of the field of view.