Field curvature is a simple lens aberration where the sharpest focus of the lens is on a curved surface in the image space rather than a plane. Objects in the center and edges of the field are never in focus simultaneously.
Most microscope objectives are corrected for field curvature, and are given the designation "Plan" or "Plano". One method to reduce this aberration is to insert a field stop (iris) in order to remove edge light rays. This method unfortunately greatly decreases the light collecting power of the lens. Some lenses are not corrected for a flat field in order to increase the light transmittance, or the span of transmitted wavelengths. These lenses, however, can have severe distortion as exemplified by the following Olympus lens:
|UApo/340 Iris Lens. This lens is highly color corrected and transmits down to 340 nm.
Note that the NA varies from 1.35 to 0.65.
The object images is a field of fluorescent beads of 2 diameters: 10um and 100nm.
With the iris OPEN, NA 1.3:
Notice that with a closed iris the distortion is greatly reduced, however the light gathering power of the lens is also reduced. Under dim fluorescence conditions the decrease in pixel intensity may be unacceptable.