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Chapter 9:
Microlens Formats

Most camera lenses are designed for a range of 500 mm to ∞. This range accommodates both a head shot and a landscape. A typical camera lens for a 35-mm-film format is 52 mm in focal length. According to Eq. (3.7), a magnification of 0.1 requires an object distance of 11f, or 550 mm. This may also be considered a 10X reduction. The performance of a typical camera lens degrades rapidly beyond a 0 to 10X reduction range.

The terms macrolens and microlens are interchangeable. In photography, a macrolens converts a small object into a large image within a print. In microscopy, a microlens looks at a small object. They both operate within a reduction range of 10XR to 1X. A magnification range of 1X to 10X is achieved by simply reversing the lens. Nikon frequently employs microlens as the name for a lens with near-unity magnification.

9.1 10XR Double Gauss

Figure 9.1 displays a 10XR double-Gauss lens for application to a film format with a 35-mm diagonal. The lens, as described in Smith, indicates a large spherical aberration and a decreasing relative illumination with field height. The decreased relative illumination indicates f/# inflation with field angle. Rays of the off-axis field must be clipped to manage astigmatism. Inflation of f/# with field angle is a characteristic of a double-Gauss lens. The actual field is adapted to a sensor with a 12-mm diagonal, or 6.0-mm field height, or 6.9-deg field angle. The f/# inflation at 6.9 deg is < 1%.

The ray-intercept plot indicates enormous aberration at a 6.0-mm field height. The tangential error indicates 154 μm of spherical aberration. The sagittal error indicates 51 μm of spherical aberration. The edge spread is nearly 40 μm. The encircled energy at 10 μm is much less than 30%. The flat spot of the ensquared plot occurs at much less than 5 μm. The diffraction limit might be suitable for a 10-μm pixel, while the aberration limit is certainly not.

The low f/# indicates a high brightness; however, the aberrations indicate poor contrast. A full-pixel object at > 40 μm in width should display a brightness in accord with the f/#. However, edges will spread to > 20 μm wide. A 40-μm-wide stripe transforms into a round peak at 80 μm in width. A subpixel object will spread into a mound of half height at 40 μm in diameter.

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