Page 72 - Fundamentals of Light Microscopy and Electronic Imaging

DESIGNS AND SPECIFICATIONS OF OBJECTIVE LENSES 55

Nikon
Plan Fluor
100 Numerical aperture/
Magnification X/1.30 Oil immersion medium
Application DIC H
∞
Lens-image distance/ /0.17 WD 0.20
Coverslip thickness Working distance
(mm) (mm)
Color-coded ring
for magnification

Mag. 1X 2X 4X 10X 20X 40X 50X 60X 100X
Light Light Dark
Color code Black Gray Red Yellow Green White
blue blue blue
Figure 4-10
Key for interpreting the markings on the barrel of an objective lens. Markings on the lens
barrel indicate the type of lens and correction, initial magnification, immersion medium,
numerical aperture, lens-image distance, and required coverglass thickness. For quick
reference, the color-coded ring, near the thread, denotes the initial magnification, while the
color-coded ring near the front lens denotes the type of immersion medium (black-immersion
oil, white-water, orange-glycerin, yellow-methylene iodide, red-multi-immersion).

Image Brightness

Notice in Table 4-1 that the ratio of numerical aperture to magnification determines the
light-gathering power of a lens and hence the image brightness B. B is defined through
the relationships

2
B (NA/M) (transillumination mode)
and

2
4
B (NA /M ) (epi-illumination mode)
where M is the magnification, and NA is the numerical aperture, a geometric parameter
related to the light-gathering power of an objective lens. Numerical aperture as a primary
determinant of the spatial resolution of an objective is discussed in Chapters 5 and 6. The
values for magnification and NA are indicated on the lens barrel of the objective. A
60 /1.4 NA apochromatic objective lens gives among the brightest images, and because
its image is color corrected across the entire visual spectrum and is flat and substantially