The final part of this article demonstrates application of the optimum aperture concept to digital imaging with a sequence of increasing magnification images obtained with a scientific grade digital camera, the Kodak (now Redlake Imaging) MegaPlus 1.6i/AB. This camera is equipped with a Nikon “F” lens mount so it can be used with Nikon 35 mm camera lenses. My previous article “Pixel Array Size Requirement to Replace Photomicrographs on Film” demonstrated that the 1024 x 1534 pixel image recorded by 9X13mm grayscale sensor of this camera can meet an Abbe criterion of 500 times the N.A. in small format print equivalent to a 10X enlargement of a negative or slide. The camera is shown mounted to an Olympus 35mm camera bellows in Figure 14.

click image to enlarge (101K) Figure 14

click image to enlarge (139K) Figure 15

Figure 15 is a close-up view of the Olympus Auto Bellows and the adapters made to mount the MegaPlus camera and Zeiss macro lenses to the bellows. My specialty later in my working career was failure analysis of gears. A test gear with a tooth broken out by cyclic fatigue loading near the tooth tip is shown on a copy-stand in Figure 14. The fracture surface left when a tooth was broken out of this gear is shown at increasing magnifications in Figures 16-19. The scientific interpretation of the topographical features of the gear tooth fracture surface is called fractography. This test gear of medium carbon steel has been heat treated using a new process called contour induction hardening. The arrow in Figure 16 indicates a smoother, circular region of slow crack growth from each cycle of loading with radial markings indicating that the tooth had a primary fatigue crack origin in the center of the circular area in the interface between the hardened surface zone and the much softer core. The origin region is at a nonmetallic inclusion stringer evident in Figure 19. Figures 16 and 17 were recorded using a 60 mm f/2.8 Nikon Micro Nikkor macro lens instead of a bellows mounted lens. The recording conditions were with camera magnifications of 0.5X and 1X at f/11. Figure 18 was recorded with the Zeiss 100 mm f/6.3 Luminar lens with 2.5X camera magnification using an aperture setting of f/6.3. Figure 19 was recorded with the Zeiss 63 mm f/4.5 Luminar lens at 5X camera magnification using an aperture setting of f/4.5. Small format photographic prints of these digital images for Figures 16-19 would have, respectively, the following Abbe criterion values for total magnification: 330N.A., 440N.A., 440N.A., and 540N.A. The plastic dovetail inserts of the Olympus Auto Bellows were found to be cracked a few years after the gear fracture was photographed.    

click image to enlarge (138K) Figure 16	click image to enlarge (184K) Figure 17
click image to enlarge (161K) Figure 18	click image to enlarge (229K) Figure 19

All-metal heavy duty sliders and brackets for the lens and camera were made in my home shop to correct this problem. The modified bellows is shown in my Microscopy Today article “Heavy Duty Camera Bellows for Digital Imaging” along with resolution test results for the Zeiss 63mm f/4.5 Luminar lens and the Olympus 38mm f/2.8 macro lens ³. The transition from photomacrography with a view camera and 4X5 Polaroid film to digital imaging with the MegaPlus camera occurred very rapidly in early 1995 in our metallography laboratory because of savings in time and elimination of film cost. Polaroid film is still used in the laboratory with the Zeiss Ultraphot II for low magnification photomacrographs obtained with the Luminar lenses and their vertical illuminators. Polaroid film is also used with a view camera system dedicated to brightfield imaging of complete metallographic samples (see "Brightfield Illumination of Complete Metallographic Specimens")⁴. The Polaroid film images are subsequently scanned to digital files.