Here's the second part of our series on how to view microscope images.
Figure 5, on the other hand, has the design from the edge of the window in the taxi. It was always a miracle on this trip, on the rough roads of northern India in 2008, to get a decent picture while the car was moving. There is more light, and we are not trying to focus from a nearby blade of grass to the far mountains. Yet not only the window, but branches outside the window, are out of focus. I must have had to open the aperture to get enough light for the camera to capture the photo in a smaller amount of time. Just like the human eye, the smaller the pupil, the easier it is to focus. The problem is that if the opening is too small, then the total amount of light is not sufficient to see what is in focus.
Back to the screw now. Figure 6 shows the thread crests at the discolored area. We can also clearly see some scrape marks. But we’ve doubled the magnification again, and now we can’t even focus on the entire tiny area. There are now software packages that take multiple images and then “stitch” the portions of each image that are in focus to create an entire photo that is in focus. These can be handy if you have one. But you can’t view whatever you are looking at directly. You can’t turn it and rotate it to see what that little sparkle is as you are moving it. You can only view the computer-massaged image.
Figure 7 is the same magnification as Figure 6, but now we have moved toward the top of the screw, so a wider arc is in focus. This is the exact same screw, and the photos were taken a few minutes apart with the same microscope and camera. Now look at Figure 8 (again the same magnification as Figures 6 and 7), which is the exact same area as Figure 7. But now I have focused on the thread roots. If you were just given Figure 8, I think it would be very difficult to realize you were looking at thread roots, unless you know how tricky micro photos can be to interpret.
The scanning-electron microscope gets around the depth-of-field problem. Figure 9 shows an image of approximately the same magnification as Figure 1. Compare the two marker bars of 2 mm each. Figure 10 was obtained at a magnification setting five times as high. It’s comparable to Figure 3. Yet the focus is not too bad. I was having problems with my SEM at the time, so it’s not as good as it could have been, but even this relatively bad SEM photo is in much clearer focus than the optical.
For reference, here's Part 1.