Build a microscope!

Step 5 Optics

Back to the microscope: How does it work and how much magnification does it have?

One way to look at it: The monocular has reading glasses! Example: the enlarger lens is 50 mm (2 inches, most common.)

The monocular magnifies 8X, but it cannot see nearby! So, with the help of a '+10' lens (= 50 mm), it can see really close. But a 50 mm lens magnifies x 5, so total magnification is 40 X.

Another way: To view a small object, a 50 mm loupe is used. Magnification is 5 X. This is magnified 8 X by the monocular, which makes it 40 X.

Now, check the focal length of the objective, most common is 50 mm or perhaps 70 or even 100 mmfor larger models.

1x magnification = 250 mm. I have not found out why, but for all practical purposes, it works.

The formula: Magnification = 250 / focal length

So, if the focal length is 50 mm: magnification = 250 / 50 = 5 X.

( so a 100 mm lens has 2.5 X).

The magnification of the microscope in the example is : 8 X 5 = 40 X.

This way, if you need a larger magnification, an objective lens with a shorter focal length would do the trick. 28 mm would give (250 / 28) X 8 = about 72 X.

This type of optical design cannot handle much more: contrast and sharpness will suffer.
After all, the optics were not designed for it!

I tried to use a 8mm movie camera zoom objective (8- 28 mm). At lower magnification (72 X), the image is acceptable, but the higher magnification (301 X) does not reveal any more detail, it just makes the image fuzzy and restricts the field of view. One of the problems using zoom optics is the amount of optical elements: every glass/ air surface scatters about 5% of the light.
Zoom optics often have more than 10 of these surfaces, this does add up considerably and lowers contrast.

One more thing to consider: When using camera objectives, they have to be turned around!
The outside part is designed to look away, to near infinity. The inside part is designed to give a sharp image at a fixed length. Here the objective lens is used to look at a fixed length and project a parallel light bundle (= infinity), to be picked up by the monocular.
Enlarger objectives, as well as copier lenses are designed symmetrically, so they don't need turning around. For practical reasons (space) it might be a good idea to do it anyway.