This microscope can be made in a few hours. It is cheap and nice (not a toy)!
Possible magnification range: 10-100x (dependent on objective lenses)

Materials:

discarded photo enlarger
8 X 21 mm binocular
some pieces of scrap wood
Optional: some discarded camera lenses, piece of aluminum sheet.

Costs: 15 - 20 E / $

I built the microscope on this picture some years ago, but made another one for this instructable. So I did not spend much time to put a nice finish on parts.

Step 1: The Stand

The stand will be made from a part of the enlarger:

Take the mechanism part of the enlarger off the base. The lower part, below the film holder will become the base of the microscope. Take it apart from the light source and condenser.

The rest of the enlarger is no longer needed. It might serve as a stand for copying documents.

Step 2: Turn the Enlarger Into a Microscope Stand

Remove the bellows and the red filter, and turn the whole thing around. The focusing knob is now moving the part with the objective lens up and down.

Perhaps the base needs to be supported with a piece of wood to stand upright. The hollow part of the base plate, designed to allow the light to get through, needs a wooden insert. This will be the object table. Do not fasten it, because transparent objects have to be viewed with light from below, you can use a glass plate as object table for this.

Step 3: Taking the Binocular Apart

8 X 21, or 10 x 25 binoculars are readily available and really cheap, but do contain nice optics! Do check them out before buying: it should have a nice field of view, and a clear image. Quality can vary widely within a batch .

Take the cord out, it is fastened by special screws. Underneath is another screw, unscrew it.
The small telescope (monocular) can now be carefully taken from the connecting/ focusing bridge, 2 small steel balls will come out as well.

For this project, it is sufficient to just use one of the monoculars. However, to get some understanding of the optics, why not take the other half apart .

Step 4: What's in It??

Each monocular contains:

Achromatic objective. Corrected for basic image faults.
2 tiny prisms in an assembly: this turns the image from upside down to normal.
A nice 'Kellner'- design eyepiece. very nice to be used as telescope or microscope eyepiece.
The expensive ' wide field' eyepieces sold for microscopy are often Kellners. The field is about 60 degrees.

The focusing mechanism of each monocular is connected to the focusing bridge with a tiny protrusion near the front. This would move the objective lens up and down. This protrusion part can now be unscrewed.

You can fold the rubber part of the eyepiece back from the aluminum ring which separates the rubber cover of the eyepiece and that of the body to be able to take out the eyepiece. An aluminum tube now appears. 5 small screws hold the eyepiece assembly and the erecting prisms.

When this is unscrewed, the eyepiece assembly can be taken out. The erecting prism assembly comes out separated from the eyepiece. Pull the rubber back.

After this is done, The rubber cover can be turned back from the front. The objective lens, in a plastic mount, can be pushed out. Carefully wipe the abundant grease away, it is a pain to clean it from the optics!!

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.

Step 6: Assembly

Place the monocular on top of the objective lens. Make a small wooden block or any other object to space the monocular on top of the objective lens. perhaps a plastic cap, or something similar is needed to align both parts. You can use the thread for the red filter to fasten it. (Every magnifier is different, a spacer example is the the wooden part of the intro.)

If you need to add an objective lens (they are often 'missing' in 2nd hand stores) you need to make an aluminum disk, which can be set in the mount by a screw. See step 7 for pic, the zoom lens.

Now tape or clamp the monocular in place. READY!

Try the microscope on an insect or a small flower. Enjoy the view!

But no kidding; you can do science with it!

Step 7: Low Mag Variation

For detailed tinkering, like electronics work, you want more working space and 40 X magnification might be too much. Here the objective lens of the other monocular comes in handy:

Carefully cut the plastic mounting so it is not much longer than the thickness of the objective lens.
Move the rubber casing of the intact monocular back from the front.
insert the spare objective lens in front of the objective, and put the rubber back like it was. That's it!

The magnification will now be about 10 X, the working distance about 200 mm (8 inches).

The microscope mount might be too small now, but hey, you can use the enlarger stand!

Step 8: Accessories

For see through use, you can put the microscope on a clear plastic box with a mirror. A petri dish is useful for pond critters.
Photography is very possible. Focal depth is very limited, but software can take care of this: CombineZ 5.0
Illumination works well with a halogen reading light. Small critters hate the heat, so using a white LED is another option.

Left is an 8 mm moviecam zoom lens, mounted in an aluminum disk. Right is a 75 mm enlarger lens.