Many of you had emailed to ask me how I keep the focus fixed in any of my movies. This is specially relevant when:
1. You want to watch a process for a long time.
2. When you would like to do quantitative microscopy (when you analyze what you captured; such as a ciliate trajectory).

Here is a trick that I use – that uses a small stack of A4 size paper.
Method:
1. I start by putting my iPhone or my iPad on a object; such as a matchbox of plastic tape roll. Thus you have set the height for your imaging sensor (let’s call that h1).
2. Now I attach my foldscope with magnets to the iPhone/iPad. Because of the weight (small) of the foldscope – the entire microscope is hanging down (holding for its dear life 🙂
3. You need to choose the tape roll/matchbox height h1 to be greater than height of the foldscope – so the lowest part of the foldscope (which is the watch battery) is not touching the table surface.
4. Now insert the paper stack slowly under the battery while observing your screen. You can initially insert a certain number and cross the focal plane. You can start removing one paper out of the stack – to get the exact focus height you want. Try this with a sample in the microscope; so you know what you are looking for.

It’s as simple as that – use “paper” to make your precision focus. Now depending on how thick a paper stack you use, you can tune the steps (height change per removal of one sheet).

If you need any clarification; just leave a question in comments below.

Happy foldscoping!!
Cheers
Manu