When I first heard about Foldscope from a friend in 2016 I became curious. Until my Foldscope arrived in early 2018 I started experimenting with ball lens microscopes myself. Please see a photo of some of my models. When I first used my new Foldscope, I was impressed with the good image quality. I attribute this to the aperture around the lens. My congratulations to Prof. Prakash for his very good design.
If you look at my microscopes, there is just the lens in a fixture, but no aperture around it. We are all familiar with apertures in cameras. It’s the same thing here. As no lens performs perfectly, the ball lens microscope and the camera suffer from aberration. I’ve included a picture of what aberration looks like in a ball lens microscope without aperture. Only the center is sharp. The outer regions are very blurred. The fact that Foldscope users don’t have such problems is due to the good optical design by Foldscope engineers. The more you know about all the delicate features, the better you can appreciate a well built microscope.
For those interested to know more about the supplies used in the self-made microscopes, I give the references here. The lenses are from Swiss Jewel and are available in different sizes and different materials. The smaller the lens, the higher the magnification. A saphire lens has more refraction than BK7, yielding higher magnification as well. However, it becomes also more difficult to adjust the focus at higher magnification. For some of my microscopes, I used the 3D printer files from PNNL https://availabletechnologies.pnnl.gov/technology.asp?id=393 available free of charge. For other models I just punched a hole into a soft material.
But there is nothing like the Foldscope! It is the best ball lens microscope.