!! Please note this is a shameless cross post from expereiment.com I just want more input if possible!!
As I prototype the field dissection microscope I am facing some lens related questions, and more importantly lens related design decisions.
I have started off with things I had around the workshop from previous Openflexure prototyping:
In the first version I simply unscrewed the PiCamera2 lens until it almost fell out. Then took an image.
I think the magnification is about 0.25 between the woodlouse, and the image on the camera sensor. Now there are a few of problems here:
- Very hard to repeatable set the lens in the same position
- May need a bit more magnification in some systems
- Working distance is super short (~1cm or so), which is really annoying
One thought is to make a system with a second lens. For this I make a modified super short lens spacer for the picamera lens, and a separate holder for another lens, these can be held apart by a little frame
The plan was to make a system with two lenses. As the camera sensor is smaller than the bug, we need to demaginfy the bug somewhat, to make a virtual image, that we then image with the picamera lens:
The eventual plan was to get “pancratic magnification” or adjustable zoom, my eventually modifying the system so that the picamera lens can be moved a few mm. I decided to start with the picamera lens about 6mm from the sensor giving it about a magnification of 1
Starting with the 50mm tube lens from the OpenFlexure microscope I ended making a tube that spaced this lens about 75mm away from the pi lens. This gives a magnification of about 0.33, and a working distance of about 180mm. This working distance is huge, making the microscope super unwieldly (which we can’t have, we must be able to wield with ease!). I was able to see my woodlouse (well an new woodlouse, Woody the Second we shall call them). Though it was awkward enough I didn’t save a snapshot.
In desperation I also tried the 6mm focal length plastic lens from the OFM illumination. The lens to lens distance here needed to be really accurate, and without an adjusting mechanism I couldn’t really get a focus, but the lens is clearly awful so I didn’t proceed.
Well this is the question!
First of all, I think the next stage for me should be to design a stand, tweak the mechanics some more, and write some docs. This way others can build something an have a play.
As for what is next for the optics, I would love some feedback. I am much more of an mechanics person than I am an optics person. So I am just assuming everything is a perfect thin lens. Which will only get me so far. I have some ideas, but there is a trade off on cost/openness/quality/availability. I am not sure which is most interesting for the community.
What do people think of these options:
- Find a plain lens with about 20mm focal length and try again. Both as a 1-lens and 2-lens system. My back of the envelope calculations tell me should give me a more sensible size and working distance.
- Get M12 lenses perhaps this kit. These are easier to adjust. However there is lots to think about here:
- The kit above seem to be six lens groups. If they are optimised for imaging long distance, will this make a rubbish microscope. How easy are these lenses to get, and can we get identical ones in a few years?
- I think single lenses are available in an M12 housing, but they are fairly expensive, and might perform even worse!?
- M12 mounts are available for lots of cameras. Allowing experimentation with PiCamera2, PiCameraHQ, and lots of other cameras
- Just get a cheap C-mount adjustable microscope lens and put in on a picamera HQ. The downside is the lens is now a black box, and this lens might be hard to buy in some locations. The up side is it should give me time to concentrate on nice mechanics.
All comments welcome!