There are two basic approaches to using a camera with a microscope. One, called the afocal method, uses a camera instead of the human eye to focus the image. If you hold a cell phone camera (or other camera with a lens attached) over the eyepiece and take a picture, that's the afocal method. The camera lens is needed to bring the image onto the camera sensor. The other (projection) method involves projecting a real image directly onto the camera sensor, without any lens attached to the camera. This is a deep subject with tons of permutations; I learned to my surprise that it is not necessarily true that you will get a well-functioning setup if you use the manufacturers recommended parts and pieces. There is extensive material about microscope camera setup on the microbehunter.com forum that I used to come up with the system I have.
For macroinvertebrates, which range in the size roughly from 500 microns (half a mm) up to 25mm, a stereo / dissecting microscope with a magnification range between 6x and 40x (with 10x eyepieces) works well. Compound microscopes with high magnification ranges are less appropriate.
The size of the camera sensor comes into play when using the projection method. The camera, and therefore sensor size, should be chosen to approximately fit the image projected from the microscope camera port: too large a sensor will result in the image not filling the frame, while too small a sensor will lead to cropping of the image. The method I use is 'direct projection' meaning I chose a camera with a sensor approximately the same size as the image formed by the microscope camera port. The physical parameters of the Wild phototube place that image about 1/2" above the end of the port. A mirrorless camera, as compared to an SLR for example, has a flange focal distance short enough so the camera, without any lens, can be positioned so the image from the microscope lands directly on the sensor without any intervening lens. I use 2 different mirrorless cameras - a Canon M200 with an APS-C sensor, and an Olympus Pen E PL7 with a micro 4/3 sensor. Both have tilt touchscreens for final focusing. Both are obsolete and available relatively cheaply used.
Lighting has a lot to do with the quality of images captured, both for color-balance and illuminating details, as well as managing chromatic aberration (CA). (see microbehunter.com forum for more about CA). I use a combination of halogen lighting delivered via fiber optics and LED lighting under the stage for a balance of both. The Dolan-Jenner dual fiber optic halogen light supplies are old, reliable and can be found cheaply.
The Wild stereoscopes were manufactured from 1958 to about 1994, mine is around 40 years old. They are wonderful.
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