Microscopes have been around for about 400 years. The first ones were just magnifying glasses with a really short focal length. Later, the compound microscope was developed. It is comprised of an objective and an eyepiece. This is the most common microscope today. The components used to make a microscope normally adhere to standards such as DIN.
The DIN standard specifies things like the distance between the object and the image formed by the objective and the distance from the object to the flange. Here’s a picture that shows the dimensions:
As you can see, the distance from the object to the image is 195mm and the distance from the object to the flange (the place where the objective screws in) is 45mm. The tube length (distance from the flange to the end of the tube where the eyepiece is inserted) is 160mm. Note that this means the image is 10mm inside the tube; this means that if you want to get the image onto an image sensor, you need a relay lens.