Mike already explained the use of coarse and fine focusing controls on a microscope. In older microscopes, the focusing controls consists of two sets of knobs, as in this 1950’s Olympus GB microscope.In that kind of setup the travel of the fine focussing is limited usualy to some 2 mm. Once the fine focussing has reached it’s upper or lower limit, the tube needs to be lowered or raised to a position somewhere in the middle. Thats why the limits are marked on the stand .In more recent stands, the 2 (x2) knobs are mounted “coaxialy”, meaning that the two knobs are mounted on what appears to be the same axis. In those systems the fine focussing works over the entire range of the coarse focussing travel.In reality the system consists of 2 axises, a thinner one, mount within a thicker one.The thinner, internal axis is the fine focus. Using a planetary gear with 3 balls the movement of the fine focus control is transferred to the coarse focussing mechanism albeit that the movement of the fine focusing is reduced about 10 times compared to the one of the coarse focusing.
Coarse adjustment is used to focus the image on the microscope under low or medium power by using larger motions of the lens. (Note: Coarse adjustment should never be used during High power) The coarse adjustment knob on a light microscope focuses by moving the lens quickly. The fine adjustment knob moves it slowly. Always use the coarse adjustment first and always start on low power. The coarse adjustment allows you to find the item you are looking for easily, and the fine adjustment is to focus more clearly on the item you are looking at. The coarse focus moves in faster, whereas the fine focus is much slower therefore you see more detail.They both move the objective lens towards and away from the specimen, but at different sppeds. 1 turn of the coarse adjustment knob equals many turns (10?) of the fine adjustment knob. Fine adjustment is used to focus the image on the microscope of only high power by using very tiny motions of the lens.
