To parfocalize a zoom system, you must first establish an image on your monitor which is in focus.  To do this, it us usually easiest to set the zoom to its lowest magnification setting.  Next, adjust the entire zoom system or specimen up or down until the image comes into focus (it is helpful to know what the “working distance” is for your system, then measure that distance from the bottom of the zoom to your specimen).  Once you have an image in focus at low magnification, then zoom to the highest magnification setting.  Adjust the focus on the zoom (if you have an internal focus bottom module) or adjust the entire zoom system or specimen up or down to establish a sharp focus at the highest magnification setting.  Now that you are in focus at the highest setting, zoom back down to the lowest setting and do not adjust any focusing modules.  If the zoom is in focus at the lowest setting, you system is parfocal and you are finished with this procedure.  If the image is not in focus at the lowest power, you must adjust the “Adjustable C-mount” on top of the TV Tube (attached to the camera).  While at the lowest magnification setting, loosen the set screws on the knurl and begin turning the top knurl to either increase or decrease the space between the two knurl.  This needs to be done without altering the distance between the specimen and the bottom of the zoom (no change in working distance).  While you are adjusting the gap distance between the two knurl, you will begin to see the image coming into a sharper focus.  Once the image is sharply in focus, lock the knurl into place.  Now you can zoom back up to the highest magnification setting, make any focusing adjustments with the entire zoom, specimen or internal focus modules, get the image sharply into focus and then zoom back to the lowest magnification setting.  Your zoom should now be parfocal. 

Above you have a narrative of the parfocalization process, below are specific points to the process: 

1. Determine the working distance of your system prior to starting.
2. Set up zoom to the specified working distance by measuring.
3. Set the zoom to low magnification and focus using internal focus or by moving entire system up or down.
4. Zoom to high magnification and refocus using same approach as in step #3.
5. Zoom back to low magnification (do not adjust any focus).  If image is sharply in focus you are finished.  If image is out of focus, proceed to step #6.
6. Loosen set screws on knurl on top of TV Tubes.
7. Adjust gap distance between knurl by turning in either direction (increase or decrease gap size).  Determine which direction brings image to a better focus point.
8. Once image is in focus, lock knurl.
9. Zoom back to high magnification.  Adjust focus as described in step #4.
10. Zoom back to low magnification….Your image should be in focus and the system should be parfocal.  If not, repeat steps #4-9.

Very Important:
In order to have a parfocal zoom system, you MUST have a C-mount camera and not a CS-mount camera. 

What’s the difference you ask???

C-mount and CS-mount cameras are very similar and if you aren’t educated in the one major difference then you would never know you were using a CS-mount camera.  A C-mount camera has a 17.5mm distance from the camera sensor to the edge of the threads (“face to sensor”).  A CS-mount camera as a 12.5mm face to sensor distance.  The thread size and pitch are identical so any C-mount lens you use will thread into a CS-mount camera, however the 5mm difference will not allow the lens to work as it was designed (a zoom lens will never be parfocal). 

Many camera manufacturers advertise their cameras are both C-mount and CS-mount compatible.  This means they are CS-mount cameras and they supply a 5mm spacer ring with the camera which can be screwed into the camera to increase the “face to sensor” distance to 17.5mm, making it a C-mount camera.  If your camera is claiming to be both C-mount and CS-mount compatible, make sure you keep the small 5mm spacer ring which is packaged with the camera.  If you loose this ring, contact your camera supplier to order another one.
 

Parcentering the Zoom Image:

Not only are all Optem® zoom systems designed to maintain focus throughout the zoom range, but they are also designed to zoom on center.  Due to manufacturing tolerances in the many different cameras, you may be required to adjust the position of your camera to have your optical zoom system parcentered.  This is done by setting up a parfocal zoom system and focusing on an object at the highest magnification. Note the detail of the object that is in the center of your field of view. Then go to the lowest magnification on the zoom system.  Adjust the three screws on the camera mount to place the detail you had in the center of your field of view at the highest magnification to the center of your field of view at the lowest magnification.  Your optical system should now be parcentered.