Procedure for Making Camera and Microscope Eyepieces Parfocal
Charles Krebs 12/10/10 v1.5
The procedure discussed below is the method I use to make a SLR camera (or other
lens-
Preliminary Preparation:
Be sure microscope viewing eyepieces are properly set up, and adjusted for your eyesight. If, on your microscope, the focus will change with a change in interpupillary distance setting, or there are individually focusing eyepieces, be sure they are properly set and these settings remain constant. If they are prone to move accidentally you might consider taping them in place so that the settings do not change. If they are inadvertently changed after following this “parfocal” procedure, you may need to run through it again.
It is best to use a 4x or 10X objective for this procedure. Higher power objectives
will give less accurate results. It’s somewhat counterintuitive, but the higher the
power of the objective, the greater the depth-
If you will be using an eyepiece designed as a “projection” or “photo-
If you will be using a normal viewing eyepiece in the trinocular tube, start at Step
1-
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Step 1-
First set up a static subject slide that has a very fine detail and high contrast. Focus carefully on a specific detail through the eyepieces. DO NOT CHANGE THE FOCUS DURING THE REMAINDER OF THIS STEP.
If the trinocular tube is adjustable in height (some are, most are not), remove one
of the viewing eyepieces and place it into the trinocular tube. While viewing through
this eyepiece, adjust the trinocular tube height up/down until the subject appears
in sharp focus. Leave the trinocular tube at that height. Put the "viewing" eyepiece
back into the eyepiece tube, and place the photo-
If you have a camera body that has a “live-
Now, place the camera body above the trinocular tube, and while looking through the camera viewfinder raise/lower the camera body until it appears you have achieved best focus in the camera viewfinder. (Not as easy as it sounds... but at this point, being as close as possible is good enough). We're done using the camera viewfinder for focus from now on.
Go to STEP 2-
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Step 1-
This procedure uses a regular viewing eyepiece in a way that causes it to project
an image into a camera body. It does work, and the results can be of good quality
so it may be worth a try if you want to use a DSLR without the lens. You are actually
using the eyepiece in a manner quite different from its intended use. If possible,
it would be better to locate appropriate “projection” photo-
First set up a static subject that has a very fine detail and high contrast. Focus carefully through the eyepieces. DO NOT CHANGE THE FOCUS DURING THE REMAINDER OF THIS STEP.
Place the eyepiece you will use as the photo relay into the trinocular tube and while viewing through it, adjust the trinocular tube up/down until the subject is also in focus through that eyepiece.
This is your starting position for the trinocular eyepiece. It will never be positioned
lower than this point. At this “base” position, it will provide a “virtual” image
that would require a lens to convert it to a “real” image capable of being recorded
on a sensor or film. The way we can accomplish this without adding additional optics
is to “pull” this eyepiece “out” slightly so that the distance between this eyepiece
and the objective is increased. (This places the intermediate image formed by the
objective slightly in front of the eyepiece diaphragm, and it will now project a
real image.) If the trinocular tube is adjustable, you can do this by increasing
its length to some degree. If it can not be adjusted you will need to place shims
or spacers under the eyepiece to elevate it. After this eyepiece has been elevated
you should no longer try to assess focus or magnification by looking through the
trinocular eyepiece with your eye. (Reminder... the microscopes actual focus should
not be changed). You now need to place a lens-
As the eyepiece in the trinocular tube is moved up (increasing the distance between this eyepiece and the objective) the relay magnification will decrease. The camera must be moved so that image as seen through the camera viewfinder looks as if you have achieved best focus on the camera focusing screen. (Not as easy as it sounds... but at this point, being close is good enough).
If the magnification still appears to be too large through the camera viewfinder (that is, you are recording much less of the subject than desired compared to what can be seen through the viewing eyepieces) you must increase the distance that the eyepiece in the trinocular tube is raised… which will then necessitate lowering the camera until it looks as sharp as possible through the camera viewfinder. (see Note 1 at bottom)
(If you have ever used a slide projector the following analogy might be helpful. Think of the camera sensor as the screen, the eyepiece in the trinocular tube as the projector lens, and the microscope slide as the slide. If you focus the lens on a slide projector so that it moves farther way from the slide, the screen (sensor) must be moved closer to be in focus, and the projected image becomes much smaller).
Once you have “roughed in” the magnification and positioned the camera so that it appears that you have achieved best focus on the camera focusing screen you are ready to move on to the next step. We're done looking through the camera viewfinder for focus from now on.
If you have a camera body that has a “live-
If your camera does not have “live-
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Step 2-
Take a series of three pictures.
1) The first should be taken at what appears to be the best focus through the viewing eyepieces.
2) For the second shot, move the fine focus and raise the subject up towards the
objective just the slightest amount so that it now appears very slightly "out-
3) For the third shot, go back and set visual best
focus. Then using the fine focus, very slightly "lower" the subject stage so the
subject now appears very slightly "out-
Now examine
the three images very carefully at 100% or 200%. Find the one in which the subject
appears best. (Chances are none will be that great, but you will be able to tell
which is "closest".
-
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Run through
this 3-
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Step 2-
Mount the camera onto or above the trinocular tube. With “live-
It’s always best to try a few different subjects and check to be sure that you consistently get proper focus in the viewing eyepieces and camera simultaneously.
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Note 1: If available, it might be preferable to use a “viewing” eyepiece of lower
than 10X power. This would require raising the eyepiece a shorter distance in order
to get acceptable relay magnifications. My personal experience with this method is
limited. I have always used Olympus NFK photo-