I think I would prefer to call this item a zoom rail vs a focus rail. Basically the rail lets you get in close or far away (a rough focus, if you will) I then turn the lens assembly for fine focus it.
I missed this when you posted back in March.
Indeed the rail is a "focus rail", while the adjustment ring at the lens adjusts the "zoom". It is true that when in a fixed position of the focus rail, adjusting the zoom ring will adjust focus, but adjusting the zoom ring also
changes the magnification, or zoom. Think of it this way...you can't change the magnification/zoom by moving the scope body up/down, can you? Moving the scope body up/down only
changes focus.You have to change the scope zoom ring in order for the magnification to change.
Why is this important for coin photgraphers? Well, let's say you shoot a bunch of Dollars, and then a bunch of Dimes, and then go back to shooting Dollars. You'll probably want the second batch of Dollar images to be the same size as the first, right? This means you will need to have the same magnification, and thus the same zoom ring setting on the scope body.
Now if that hasn't confused you, then unfortunately the method to get that same magnification each time might put you over the top...
To calibrate a macro photo system, you first decide how much of the frame (vertically) you want the coin to fill. I usually choose 90% or 95%. You can do this most easily using a ruler with a fine scale. You must iteratively adjust the vertical focus rail and the zoom ring until the coin fills the desired amount of the frame, or until the ruler reads the correct amount. "Correct" for a Dollar to fill 95% of the frame would be 38/0.95=40mm. So adjust the zoom and focus until the frame height captures 40mm of the ruler. Then place your Dollar on the stage, and adjust the focus rail (NOT the zoom ring) until the Dollar is in good focus. Take a marker or pencil and mark the focus rail to indicate this is the position for Dollars.
Repeat the above procedure for each size coin/medal/etc that you would want to photograph.
Now comes the part that is counter-intuitive to the above discussion, but indeed is correct...let's say you have been shooting Dollars, and then want to shoot Dimes. You will move the focus rail to the position for Dimes, but you won't move it after that, instead using the "zoom" ring to focus the shot!! The reason this works is that for a given size coin to be in focus, both the zoom ring and
the focus rail have to be in their exact positions for that size coin. To get repeatability, you can fix the focus rail and adjust the zoom, or fix the zoom and adjust the focus rail, and you will get the same result, but IMO it is virtually impossible to fix the position of the zoom ring, but fixing the position of the focus rail is relatively easy.
So if you're still with me, there is one more caveat...you must put the coins at the same height each time. This is a problem for slabbed vs raw coins. If you are shooting both slabbed and raw, then you might consider having a thin (a few thicknesses) piece of felt or similar to put the raw coins on to bring them up to the same level as a slabbed coin. Alternatively, you could calibrate the system for slabbed coins at 95% of the frame height, and then just let raw coins be a little smaller since they will be a little farther away from the scope.
Hopefully the above makes sense to all...Ray