Nettdi, What is the black ring at the front of your camera marked with? C or CS?
C = 17.5mm back to the sensor
CS = 12.5mm back to the sensor
The 1.25" nosepiece for this camera is usually 20mm but sometimes 25mm in length.
There are a lot of different numbers about the correct spacing for these (and similar) .5x 1.25" focal reducers. What Clouzot has mentioned is the common information for the GSO sourced reducers (50-55mm), but I've seen a few similar reducers with different spacing.
I would actually check the focal length of the focal reducer. Measure the focal point with just the reducer from a bright infinite source (the moon is a good choice - do not use a light bulb in the same room) and then use half of that measurement as the spacing the reducer needs.
Something else to check. The focal reducer should when mounted on your camera have the lens that faces outward (away from the camera) curved outwards (a convex surface). There have been examples of inexpensive focal reducers being assembled incorrectly. While the curve is not obvious it's quite easy to check, just gently place the end of a small ruler (or other flat surface, even a small piece of paper or cardboard can be used) across the lens and you should see a small space towards the edges .... or the ruler should be able to rock back and forth a little bit.
If the lens is assembled wrong the lens surface facing forward will be concave (curved inwards) and a flat end of a ruler will not rock when placed on it (or you will notice a space in the middle of the lens to the surface of the ruler).
If you need to flip the lens over the thin retaining ring can usually be carefully un threaded using two small (jewelers) screw drivers or other small sharp ended tool. If you do remove the lenses to flip them make sure to be careful to keep the lens together (they are often in two pieces) and don't rotate them, just ease the lens out, flip it over and return the retaining ring. Be gentle with the lenses and keep them straight when removing and re inserting them to prevent chipping the edges.
Edited by mclewis1, 02 July 2020 - 10:44 AM.