Image attached.
Measure from Point A (Lip), or Point B (end of threads)?
I found different places online that say one and the other. I used to know this but now I’m confused.
(I know the other end is the camera sensor.)
Thank you.
📷 backfocus.jpg
And is it universal for all reducers?
I use three reducers from different brands, and the instruction manuals always specify from point A
From point A. The threads will be inside, or overlap, with whatever adapter you screw on to it so including them in the math doesn’t make sense.
I have used this reducer for several years and agree it is point A. The threaded connection gets buried in the image train and does not add to the length once it is attached.
They are not all measured the same. My Celestron EdgeHD 8 reducer was measured from the center of the Focal Reducer (FR) glass (facing camera chip) at B (link to manual). I measured the distance from B to A then marked that value from threads at A on the outside of the next connector, marked that in silver sharpie then used that mark to calculate back focus (105mm ±0.5mm per the FR specification) using digital calipers to the recessed chip location on the camera (17.5mm on my then new QHY294M).
No FR, BF distance for EdgeHD 800 increases to 133.35mm. I switched to a refractor and BF issues were replaced with focuser limit issues.
CS & GB! Kip