"Back focus" for a camera means the distance from its sensor to some reference plane on the front of the body. The problem is which plane: the bare body's front face, the face of a tilt plate, or the end of an adapter that happens to be in the box.
The pair problem
Some manufacturers publish two numbers separated by a slash and leave you to work out what they mean. ZWO quote the ASI533 as "17.5mm/6.5mm". Those are the same camera: 6.5 mm is the bare body, and 17.5 mm is the body with the 11 mm T2 extender that ships with it screwed on.
If you take 17.5 and then also fit the extender, you are 11 mm long. If you take 6.5 and forget the extender is already on the camera, you are 11 mm short. Both are easy mistakes and neither is visible until you look at your corners.
The model-number problem
A model number is not a specification. The QHY268C had a back focus of 17.5 mm and then, from January 2023, a short-back-focus version at 14.5 mm — same name on the box, 3 mm apart. The QHY533 is worse: the mono body is 14 mm and the colour body is 17 mm, under one model number, and they take different threads.
Any figure you find on a forum from 2021 may be describing a different physical object from the one you own.
What manufacturers actually print
A sample from this site's dataset, quoted verbatim, with whether the manufacturer states the plane the figure is measured from:
| Camera | Published, verbatim | Datum |
|---|---|---|
| Player One Ares-C Pro | "Back Focal Length 17.5mm" | datum not stated |
| Player One Artemis-C Pro | "Back Focal Length 17.5mm, 12.5mm( without sensor tilt plate)" | states the datum |
| Player One Neptune-C II | "Back Focal Length 12.5mm" | datum not stated |
| Player One Poseidon-C Pro | "Back Focal Length 17.5mm, 12.5mm( without sensor tilt plate)" | states the datum |
| Player One Saturn-C SQR | "Back Focal Length 12.5mm" | datum not stated |
| Player One Uranus-C | "Back Focal Length 12.5mm" | datum not stated |
| QHYCCD QHY183C | "17.5mm（±0.2）" | datum not stated |
| QHYCCD QHY183M | "17.5mm（±0.2）" | datum not stated |
What to do about it
- Take the figure from the manual, not the product page. Several manufacturers omit back focus from the product page entirely.
- Take the SMALLER number of a published pair, and treat the adapter that makes up the difference as a separate part you can choose to fit or not.
- Check whether your body has a tilt plate and whether the published figure includes it.
- Confirm mono and colour separately, even when the model number is identical.
- If the manufacturer does not state the plane, record that it did not. An unstated datum is a real piece of information about how much to trust the number.
Every camera page on this site quotes the manufacturer's line verbatim and says whether the datum was stated, so you can judge the figure rather than take our word for it.