One Way Autofocus


I am using a home-made ribbon-based automatic focus system for my telephoto lens. The problem with it is that it has backlash in both directions. Meaning that there is a 30-50 step lag whenever I switch focusing direction.
Is there any way to adjust autofocus so it can only run in one direction?
For example, start at step 1000 and calculate HFR for 15 steps in one direction. Or is the best way to do it manually?


No there isn’t. Just backlash compensation is available. I thought it worked both ways though? Perhaps if SGP supported DSLR motor focus (from the camera) you wouldn’t need that at all?


Backlash compensation in SGP has an IN or OUT preference. As far as I know doesnt give u the choice for both directions. Perhaps, it wouldnt be difficult to change that!!

Ι use CCD with my lens, not DSLR.


Made it a feature request for you :smile:


Maybe I am just being dense here… but to me any focuser, ribbon based, belt based, gear-based… if they have backlash all of them have it in both directions. The point is you just pick one direction to unwind it because the assumption is that it will be “engaged” in one direction.


Sure…but when I engage autofocus both directions will be engaged…as far as I understand it.
Suppose i am at step 1000 and set a number of points each 10steps the focuser will go to position 10xa and reverse…going inwards. Thats one change.
After the sequence ends it will reverse again towards the point of correct focus. So i have 2 directional change.
Is it no so?


This request has come up before and I don’t think it is needed. The only thing needed is to make sure the sequence of steps during autofocus goes the correct way - and right now SGP does not guarantee that because it always does autofocus from high to low.

But if you ever need to do a sequence of steps it shouldn’t matter if you do it from high to low or low to high. You should always do it in the “best” direction for your system - which usually means pushing against gravity. As long as you always do stepped things in that direction the existing single-direction backlash compensation will work fine. You just make sure never to do a sequence in the wrong direction.

So there is still a pending request for autofocus to go either from high to low or low to high - depending on the compensation direction. But I don’t think there is a need to allow steps to go up or down without backlash compensation - because the position values will be different going up vs. going down - due to backlash. Doing everything one way only is a way to make those numbers solid and repeatable.



If you are at position 1000, and you choose 7 points, and 25 step per point, and 5 steps for In backlash;
Sgp autofocus routine does:
Go to position 1000+25+25+25+5-5 =1075 (backlash removed).
Go to current position -25 = 1050 (no babklash, same direction).
Go to current -25 = 1025 (no backlash, same direction).
Go to current -25 = 1000 (…).
Go to current -25 = 975 (…).
Go to current -25 = 950 (…).
Go to current -25 = 925 (…).
The focus point is computed and is, by example 1012.
Go to the focus point 1012+5-5=1012 (backlash is removed again).

Backlash compensation well done in only one direction !


yes i can do it that way but the final goto will be manual…not automatic…no?


OK… say you are at position 1000. No focuser movement has yet occurred and your ribbon is right in the middle of the backlash “slop” (meaning some small amount of movement in either direction will result in no actual physical change.

Then… you set your backlash comp in SGPro to IN and 500 steps. Your AF routine has 9 steps and a step size of 10. This means your AF range will be 90 steps, 45 outside of the current pos and 45 inside the current pos… focus is hopefully somewhere in between.

So… your first movement is OUT by 45 steps, but backlash comp will adjust this to 1545 and then back to 1045. At this point, inbound movement is fully engaged. SGPro will take a focus frame here and then move in 10 steps and repeat until the range is complete. At this point SGPro will determine the focus position say… 990. The focuser will move out to 1490 (getting rid of the backlash from direction change), then finally to 990.


I got what u and Michel said…just was sure I was missing something. I wasnt aware that the compensation feature besides removing the backlash by adding the extra backlash steps also reversed direction by itself. Now it makes sense.
Thanks a lot for taking the time.