Trap focus accuracy and stability

[SD]

I am using a 50D with 70-200 F4  with ML 2.3.

I put this arrangement to use with Trap Focus in CONT mode. I leave the thing sitting in out for a couple of hours and come back for my data, all well and good, I am getting  a stream of data but wish to really clamp down on edge cases.

Questions:

Obviously if my focus plane is say 20 metres away, it is not limited to those objects exactly at 20 metres, there is a range, from say 18 to 22 metres that will trigger the focus indicator in the camera and hence the shot.  The accuracy of this range is an issue for me.

If I use a 70-200 F2.8, will the inherently smaller depth of field hence narrow that trigger range?

Would a different camera body help?


   Thanks, SD.

[Francis]

Select a specific focus point perhaps? If you were using a larger aperture like 2.8 or a full frame camera like the 5d, your DOF would be shallower and thus your zone that can trigger the trap focus would be smaller.

Is 20m the distance you are actually working at or is that just a random #. At 20m you are focusing at infinity probably. On my 70-200 infinity is at about 13m or so. So if this is the distance you are working at, perhaps you get closer to limit the  trap focus.

[SD]

20 metres  was a random number I just used for illustration.

The distance is maybe 6-10 metres. It is the case that my setup is very sensitive in that accidental triggers by things further away or nearer than my desired focus band seem to happen quite a lot.  The focus band I desire is maybe 1-2 metres deep and I frequently have to deal with false positives from one side or another with the F4 lens.

I am working up to just getting that F2.8 lens and wanted to check my sanity on these ideas first, thanks for the input.

Another question is noise in the focus assessment done by the body. Does anybody have measurements on this stuff?

   SD