Author Topic: FWHM focus feature?  (Read 1883 times)


  • Just arrived
  • *
  • Posts: 1
FWHM focus feature?
« on: September 03, 2013, 06:58:42 PM »
Hi Folks,

First, I have to say, I *love* ML - it has provided some absolutely super extensions to my 550D functionality.

I use my camera mainly for astrophotography, usually coupled to a high-quality 80mm apochromatic refractor, but often with a regular Canon AF lens which I always end up setting back to MF in order to achieve an accurate star-focus. This takes a bit of trial and error to achieve focus and I'm never quite sure if I have the sharpest point or not.

When shooting "tethered", I use a Windows application called BackyardEOS - some folks here may well be familiar with it. One of the focus-aid features it presents is a measure of the FWHM (Full Width at Half Maximum) - this is a measure (in camera pixels) of the width of a chosen object (typically a star, but a planet would work, too) across the line where the intensity is equal to half the maximum (i.e. the brightest part, typically the center). This feature is incredibly, incredibly powerful - I can be assured I have the sharpest possible focus each and every time by tweaking the focus in order to minimize that number.

I think (and I realize I may be in a minority here) this would be a great feature to build into the focusing aids in Magic Lantern.



  • Developer
  • Hero Member
  • *****
  • Posts: 3218
  • 60Da / 1100D / EOSM
Re: FWHM focus feature?
« Reply #1 on: September 04, 2013, 05:28:44 PM »
For astro I use LV display gain + FPS Override + 10x zoom to get focus, works like a charm, it's very fast, and you can be very confident that you've achieved optimal focus. IDK if a complicated algorithm to calculate this for you is a necessary or even possible b/c of the limited CPU/memory resources of the cameras.

I made something similar to this request myself before I knew about the FPS override + LV gain trick.