I just tried the collimating my own copy of the 24-105L using the technique described above (except using the star Canopus rather than a artificial light source) and the improvement was stunning! My lens is now a great performer wide open and at all focal lengths as a result. Attached is a comparision at the 105mm f4.0 - the weakest setting for this lens - showing before and after. Both shots are near best focus possible and are crops at 100% from a 50d.
Dissassembling this lens is not for the faint of heart and requires some patience - it's not for everyone!
Yeah me neither, I'm just not that game even though mine has a soft spot at the bottom right of the image. I'm sure ill talk myself into it though lol.
I'm very impressed Terry! Is that example from the corner or centre of the frame? Been awhile since I used that lens for any astro work but I'm pretty sure mine doesn't look like that.
I've come to the conclusion that I had an exceptionally bad copy of the 24-105L lens. I believe most 24-105 owners would not need to go through this process.
Here is a centre crop and reduce fullframe at 24mm f4.0 using a 50D. This was a single 30 second exposure with cloud and moonlight interference. There is some minor coma at the edges, but IMO quite acceptable for nightscape shots. Comments?
Btw...the previous comparision was also done at the centre of the frame and at 105mm f4.0, but was slightly out of focus in the post shot. In the before shot the image was very mushy and this was the best I could do at the centre of the frame!.
Looks a bit like what I get with my 24-105L, but not as bad, and only in the corners...Interesting. Huge difference, I bet you're happy with that!
Erik