Fluorecent and AF inaccurracy

Why on earth would fluorescent light make a lens backfocus horribly when it is pin sharp in any light but fluorescent? My 24-70L does just that. Couldn't work out how it could be backfocusing so badly sometimes, sent it off 6 times to Canon CPS but they always sent it back and said it was calibrated perfectly. Last night I shot a Barmitzva and half the shots were off. It dawned on me that the focus problems were always with fluorescent light, so I did a test on my two 5D's. 10/10 with each camera were OOF with fluorescent, in a dimmer tungsten light 10/10 with each camera were perfect.

Anyone know how or why? Am I wasting my time sending it back to Canon yet again?

Don't know why, but I see the same focus error under flourescents in close focus test of the 50L.

Take it outside or under incadescent and it is much better.

I'm hoping to hell it isn't all of these lenses, I've been looking at photos taken with a rental one I once had to use and it seems fine.

Just guessing, but if it is normal domestic (4 ft tube type) fluorescent lighting, not the high frequency electronic type, then I suspect it is in connection with the 'flicker' as compared to tungsten lighting. I think the low energy fluorescent lights operate at a higher frequency, so it may focus OK with them.

Best wishes,

Ray

Well I'm just back from a 12 hour wedding, the majority of it under fluorescent lighting with a friends 24-70L, every single image sharp and in focus.

Ray, one of the worst things I ever did was to sell my original 24-70L to you, the 2nd one has been a nightmare from start to finish.

the camera - not the lens

I can't say for sure why the problem is occuring but since all AF determination is done in the camera rather than in the lens (the lens just moves to where the camera tells it to move) I would say it must be a problem with your 5D. Furthermore, as it seems to be common to two different examples of 5D I'd suggest it is a generic 5D fault.

Send the cameras in (one at a time) but you might as well keep the lens at home.

I wonder if there is a frequency (or band of frequencies) of light that is missing in the fluoro output that the camera AF is most sensitive to, so that it has relatively little light to work with. If so, then I guess it would have to be at the lower frequencies that look warmer to our eyes. Unlikely, but something is upsetting the AF. The overall colour temperature of fluoro lighting (4000-5000K) is approaching that of daylight but I haven't seen a frequency spectrum for it for so long that I forget what it looks like.

I don't see how simple flicker can be doing it as many light sources flicker.

Ditto

My 5d with my 28-70L does the same thing. Outdoors its perfect, indoors low light flourescent, OOF backfocus , front you name it. Its been at canon once . Did not change anything. Ive considered selling it to replace it with prime lenses.

An issue addressed by folks at DPReview.

This question keeps coming up over at DPR and I have sent them to this thread to get some info. Coming back with some more detailed information may be a help for others. Speculation leads to rumor so let's find out the facts foir this subject. I know there are those here at OPF that know the answer.
Don

maybe Chuck will jump in and settle this

I'm glad I'm not the only one who has seen this phenomenon. It was apparent with both my 5D's and the 24-70L and 50/85mm lenses. I sent my 24-70L in with camera, it is now accurate under most fluorescent though by no means all, with the center point only. The 85mm with it's 5D is now accurate, again with center point only under all lighting. The off center points are worse than useless, worse than when I sent them in. I haven't seen the problem at all with any of my f4 zooms such as the 17-40L, 24-105L and 70-200 f4L IS. Could be something to do with the frequency of some types of fluorescent lighting (open strip lighting seems to be the worst while diffused doesn't seem to be a problem) Sigh...

What I do know is that I'm getting far more in focus and sharp enough shots using f1.8 (85mm) and f2.8 (24-70L) with focus recompose and the center point than using any of the off center points. Given that focus recompose at f1.8 is not exactly a good idea I think it says it all for canon's non pro level AF sensors...

Thanks for jumping back in on this Ben. I actually have never noticed the problem on my own 5D and 24-70. I was trying to help someone else and I remembered somewhere I had read a thread that mentioned it. It also introduces others to OPF by sharing across the net.
Perhaps Chuck will respond on this phenomenon if you approach him?

I did correspond with Chuck at the time, he tested a whole bevy of lenses in fluorescent lighting and sent me the results which didn't have the problem.