will be shooting a fashion show with the XL1S standard lens, last year shoot same fashion show at 1/60, lighting was not the best want to trie to shoot at 1/30 , anyone used this setting with people walking, strobe effect? any imput will be appreciated.

At 1/30 there will be some strobe effect with movement, especially faster movement. Give it a try with some friends as models and see if the effect works for you or if yo uneed some more light.

In addition to the potential strobe effects, you will lose 50% vertical resolution.

50% vertical resolution loss? How will this happen?

Because only one field is being used in those "lower than system" refresh modes (1/50, 1/60)

As far as I'm concerned the possible 50% resolution loss is totally academic. The slow shutters speeds are visual effects and the loss isn't noticable when viewed on a television. I suppose this is due to the bluring induced by movement.

Andre, do you also know why they use only one field? It doesn't
make a lot of sense at the moment here. I'll see if I can do some
resolution tests this weekend....

I haven't researched this topic, so i'll throw this question in here. I've heard (read?) that some cameras use interpolation for the second field, rather than repeating the same field twice. While this would still lower the resolution, it would be less than the 50% loss caused by repeating fields.

Interpolation is not an increase in resolution.

Basically, 1/30 is the same as frame movie mode. There's not enough time to multiplex two image captures into two fields per frame at this shutter speed(30 fps > 1/30 sec per frame+ processor overhead). So one field is captured, and replicated into the second field.

And what if you are in frame mode and have such a low shutter
speed, what then?

same same

Frame mode and low shutter speed settings are two different things. Real frame mode in camera's captures the two fields during the same timeslot (which could be in contrast to the long shutter settings very short,like1/1000..) and dumps the data into a full frame buffer memory. This memory is being read out in interlaced mode. The end result is two fields (interlaced)belonging to the same "shot". Pseudo frame modes, because they don't have the full frame budder memory, try to imitate frame mode by R/B and G combinations or by line repetition or interpolation on a single field, resulting in somewhat better vertical resolution and/or artifact reduction. Slow shutter modes integrate the light during a longer time (like 1/30 or longer) and output (repeat) one field during the time the next image is being integrated on the CCD. Slow shutter images allways show motion blurr(long integration time on the CCD) and can have refreshes going down to 3 times.sec. Frame mode however can have very short shutter times if needed.

You can't access the slow shutter speeds in frame mode so it shouldn't be a problem

Either my mind is completely wrong here or something else is.
If I have my Canon XL1s in frame mode I can do 1/25 and even
a bit lower (I have the PAL model)....

Frame mode doesn't exclude long shutter settings. The difference with long shutter settings in intelaced mode is that in frame mode the two fields are continiously read out (no res loss) during the time the next integration period of the CCD is running.