Out for a days shooting hang gliders and someone asked if I could take a few shots of them on their flash card.

I said sure why not and put their card out of a Nikon D1X into my 20D. Switched to JPEG and took a couple of test shots and all ok. Could see the Nikons images on the card also.

Anyway, after I put my card back in the folders jumped from 112 (frame 1293) to 253 frame (5369). So where did the extra 4000 odd frames and 141 folders come from??

The guy with the Nikon came back and said "can you see the shots", his camera couldn't see anything on his CF card including his original shots. I said "yes" and I put his card back in my 20D and he could see all the shots.

Weirdness?

This is normal.The reads the numbers and adjust to it.If it is a larger number then it starts counting from there on.

There was a clash with folder and/or frame numbers.

You can remove the card and reset your numbering in the camera. Use your pc to set the folder and image number to whatever your last photo was, then tell the camera to use continuous numbering again and re-insert your card. It should start where it left off.

Chances are that the other guy will get to see your photos when he looks at them with his PC but his nikon camera will be looking in a different folder from where your canon camera put them.

Thanks Alan, I didn't know you could do that.

Craig of Oz....
The easy (or not so easy) ONLY way I know to fix this is remove and reinstall the clock/date/setttings battery, which is inside the battery compartment. Reformating cards does not restore the old numbering.

Or, as above, you can set the camera to start recounting from zero each time.
That cameras do this makes sense. They will start counting from the highest last number on any card to avoid file number conflicts with downloaded images. The camera has no way to know who's card you put.
Jack

Thanks guys,

It ended up easier than I thought it would be. I just set the camera "file numbering format" from "continuos" to "auto reset" and put a formatted card in it. It went back to shot number 1 and folder 100. I then put the numbering back to "continuos" and put another card back in with a folder number I created and some shots that had the big numbers on them renamed them back to where they should have been. The camera started counting back from the renamed file and folder numbers.

Regards, Craig.