How do I change the duration of a still image in iMovie to less than 0.1 second?

To elaborate on Karsten's answer, if you are working with very small durations, you will want to go to iMovie/Preferences and put a check mark next to "Display Time as hh:mm:ss:frames". This will let you edit to the nearest 30th of a second, rather than to the nearest 1/10th of a second if it is not checked.

I have found there is a practical limit of about 3 to 5 frames where iMovie dies not want to go shorter when using the Duration popup box.

To go shorter in this case, you can

1) Slide the thumbnail slider to 1/2 second so you can select individual frames with your mouse.

2) Select 1 frame, 2 frames, 3 frames, or whatever you need with your mouse. Then Command-C to copy. Command-V to Paste. Then delete the original clip. You are left with your 1 frame copy.

I am not sure that was clear, so ask if I didn't explain it well.

AppleMan1958 wrote:

… I have found there is a practical limit of about 3 to 5 frames where iMovie does not want to go shorter when using the Duration popup box.

for stop-motion there are perhaps better tools avail

istopmotion is a commercial app (starting at 40€ in the AppStore) offering additional tools for the animation master, such as onioning or tilt-shift.

http://www.dayofthenewdan.com/projects/time-lapse-assembler-1

is some very basic stills-assembler, stitches stills together only, therefor free ..........

the time format in video is

min:sec:frames

00:00:01 would indicate 1 frame 'length' = 1/30th of a second (assumption is, your working in a 30fps project)