Why when shooting slow motion do you change the camera speed? Why not just do it in Post (I.E. Final Cut Pro)?

I shot my first wedding on my HD100 at 1280x720 at 24p. In post I wanted to slow some shots down for effect. Having access to Shake (which in my opinion is far superior at handling retiming than fcp), I slowed the footage down. What I got was very fluid slow motion- and a clear ripply shell around my moving subject, which happens when you blend frames in shots with lots of motion. It's better to shoot at a higher frame rate and then slow it down in your timeline than to tweak a lower frame rate in post, because when you shoot at say 60p you have a lot more frames to work with and you don't have to worry about your compositing software having to overcompensate for pixels that aren't really there.

The speed in a clip is related to how many frames there are per second. If shooting 24p, there are 24 frames per second that make up the action. To make a clip "slow mo", you need to add in more frames to make the action last longer. This is done in 2 ways: letting Final Cut (or other app) add in new frames between the solid frames that you shot (retiming) or by shooting more frames to begin with.

Retiming:
You have a clip at 24p and you want to slow it down 50%. Final Cut will spread out your actual frames and place in new frames in between with frame blending. You can scrub frame by frame to see whole frames and blurry frames in sequence. There are apps like Shake or After Effects, however, that use what's called "optical flow" technology. This takes a lot longer, but it means that the app will look at the frames you have and decide what SHOULD be in the missing gaps based on the actual images. It creates new WHOLE frames instead of just BLENDING. It produces pretty good results.

High Speed:
The best way is to just shoot more frames to begin with. If you shoot 60p and drop that into a 24p timeline, you will have those frames that originally took up 1 whole second now spread out into 2.5 seconds. That means you now have 40% slower footage without frame blending or the hassle of outputting into a complex compositing software package to use optical flow. A lot of those commercials showing glass breaking or bullets shattering water bottles are shot at 1000 frames per second and then dropped into a 24p timeline. That will get you about 42.5 seconds for each 1 second shot.

Tip:
I have slowed down clips in Final Cut many times. The trick is to not go too far. I find that 70% or above still looks good with frame blending. I would not go below 60% or it could look really stiff.

Hope that helps.

Which JVC model are you using? If it's the 110, 60P is only available in SD mode - which works well, but it's no longer 1280x720. Optical flow is feature brought over from Shake and built into Motion 3, so try importing a few clips there for slow mo, and render out (which will take a while, but worth it).

Just don't try to render any clips where a moving object is filling the majority of the screen or you will have artifacting around the edges when you frame blend.