Frame Rate Conform and Shutter Angle

Shoot at one frame rate, play back at another, and the clip changes length. That is the whole of slow motion, and the arithmetic is one line:

Slow-motion factor = capture fps ÷ playback fps

Playback duration = clip duration × slow-motion factor

Shoot 120 fps and play it on a 30 fps timeline and every second you filmed becomes four. Ten seconds of action runs for forty. Nothing is invented and nothing is thrown away: the same 1,200 frames are simply shown more slowly.

Conforming is not converting, and mixing them up is the usual mistake.

What this calculator does is conform, sometimes labelled “interpret footage” in an editor. The frames you shot are relabelled to a new rate and the clip stretches. Quality is untouched because no frame is guessed at.

Converting is the other thing, and it is what people mean when they say a 24 fps film has to air on 30 fps television. There the duration must stay the same, so frames have to be manufactured. The old broadcast answer was 3:2 pulldown, spreading four film frames across five video frames. The modern answer is optical flow, and it produces artefacts around fast edges no matter how good the software is. If you have the choice, shoot at the rate you will deliver at.

Standard rates and what they are for

If all you need is the finished clip length or a frame count, the frame rate converter does that without asking about shutter.

Those odd fractional rates are not a typo. When NTSC gained colour in 1953 the subcarrier interfered with the audio, and the fix was to slow the whole system by a factor of 1000/1001. Seventy years later every US camera still carries the scar, which is why 29.97 exists and why a timeline set to 30 will drift against one set to 29.97 by about one second an hour.

The 180-degree shutter

Whatever you capture at, the shutter should be roughly double it: shutter = 1 ÷ (2 × capture fps). At 24 fps that is 1/48, which most cameras round to 1/50. It comes from the rotating half-disc shutter in a film camera, and it is the blur people read as “normal” motion. Go faster and the footage looks stuttery and hard, which is the deliberate effect in the Omaha Beach landing in Saving Private Ryan. Go slower and it smears.

The catch is that a fast shutter needs light. At 120 fps you want 1/250, which is three stops darker than the 1/30 you might use for stills, and that is on top of whatever the slow-motion mode already crops or dims. Bring more light than you think.

Worked example

You film a skateboarder for 8 seconds at 240 fps and cut it into a 24 fps edit.

• Slow-motion factor: 240 ÷ 24 = 10×
• Playback duration: 8 × 10 = 80 seconds
• Frames captured: 8 × 240 = 1,920
• Shutter to match: 1 ÷ 480, so set 1/500

Eighty seconds is a long time to watch one trick. Slow motion nearly always wants trimming afterwards, and the count above is the honest way to find out before you are in the edit.