Shooting car scene

Viciam · Oct 12, 2017

hi guys,

What is best setting to record a car from side view because last time i did it, it seems the wheels are not spinning and the car is just gliding along the road.

Im guessing it has to do with frame rate/shutter speed? I believe i i was on 30fps and 1/60 shutter speed. I obviously want to capture the wheels spinning

Also i want it to have a fast action movie type of feel. So should i use 30fps but with higher shutter speed?

Hollow Dog · Oct 12, 2017

You're right that would be controlled by shutter speed.

As the wheel RPM reaches the shutter speed the wheel will appear to be in a stop condition.

Viciam · Oct 12, 2017

Hollow Dog said:
You're right that would be controlled by shutter speed.

As the wheel RPM reaches the shutter speed the wheel will appear to be in a stop condition.

Ah i see. So what do i need to do?

MAvic_South_Oz · Oct 12, 2017

Experiment.
Either different shutter speeds or maybe easier different vehicle speeds.

Hollow Dog · Oct 12, 2017

Viciam said:
Ah i see. So what do i need to do?

I believe as a general rule a faster shutter speed for a slower-moving wheel and slower shutter speed for a quicker moving wheel will generally show more wheel movement. (blur)

BlakPhoenix · Oct 12, 2017

This is not controlled by your shutter speed, but by your frame rate. Shutter speed only controls how crisp a single frame appears, frame rate controls the motion between frames. To ensure a wheel continues to keep turning (in a forward motion), the frame rate must be faster than the wheel rotation rate.

But wait! There's more... Many car wheel are actually displayed in a repeating pattern. This means that the frame rate has to be faster than the repeating pattern (or a multiple of this), not the entire wheel rotation. Or written another way, the time between frames must be less than half the time it takes the wheel to rotate so far as the pattern begins to repeat (usually when the following spoke is in the same position the last spoke was in the pervious frame). The reason that it is half the time, is because if you get close a full pattern rotation then the wheel begins to look like it's rotating backwards.

But have no fear, this can all be worked out beforehand you just need to know a few things first:
Diameter of the tire (not the wheel, but the whole tire)
How many times the pattern repeats on the wheel (usually how many spokes there are)
Speed of the car
Frame rate you will be shooting at (we will assume 30fps)

~~~~
In the example below I will assume that the tire diameter is 17" and the pattern repeats 6 times.

Let work out the tire circumference:
pi x 17 = 53.4 inches

Now let's work out the repeating pattern distancing:
53.4 / 6 = 8.9 inches

Divide this by two so that the wheel moves forward in motion:
8.9 / 2 = 4.45 inches

So now we know that we have to take a new frame before the car has travelled 4.45 inches. So since we're shooting at 30 fps:
4.45 x 30 = 89 inches per second = 5.1mph (8.18km/h)

And there you have it! For a 17" diameter tire with 6 spokes, you must stay below 5mph to have the wheel rotating forward.

But that's not very fun is it? 5mph is far too slow!

~~~~~~
Let's go faster!!! We don't care if the spokes rotate more than once between a frame, so long as the next frame appears to show a following spoke, it can actually be any spoke!

So here we can get into multiples of the same speed. So here you can just reduce the number of repeating patterns on the wheel all the way down to the top speed the car can be going of ~45mph (72.5kmh), where the entire wheel almost rotates before the next frame. But we can go faster still, why limit ourselves to one full rotation, we can go hundreds of rotations between frames!

tl:dr
Really it's all about timing your frame with the position of the wheel. See the above formulae to work out at what speed your tires will align well with your frame rate and enjoy the maths & science!