Why is the FX30 more noisy when shooting at 100/120fps?

[alisterchapman]

I love the FX30, it's a great little camera and it produces a lovely image. But one thing a few have noticed is that if you shoot at 100fps or 120fps it can be a little more noisy than it is at other frame rates. 

When shooting 4K/UHD up to 60fps the FX30 downsamples from 6K of pixels to a 4K recording. This oversampling brings a nice noise reduction, the equivalent to almost 1 stop of exposure (around 4-6dB). When you shoot at 100/120fps the camera reads 4K of pixels, there is no downsampling, so the images will be noticeably more noisy. This is just a limitation of how this camera works, my guess is that it doesn't have enough processing power to convert 6K to 4K at 120fps or perhaps the sensor can't be read at 6K at 120fps. The plus side is that the normal 4K recordings downsampled from 6K really are very good indeed and packed full of detail and texture.

Plus, if shooting at 120fps and you choose to use the near equivalent of a 180 degree shutter -  1/250th, you will need 5 times more light to get the same exposure compared to 24fps and 1/48 (180 degrees). So to get the same exposure you need to open up the lens by 5 stops or increase the light level by 5 stops. Any less than this and you will be under exposed and that will make your footage look more noisy.

When I shoot at 120fps I will often use 1/125  to gain back 1 stop, then expose nice and bright to help eliminate the extra noise.

[julienjarry]

@alisterchapman are you saying that it is noisier even when exposed correctly? Or noisier because generally underexposed (which I know you know). Thanks!