Hi
Quick technical question. I notice when i am filming at 4K 30FPS and if i increase the FPS to 60 that the picture on my phone and i assume the finished video zooms in.
Why is that when all i am increasing is frame rate?
Thanks
The crop in is due to limitations of the sensor to handle the increased demands from doubling the frame rate from 30 fps to 60fps
TLDR:
The
Air 2s sensor is 5472 pixels wide but 4k only requires 3860 pixels wide. There are several ways cameras can deal with having a sensor that has more pixels than necessary for the video format.
When possible the best thing to do is to read out all the pixels and then downsample the image (combine pixels) to produce a picture that has the same field of view as the full pixel image while improving noise and sharpness. The down side to that is it takes time for the camera to read out every pixel and also this requires more processing power.
Remember that these cameras are rolling shutter cameras, they don’t read all the pixels at once, it reads then line by line. Going from 30 fps to 60 fps reduces the time between frames by half so the time between when the last pixel is read out could possibly be after when the first line of the next frame has to start being read out which is a technical limit. More rows=more time
Going from 30 to 60 fps also doubles the number of pixels the camera has to process per second and the camera may be limited by how many pixels it can process per second. Without the technical specifications of the sensor and processor there’s no way to say for sure but either one or both of these reasons could be why the camera has to change the scan mode for 60 fps. My guess is it’s the data rate that is the culprit.
So with one of the above technical limits to achieving 60 fps the best solution is to readout fewer rows and pixels for each frame. Some cameras only allow higher frame rates in 1080P for example but as mentioned the
Air 2s sensor has many more pixels than necessary to achieve 4k so the camera crops into an area of the sensor that has exactly the number of pixels necessary for 4k which means it has to read out fewer pixels to get you the same 4k resolution. Fewer rows to read out means each frame can be read faster and solves the issue of timing. Fewer pixels that don’t need to be combined to get to 4k also saves a ton of processing power for the camera to be able to handle double the frames.
/TLDR
It’s a pretty common way for cameras to handle higher frame rates especially when the sensor has more pixels than needed.
