Composing for crops
Avoid framing too tightly before switching to Live View video mode
All of Canon’s current Dslrs feature Full HD 1080p video recording, but their imaging sensors are packed with many more pixels than the 1920 x 1080 pixels needed for Full HD resolution. Take the 5D Mark IV for example, which is capable of capturing still images at a resolution of 6720 x 4480 pixels.
Full-frame sensors like the 5D’s make it easier to get a shallow depth of field to give movies a more cinematic look, but that quality would be lost if the sensor was simply cropped to give Full HD resolution.
To get around this requires some technical sleight of hand called ‘pixel binning’ often used by camera engineers to enable the full width of the full frame sensor to be used. Pixel binning achieves the lower resolution required for Full HD by treating neighbouring groups of pixels as one individual pixel.
Now, if you switch to 4K video on the 5D Mark IV, the image appears cropped. It seems weird that when you shoot the higher resolution 4K you end up with a smaller recorded area of the scene.
The reason for this is that Canon have opted for a 1:1 pixel crop. In other words, the 4K image is made up from 4096 x 2160 pixels at the centre of the sensor. This might give a sharper image, but the effective 1.74× crop factor means that lenses appear to give more reach, which can make it more challenging for wide-angle work.