Five years late to the game, I am picking up a Panasonic GH4 for filmmaking. This is for a specific project, which I needn’t discuss here. I have just spent some time making sense of the crop factors and the 4K windowed video, and here is what I have figured out.
First of all, “crop factor” has to do with differences in the physical dimensions of the image sensor. A “full frame” image sensor is one that is the same size as the film frame of a 35mm still camera*, about 36×24mm. Most digital cameras use a sensor that is smaller than that. The result is that the smaller sensor sees a smaller portion of the image projected by the lens. This is exactly as if an image from a full frame camera had been cropped to a smaller size, hence the name. The ratio in size between the smaller sensor and a full frame sensor is known as the “crop factor” or the “35mm equivalence factor”, and it is useful in comparing the angle of view of different lenses based on their focal length. For example, an APS-C sensor has a crop factor of 1.6, so a 100mm lens on an APS-C camera will have the same angle of view as a 160mm lens on a full frame camera.
All of that is broadly understood, but what most people don’t realize is that a crop factor is somewhat meaningless if you are comparing image sensors with two different aspect ratios. A Four-Thirds sensor (which is 17.3×13mm) is known to have a crop factor of 2.0, but that is actually the ratio of the diagonal dimension. The horizontal factor is 2.08 and the vertical factor is 1.85. So is a 12-35mm Micro-Four-Thirds lens equivalent to a 24-70mm EF lens? Well, yes, but only diagonally. How often are you trying to fit a subject diagonally across the frame? Still, it is a reasonable rule of thumb. However, the aspect ratio situation gets even more complicated when you digitally select a different aspect ratio. The Four-Thirds sensor has a 4:3 aspect ratio. If you use that sensor to record an image with a 16:9 aspect ratio, the portion of the sensor being used is even smaller (17.3×9.75mm). The horizontal crop factor remains the same at 2.08, but now the vertical crop factor is 2.46.
For 4K and C4K video, the GH4 uses a “window” of the sensor. Instead of the 4608×2592 pixels on the portion of the sensor that covers a full 16:9 aspect ratio, it uses only the 3840×2160 pixels in the middle of the sensor for 4K, and the 4096×2160 pixels in the middle of the sensor for C4K. I assume this solves a problem with aliasing or processing performance. In any case, this means the effective part of the sensor is even smaller: 14.44×8.12mm for 4K and 15.4×8.12mm for C4K. A smaller sensor means an even more significant crop factor: a 2.49 horizontal crop factor for 4K and a 2.34 horizontal crop factor for C4K.
What is the bottom line? Well, here are some horizontal angles of view for common Canon lenses on a full-frame sensor: 16mm = 96.7°, 24mm = 73.7°, 35mm = 54.4°, 50mm = 39.6°, 70mm = 28.8°. Here are some horizontal angles of view for Panasonic lenses when shooting 4K on a GH4: 7mm = 91.8°, 12mm = 62.1°, 14mm = 54.6°, 20mm = 39.7°, 35mm = 23.3°. This helps me know what lenses I will want when shooting my project.
(* 35mm still cameras and 35mm film cameras have difference frame sizes. Film moves through a still camera horizontally, so the limiting factor is the height of the frame, about 24mm high. Using a 3:2 aspect ratio, the width of the frame is about 35mm, hence the name. The same film moves through a motion camera vertically, so the limiting factor is the width of the frame, about 24mm wide. The height of each frame will then depend on the the aspect ratio and whether an anamorphic lens is used. Nothing about it is actually 35mm, except that it uses the film stock designed for a 35mm still camera.)