Crop factor and video

We have had a few threads recently on the subject of crop factor and what happens in video. As various contributors have said there does not seem to be anything definitive in the manuals, and I am afraid that for me the discussions have not always been as clear as they might.

I have decided it's time to go back to basics. What follows uses a GH4 and has to be qualified as being unique to that model, though the principles apply more widely. I've put some sample images below - boring and of no photographic merit, but they illustrate the points being made. I've reduced all to 1024px wide to reduce sizes.

My starting point is simple sensor size. Full frame is 36 x 24, 43.3 diagonal. MFT is 17.3 x 13, 21.6 diagonal.  Hence the oft quoted crop factor of 2 for MFT. This of course ignores aspect ratio. If the MFT is used in a 3 x2 mode the diagonal drops to 20.8. The effective crop factor here is now 2.08, which can be calculated simply by using sensor widths. (36 / 17.3)

Turning to video, my understanding is that FHD modes use full sensor width.  That is borne out by the test images. So comparing to a FF that uses the full sensor width we are again talking of an effective crop factor of 2.08.

For 4k video, aka UHD, the image size is 3840 x 2160 against 4608 x 3456 for the sensor in basic photo mode. The video uses the central 3840 px across the sensor width, hence reducing the image width by a factor of 4608 / 3840 = 1.2. Multiplying this by the usual 2, leads to the idea that the 4k video crop factor is 2.4. Comparing this to full frame still might though, using effective sensor width as the key parameter, be more reasonably regraded as 1.2 x 2.08 = 2.5.

Finally I looked at 4k photo mode. Here the image uses the central 3328 pixels, hence an effective width of 12.5mm. However we are back to a 4 x 3 aspect ratio, so have a diagonal of about 15.6. This is a crop factor of slightly under 1.4 compared to the MFT in still mode and hence about 2.8 compare to FF.

That's all rather boring. And to me it leads to 2 conclusions.

First, the phrase "crop factor" is a convenient shorthand. At it simplest it does offer a feel for how systems compare. But as soon as we start digging into detail it falls apart.

Secondly, I think it is irrelevant when using the camera. I compose by adjusting the zoom based on what I can see in the viewfinder, not by some devious process of thinking "what FF focal length would I use and how does that translate to this MFT camera". The one point where it might affect my thinking is knowing the scale of the impact of the 4K photo modes and possibly going for a wider lens sooner than I might otherwise do.

Finally I'll repeat my caveat that this is GH4 specific. In particular I'm unsure about the effects of using the larger 20Mp sensor in some models.

TEST IMAGES

Photo

FHD video

4K video

4K photo mode