Crop factors of current 4K cameras - correct me if I'm wrong

[Andrew Reid]

This is going into an article so it has to be right...

As others have pointed out, when a camera does a 1:1 readout of the sensor in 4K mode, you can JUST use the horizontal resolution to calculate the added 4K crop of the full frame 16:9 window of the sensor.

This way the math is really easy - just by dividing the total horizontal resolution of the sensor by the resolution of 4K can you get the crop factor.

4096 for DCI 4K or 3840 for UHD

So I tried it on a few cameras and it gives this -

Canon 1D C (DCI 4K) - 1.26x (5184 / 4096)
Canon 1D X Mark II (DCI 4K) - 1.33x (5472 / 4096)
Canon 1D X Mark II (UHD) - 1.42x (5472 / 3840)
Nikon D5 (UHD) - 1.45x
Panasonic GH4 (DCI 4K) - 2.12x
Panasonic GH4 (UHD) - 2.2x

How is this mathematically, does it stack up?

What is interesting is how little difference there is between the 1D C and 1D X Mk II in DCI 4K after all... I think most people were quoting the UHD crop when it first came out.

The field of view difference here we are not talking the difference between 3:2 full frame and cropped 4K. We are talking the difference between 16:9 full frame 1080p and the cropped 1:1 4K window.

[Andrew Reid]

The X-T2 and GH4 have extra crop factors in 4K due to their sensor readout...

Say the X-T2 extra crop is a 1.17x crop of a 1.5x sensor

What is the correct maths here? Is it -

1.5 * 1.17 = 1.75
Or
1.5 + 0.17 = 1.67

?

[Mckinise]

This defies conventional thinking about the GH4 crop but I have been estimating it to be closer to 2.5 for DCI Cinema 4k and 2.6 for UHD based upon the calculations from this article:

http://gh4crew.co.uk/?p=460

FHD 1920×1080 (16:9 Full Sensor Width) crop factor = 2.18

4K 3840×2160 (16:9 Cropped Sensor Width) crop factor = 2.18 x 1.2 = ~2.6 (test calculated: 2.616)

C4K 4096×2160 (17.07:9 Cropped Sensor Width) crop factor = 2.18 x 1.14 = ~2.5 (test calculated: 2.485)

[PannySVHS]

59 minutes ago, Andrew Reid said:

The X-T2 and GH4 have extra crop factors in 4K due to their sensor readout...

Say the X-T2 extra crop is a 1.17x crop of a 1.5x sensor

What is the correct maths here? Is it -

1.5 * 1.17 = 1.75
Or
1.5 + 0.17 = 1.67

?

 

1.5 * 1.17 would be right and so a crop of 1.75

 

just like the speedbooster thing, just with a result into the other direction:

fi: crop 2 * speedbooster factor 0.71 = 1.42 crop on our beatuiful HD m43 G6 with FD China booster!

 

22 minutes ago, Mckinise said:

This defies conventional thoughts on the GH4 crop but I have been estimating it to be closer to 2.5 for DCI Cinema 4k and 2.6 for UHD based upon the calculations from this article:

http://gh4crew.co.uk/?p=460

FHD 1920×1080 (16:9 Full Sensor Width) crop factor = 2.18

 

 

That does not make the sense it supposely might appear to do so:)

 

FHD crop stays 2, because Full Frame 36x24 Sensor is cropped just as well from the top and bottom in 16:9 mode.

Also, FOV of a crop 2 doesn´t change to the sides just because off chopping off the top and bottom of the sensor for 16:9 mode.

 

So FHD crop factor stays 2.0 for calculating your equivalent FOV in 16:9 HD with your 24mm full frame lens

 

[Andrew Reid]

Let's stick to doing the math horizontally only this time as it matches better what our own eyes see when we switch between 16:9 full frame 1080p and 16:9 cropped 4K video on cameras like the 5D Mark IV (1.74x crop for UHD).

[MattH]

19 hours ago, Andrew Reid said:

Let's stick to doing the math horizontally only this time as it matches better what our own eyes see when we switch between 16:9 full frame 1080p and 16:9 cropped 4K video on cameras like the 5D Mark IV (1.74x crop for UHD).

I'm glad you have come to this realisation as this is what I was saying for years. (However there is big exception that I have come to realise). As far as comparing field of view what sense did it ever make to compare a 16:9 video mode with the 3:2 full frame. By that reckoning even full frame video has a crop factor over full frame.  Much easier just to use the horizontal.  That way you know that a video mode with a 2 times crop factor with a 25mm lens will give you the same field of view as a 50mm on a full frame camera in video mode.  If you used the diagonals of the sensor this could be out.  One slight exception to this is DCI 4k as technically it is a different aspect video mode. By the book you should use a diagonal when comparing a 16:9 video mode to a 17.66:9 video mode. But its a lot of work for such a small difference. And as long as you say 'horizontal crop factor’ it’s technically accurate anyway. 

This is the big exception:
One purpose that you need to use diagonals for is lens coverage. Because the diagonal will always bisect the centre of the image circle: the diagonal is the diameter of coverage that is needed for that image size.  So when trying to work out whether a speed booster and lens combination will work on a particular system the diagonal crop factor should be used.

Case in point: Full frame lenses are designed at least to cater for the diagonal of 3:2 full frame which is 43.26mm.  The diagonal of full frame cropped to 16:9 is 41.3mm.  This is where full frame video's crop factor of 1.047 over full frame actually matters.  It means that you could use a full frame DSLR lens with a 0.955* speed booster on a full frame mirrorless and it would still fall within the full frame image circle when shooting video (but not 3:2 stills).  Failing to use diagonals when doing this could make one over cautious or over zealous depending on what ones intentions are.  (buy the way 0.955 is the reciprocal of 1.047. If you ever work out one of these and you want the other, just press the 1/x button on the calculator)

[tweak]

20 hours ago, Mckinise said:

This defies conventional thinking about the GH4 crop but I have been estimating it to be closer to 2.5 for DCI Cinema 4k and 2.6 for UHD based upon the calculations from this article:

http://gh4crew.co.uk/?p=460

FHD 1920×1080 (16:9 Full Sensor Width) crop factor = 2.18

4K 3840×2160 (16:9 Cropped Sensor Width) crop factor = 2.18 x 1.2 = ~2.6 (test calculated: 2.616)

C4K 4096×2160 (17.07:9 Cropped Sensor Width) crop factor = 2.18 x 1.14 = ~2.5 (test calculated: 2.485)

I agree with this and also makes sense as the crop isn't that much less than bmpcc... otherwise the stated bmpcc crops are wayyy too big.

I disagree that horizontal crops give you the best idea of FOV, as I shoot a lot of anamorphic and weird aspects. For example if I made a 32:9 video with the same width as a 16:9 video on your sheet they would have the same "FOV"... I don't think this is right. I get it (why you're taking the horizontal) but for myself it makes no sense.