Author Topic: corona camera match for a DSLR with an ASP-C sensor  (Read 763 times)

2020-05-14, 10:32:15

johan belmans

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Hi there,

for an upcoming project we will have to camera match drone footage (stills). The drone camera will be a Sony A6500 with a Zeis 16-35 mm F4 lens
We are investigating from which positions we need stills and shots.
So we are adapting the render output size and the Corona Camera settings (3DS Max) to mimic the Sony camera and lens.
- If I am not mistaken the Film width (mm) parameter in Corona Camera should be the value off the sensor size of the camera. For the Sony A6500 this should be 23.5 mm (sensor size = 23.5 mm x 15.6 mm) source: https://www.digicamdb.com/specs/sony_alpha-a6500/ .
But the tooltip mentions a value of 25.1mm for ASP-C digital SLR's. Which refers to the Classic film negative format https://en.wikipedia.org/wiki/APS-C

- Considering the focal length of the lens AFAIK when setting the film width for an APS-C camera we do not have to take the crop factor (1.53 for the A6500) into account. So 16mm stays 16 mm.

So, should we use the 23.5 mm or 25.1 mm for the Film Width and may we ignore the crop factor to get approx the correct results?


2020-05-14, 14:23:10
Reply #1

nauticus25

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Go with the 23.5mm spec from Sony. I think the APS-C value is just an approximation if you have no better data to work with. Ignore crop factor, but keep in mind that you may have to tweak the focal length a bit to get everything to line up due to lens distortion, so your final result won't be exactly 16mm. You could generate a lens distortion map for your particular lens (https://forum.corona-renderer.com/index.php?topic=21064.0), but I never had time to do that and have gotten good enough results without it.

When taking your stills from the drone, I recommend also getting a shot with the camera pointing straight down so that you have a good visual of your drone's exact location over the ground. The GPS coordinates will get you close, but this eliminates the guess work.

Good luck!
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2020-05-16, 11:24:13
Reply #2

johan belmans

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Go with the 23.5mm spec from Sony. I think the APS-C value is just an approximation if you have no better data to work with. Ignore crop factor, but keep in mind that you may have to tweak the focal length a bit to get everything to line up due to lens distortion, so your final result won't be exactly 16mm. You could generate a lens distortion map for your particular lens (https://forum.corona-renderer.com/index.php?topic=21064.0), but I never had time to do that and have gotten good enough results without it.

When taking your stills from the drone, I recommend also getting a shot with the camera pointing straight down so that you have a good visual of your drone's exact location over the ground. The GPS coordinates will get you close, but this eliminates the guess work.

Good luck!

Thanks!

2020-05-18, 13:56:28
Reply #3

nauticus25

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Sure thing. I forgot to mention that the other thing I do to dial in the rotational alignment of the camera. I line up something in the distance, like existing geometry of a sidewalk corner. Then I move the camera's pivot point to that spot. Set the camera's rotation coordinates to local, and now you can adjust pitch/yaw/roll of the camera with that distant object's alignment "locked" so that you only have to pay attention to lining up objects in the foreground. Once you get everything dialed in, lock the XYZ transforms of the camera so you don't accidentally move it when navigating your scene. I use this script for a quick way to lock/unlock your camera: http://www.scriptspot.com/3ds-max/scripts/cameralock
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2020-05-20, 15:05:48
Reply #4

maru

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Confirmed: the sensor width should be the real sensor width provided by the manufacturer, and you can ignore the crop factor. So it's just copying the values from the DSLR into the Corona Camera.

2020-05-20, 15:10:27
Reply #5

johan belmans

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Thanks Marcin! Maybe this info can be added to the tooltip.
And thanks nauticus25 for the extra tip.

2020-05-21, 00:05:38
Reply #6

pokoy

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Crop factor can't be ignored just like that... sorry but this isn't true. When you have anything else than a 36mm sensor, chances are that the focal length of the lens needs to be multiplied by the crop factor since the values are typically specific to a 36mm sensor. You should be able to get this information from the specs.
When in doubt, try to get info on the field of view of the lens on your camera. When you can get this one it'll be a safer bet than uncertain info on sensor and focal length.

2020-05-21, 00:39:20
Reply #7

nauticus25

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Crop factor can't be ignored just like that... sorry but this isn't true. When you have anything else than a 36mm sensor, chances are that the focal length of the lens needs to be multiplied by the crop factor since the values are typically specific to a 36mm sensor. You should be able to get this information from the specs.
When in doubt, try to get info on the field of view of the lens on your camera. When you can get this one it'll be a safer bet than uncertain info on sensor and focal length.

I think crop factor only applies when you're trying to compare a camera to its full frame equivalent. For instance, a 50mm lens on a Canon 70D acts like an 80mm lens on a Canon 6D. So if the Corona camera didn't have an adjustable "sensor size" and was fixed at full frame (35mm), then you would definitely have to adjust your Corona camera's focal length to 80mm in order to match a 50mm image shot on a 70D.

As a data point: The DJI Inspire 2 with an X5S camera system has a sensor width of 17.3mm. I shoot footage with a 12mm lens. My Corona camera is set to a film width of 17.3mm and the focal length is 12.124mm (slightly off due to lens distortion). The match is pretty much dead on.
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2020-05-21, 12:48:07
Reply #8

johan belmans

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In the meantime, I have send some test shots to the guy with the drone. As stated above I used the sensor size for the film width and the Focal lenght without the crop factor.
And he confirmed, with his experience, this is what he would expect.