Are you ever stuck with the problem of choosing the right tracker size for the shot? This tiny formula is useful since it works for every scenario. It takes into account the different parameters that can effect the camera angle and the number of pixels in the image.
This tracking marker tip comes from Francois Lord. Francois has been tracking since the 3d equalizer days and has spent a lot of time on set. He has devised a very creative and exact solution on how to calculate the perfect tracking marker size. This formula has been refined over the years by Francois and he was kind enough to share it with us. And now here is ....
The Flord Formula
Where :
T = Tracker size (in same units as D)
P = Prefered tracker size on screen (in pixels) between 5 and 10 pixels depending on the software you will use for tracking
B = Filmback (in millimeters) see table below for reference
D = Distance from camera (in units of your choice)
F = Focal length (in millimeters)
R = Horizontal Resolution (in pixels)
So... If you are shooting a scene with a 35mm camera and a 18mm lens, transferred to NTSC, and the background is at 3 meters from the camera... you can use the following formula...
...to find out that you need to put trackers that are 5 centimeters in diameter on the background to have them appear as 8 pixels big on screen.
Here is a list of different filmbacks that are often used in production. Please let me know if any of this information is inaccurate and I will update it.
| Format Name | x size (mm) | y size (mm) |
|---|---|---|
| 35mm (Super 35) - Full Aperture | 24.89 | 18.67 |
| 35mm (Super 35) - 3-Perf | 24.89 | 14.20 |
| 35mm (Super 35) - Transfered to HDTV | 24.89 | 14.00 |
| 35mm - Academy | 21.95 | 16.00 |
| 35mm - Slide | 36.00 | 24.00 |
| 16mm - Standard | 10.26 | 7.49 |
| 16mm - Super 16 | 12.52 | 7.42 |
| 70mm - IMAX | 70.39 | 52.62 |
| Video HDTV - 2/3" 16:9 sensor | 9.59 | 5.39 |
| Video HDTV - 1/3" 16:9 sensor | 5.23* | 2.94* |
| Video NTSC - 1/2" 4:3 sensor | 6.40 | 4.80 |
| Video NTSC - 1/3" 4:3 sensor | 4.80 | 3.60 |
| Video NTSC - 2/3" 4:3 sensor | 8.80 | 6.60 |
* I'm not 100% sure about this one yet.
Again, thanks to the Francois. And if anyone is savvy enough... I smell a really good iphone app or java application.
until next time.
Andy
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Do light levels and lens characteristics and therefore focus, not loom fairly large also? If the tracking marker is in focus in the background, along with the subject in front of it, then the formula works. But if the aperture is wide open, e.g. for reasons of low light, or you are at the end of a very long lens, then the background is likely to be out of focus, as will the tracking markers, and they won't be the desired pixel size in the shot as determined by this formula...no?
ReplyDeleteTrackerSize , App for iPhone
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