[Bf-vfx] Footage for Tracking

[Andrew Hunter]

Hey Keir,

On Tue, Jun 14, 2011 at 2:32 PM, Keir Mierle <mierle at gmail.com> wrote:
> On Tue, Jun 14, 2011 at 10:55 AM, Andrew Hunter <andrew at aehunter.net> wrote:
>>
>> Hey everyone,
>>
>> On Tue, Jun 14, 2011 at 12:27 PM, Andrew Hunter <andrew at aehunter.net>
>> wrote:
>> > Hey Everyone,
>> [...]
>>
>> I had another though that perhaps Keir can answer. Does subject
>> distance affect the calibration? Should the calibration charts be
>> taken from the same distance.
>
> What matters is that the calibration target fills the screes as much as
> possible. I can take some "good" calibration shots and post them sometime in
> the next few days.
> GOOD: Calibration target fills most of view.
> GOOD: Calibration target is shown at different angles (needed for accurate
> focal length). Perspective should be obvious (converging lines).
> GOOD: Calibration target shows the distortion (fronto-parallel view) This is
> needed for accurate distortion.
> BAD: Calibration target only fills a tiny bit of the image
>
>>
>> The reason I ask is that with wide focal lengths, where distortion is
>> most likely to occur, the subject distance greatly affects the
>> persption of depth.
>
> For wide focal lengths, you'll have to get closer to the lens.

So I can conclude from this that subject distance does not affect the
calibration beyond filling the frame? Is depth of field and depth
distortion on the perspective shots irrelevant?

>>
>> Also, in order for the chart to fill the frame on different focal
>> lengths with the same sized chart (say an 18mm vs an 85mm) the subject
>> distance would vary greatly.
>
> Yes, the distance to the calibration grid will vary. I think it's easier to
> have one size than to have multiple targets, and just vary the distance to
> the target.
>
>>
>> Would it make sense to default to a larger chart size like 11x14 or
>> offer several different sizes for different ranges of focal lengths?
>
> 11x14 is probably fine. It's important that the target is *REALLY* flat. I
> sent a PDF earlier in this thread that should do nicely.

I've gotten the pdf you send printed on thick card stock and
laminated. If subject distance doesn't matter, then 8"x11" is fine.

Cheers,

Andrew