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Canon VIXIA Series AVCHD and HDV Camcorders
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Old April 3rd, 2009, 10:10 AM   #1
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hv30 focal length

Hi everybody,

this is my first post here but a bit longer a member because most of the times i was enjoying finding answers to questions other members already asked.

now i have troubles but i can not easily find an answer to it.

i have shot some footage with my Canon HV30. Maximum Field of View ( i was not zoomed in ). Then used the footage with a 3d motion tracker. It calculates the Focal Length automatically and tells me it is about 24 mm .

when i search the internet, it tells me that the minimum focal lenth is 6.1 mm

when i change the value from 24 to 6.1 mm my 3d scene is messed up.

Somebody knows / understands something about this problem?

Thanks !
Giso (the netherlands)
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Old April 3rd, 2009, 10:30 AM   #2
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chipsize

probably it has something to do that my motion tracker sees the Focal Length in relation to the Filmback but in my case it is not really film offcourse but a chip which is smaller.

somebody knows if this is true and eventually can tell me if there are convertors which tell me the real focal length in relation to a real film back according to the 6.1 mm in relation to the size of the chip?

thanks!
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Old April 3rd, 2009, 01:44 PM   #3
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This is correct. The HV20/30 has a 1/3 image sensor, which translates a different focal length than 35mm film.

Digital Rebellion - Resources - Tools

Have not used these calculators, but you should be able to figure it out.
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Old April 3rd, 2009, 02:02 PM   #4
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See the 35mm equivalents in this thread: http://www.dvinfo.net/conf/canon-vix...-10x-zoom.html
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Old April 3rd, 2009, 04:01 PM   #5
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The question is focal length for a given film size/image area to produce the same nominal field of view. Of course the aspect ratios are a bit different; 16x9, 4x3, 3x2, etc. depending on the medium And the softwaare, is it calculating it per a 35mm still frame (24x36 mm) or a 35mm motion picture frame (closes to 18x24 mm) or something else?

The manual implies 43mm for equivalent still film, but the aspect ratio is a bit different, The 24 mm it calculated is closer to what it might be in 34mm movie film terms.

Note that the 1/3" sensor size is nominal and is relative to the standard used for the old tube-type video cameras, it is not a true measure of the sensor as you might measure it with a caliper. If 24mm gives the results you want, use that number.
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Old April 7th, 2009, 08:44 AM   #6
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Calculations

Here are some of my own calculations (in dutch) (see attachment)
the lower results are quite good in combination with the Focal Length of 6.1 mm (according to the manual)

The perspective matches almost perfect but there is a little flaw;
is this possible because the chipsize ratio is 4:3 with a max resolution (2048 x 1536 ) other than it saves it in a 16:9 ratio (1920 x 1080 ) ?

elsewhere on the web:
""The sensor size 1/2.7" does not refer to a the diameter of the sensor. It refers to the diameter of the archaic TV tube size. The actual diameter (diagonal) of a 1/2.7" sensor is 6.72mm - not 9.41mm.""

Whats this?? never heard of it!

thanks!
Giso
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hv30 focal length-focallength_berekeningen.jpg  
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Old April 7th, 2009, 10:05 AM   #7
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Quote:
Originally Posted by Giso Spijkerman View Post
"The sensor size 1/2.7" does not refer to a the diameter of the sensor.
That is correct.

See these notes for an explanation -- I wrote this for the Canon Optura series camcorders but it applies to all camcorders. Hope this helps:

From Canon Optura DV Camcorder Lineage, Pt. 1

"The nomenclature used in video camcorders is outdated, archaic and inaccurate, but for some reason the industry insists on hanging onto them. Your DV camcorder is referred to as a one-third-inch camera and lens because that's the size of the CCD image sensors inside the camera head. Except it really isn't. They're actually a bit smaller than that. One-third inch, one-half inch, etc. are tube diameters back from the days before CCD technology when video cameras used orthicon, plumbicon and saticon tubes for creating images. To make an image plane the same size as those tubes used to make, the CCD needs to be only as big as a 4:3 rectangle that would fit inside the diameter of that tube. Therefore, a one-third-inch CCD is actually a bit smaller than one-third of an inch. Then there's also the appalling practice of expressing other CCD sizes as mixed fractions, such as 1/3.4 of an inch. If the industry would simply switch to an actual millimeter measurement of the CCD diagonal, we'd all be so much less confused."
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Old April 7th, 2009, 10:35 AM   #8
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helps a lot

Thanks Chris, that explaines a lot! I just rolled in doing motion tracking stuff and i don't know a lot about cameras.

This is a motion tracking result i get from these values:
YouTube - CANON HV30 - Focal Length and Filmback values for matchmoving

in high quality and fullscreen, you can see that it is ALMOST perfect!

any idea how to just tweek the filmback size values to get the perfect results?
i tried to use the value 1/2.7" for the filmback diagonal but that is absolutely not correct.
Is it possible or is it guess work?

Thanks!
Giso

Last edited by Giso Spijkerman; April 7th, 2009 at 10:37 AM. Reason: added a sentence
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Old April 8th, 2009, 03:30 AM   #9
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final conclusion

I used the 6.72 mm diagonal chipsize values but didn't fill in the focal length...
this gave the focal length is not 6.1 but 6.05685 (rounded 6.1) but this does the job!!

http://giso.wordpress.com/2009/04/08...length-update/

thanks everbody for the help!

Last edited by Giso Spijkerman; April 8th, 2009 at 04:10 AM. Reason: link added
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Old April 8th, 2009, 05:52 AM   #10
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The reason for sticking with the tube-based measurement scheme is was no doubt based on simplifying life with respect to interchangeable lenses. By using the same nomenclature (tube equivalent size) a shooter knew the same lens would produce the same field of view on any camera, and not need to do a conversion for CCD, CMOS or tube-based camcorder.

So why was tube nomenclature using the "wrong" size? I suspect that tube based nomenclature was based on the diameter of the outer glass envelope, not the sensing element inside. That way one could tell at a glance what the size was, and there probably were very few sizes from which to select.

[TV has never been very precise - e.g., the old picture tube size was rarely the visible image size]

As long as aspect ratio remained 4:3, and video shooting was pretty much limited to professional in the TV business, all was more or less well. They had their own vocabulary and understood. It is only recently when video expanded to include wide screen aspect ratios, cross-over shooters from the still camera world, solid state technology, multi-format using different active areas of the sensor, and expanded to include a wide section of the population at large that did not do a long apprenticeship in video in that confusion has been an issue.
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Old April 8th, 2009, 08:05 AM   #11
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Thanks for your insight, Don -- your conclusions make perfect sense (as always)!
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Old June 17th, 2010, 10:29 AM   #12
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okay,
considering this nomenclature business...
(im not so logical, so am struggling- excuse me if this is easy math)
what is the method for working out the true diagonal size of the chip?
I have a HF S21 which has a "chipsize" of 1/2.6...
Id love some help!
Thanks!
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