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I'm trying to figure out the focal length of my camera. I'm trying to figure out the distance from my camera to an object using the equation listed here (How do I calculate the distance of an object in a photo?) and I had this camera taking a video recording (http://www.bhphotovideo.com/c/product/1022655-REG/sony_hdrcx240_b_hdr_cx240_full_hd_handycam.html) which says the following under Focal Length:

35mm Equivalent Focal Length 29.8 - 804 @ Aspect Ratio: 16:9 Video mode 29.8 - 804 @ Aspect Ratio: 16:9 Photo Mode 36.4 - 984 @ Aspect Ratio: 4:3 Photo mode

I'm simplifying my assumptions as much as possible and am trying to assume the video was not zoomed in at all. At this point I do not have any stills, which I understand contain more information than a video. Does that mean I should be using the "29.8" value for the Focal Length in the equation? (which seemed appropriate based on this post, How to get focal length from a camera specification which gives a value like "4.5-22.5"?)

When I do that my distance is outrageously small, and the Focal Length is the only thing that I don't have a good estimate for. Thanks!

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Your B&H link gives the link to the camera user manual, and its spec sections says:

Lens: ZEISS Vario-Tessar Lens 27× (Optical), 54× (Clear Image Zoom, while recording movies) 320× (Digital) F1.9 - F4.0 Focal length: f=2.1 mm - 57.0 mm (3/32 in. - 2 1/4 in.) When converted to a 35 mm still camera For movies: 29.8 mm - 1609.2mm (1 3/16 in. - 63 3/8 in.) (16:9) For photos: 29.8 mm - 804.0 mm (1 3/16 in. - 31 3/4 in.) (16:9)

So the focal length is 2.1mm to 57mm (actual). It also says the field of view that it shows is the same (equivalent) view of a 35mm camera with a 29.8 - 804 mm lens (which is only handy if you are familiar with using 35mm cameras, which we used for decades.)

But the focal length is 2.1mm to 57mm. You only know those two extreme end points. There are many possible zoom points in between these.

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  • \$\begingroup\$ Thanks for the response. So which of them do I take if I assume it was NOT zoomed in at all? The smaller value? \$\endgroup\$
    – Manner
    Commented Aug 11, 2015 at 18:17
  • \$\begingroup\$ It is always zoomed to some value. Guessing that such camcorders when turned on will surely have a default zoom position, but which is not specified. Wildly guessing, perhaps about 30% more than its minimum 2.1, perhaps like 2.7 mm default? From there, it will still zoom a little wider, and probably a lot longer, but it turns on at this one place. And you are saying zoom is default. ----- We also know crop factor is 29.8 / 2.1 = 14.2 ----- Message is too long, so two parts: \$\endgroup\$
    – WayneF
    Commented Aug 11, 2015 at 20:37
  • \$\begingroup\$ And I have a Field Of View calculator at scantips.com/lights/fieldofview.html --- It offers extra features, like aspect ration and crop factor. In Option 3 there, enter Crop 14.2 and "16:9 aspect ratio, camcorder". It computes sensor size to be Width=2.66 mm, Height=1.49 mm, Diagonal=3.05 mm. The Sony manual says 3.1 mm, possibly rounded, seems very close. ---- So now these methods can tell you default focal length and sensor size. ---- That is as far as my calculator goes, perhaps I need to consider adding another feature? \$\endgroup\$
    – WayneF
    Commented Aug 11, 2015 at 20:43
  • \$\begingroup\$ Oops, struggling with this tiny length issue, I forgot to add the second part... Take any still picture at your default zoom. A good Exif viewer will tell you that focal length used (from Exif data in still image JPG file). It has to be a still picture, movie files do not show variable Exif data. \$\endgroup\$
    – WayneF
    Commented Aug 11, 2015 at 20:48
  • \$\begingroup\$ Awesome, I will track down the camera tomorrow and try getting a still from the default zoom and retry my calculations. Thank you for the link and extra information. \$\endgroup\$
    – Manner
    Commented Aug 11, 2015 at 22:05

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