Hi Dave – sorry it has taken so long to get back to you.
Here is a brief explanation of the numbers. But you probably want to do some further reading.
Focal Length
The first number in lens specifications is “focal length” in millimeters, abbreviated as “mm”. The focal length of your lens will determine the magnification of the image projected onto your camera’s sensor.
Longer or “telephoto” focal lengths will produce larger images. Shorter or “wider” focal lengths will produce smaller images. Camcorders usually have “zoom” lenses, with a continuous range of focal lengths from wide to telephoto.
Image size also depends on the size of your sensor. Although it is a little counterintuitive, a small sensor will show a larger image size than a large sensor with a lens of the same focal length.
For that reason, camera and lens manufacturers will often express focal lengths in terms of “35mm equivalent”. In other words, the focal length the lens would have if used with a standard full frame 35mm film camera.
Aperture
The second number in lens specifications describes the lens’ maximum aperture – usually expressed as the “f stop”. Smaller f-stops mean a larger maximum aperture, and a more light sensitive (and more expensive) lens.
Lens Comparison
So, an apples to apples comparison of the lenses listed above would be:
$1549 Sony MC2000U,: 29.8-357.6mm (35mm equivalent) with an aperture of f/1.8-3.4
$1599 JVC GY-HM70U,: 29.5-476mm (35mm equivalent) with an aperture of f/1.2-2.8
$2698 Sony NEX-VG30: 27-300mm (35mm equivalent using a 1.5x magnification for the camera’s large sensor) with an aperture of f3.5 to 6.3
Caveat
But be careful – lens aperture/sensitivity does not necessarily equal low light performance. Your camcorder’s sensor size, sensor sensitivity, image processor and image processing algorithms are also very important.
The VG30’s large sensor means that if you put an f1.8 lens on it, it’s ability to record high quality images in low light would blow any of the small sensor camcorders away.
Another Camera Suggestion,
Since my last post, the Sony HXR-NX70 has been marked down to $1999 at Amazon. This was a great weatherproof camera at $3000, and is even more amazing at $2000.
It doesn’t have the VG30’s huge sensor, but here is what it can do:
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Suggestions for further reading
Book: “A Complete Idiot’s Guide to Independent Filmmaking”
Please see this excerpt from the book on focal length and aperture for a better explanation than I can give: https://books.google.com/books?id=rOhLfJi4cGoC&pg=PA201&lpg=PA201&dq=aperture+and+focal+length+filmmaking&source=bl&ots=q9SGXsVFom&sig=Ist-1P85d62UNv7DW-VSsj2I8Vg&hl=en&sa=X&ei=7-vjUbfTEsH-iwKM34DwCA&ved=0CGkQ6AEwBg#v=onepage&q=aperture%20and%20focal%20length%20filmmaking&f=false
Wikipedia:
https://en.wikipedia.org/wiki/Focal_length
https://en.wikipedia.org/wiki/F-number
Again, hope this is helpful,
Bill
Hybrid Camera Revolution