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I know next to nothing about cameras (other than how to push the shutter button). Up until a few days ago I thought of zoom as simply a multiple (e.g. my Canon S95 has 3 or 5X optical zoom) and had no idea what these millimeter values were.

I'm starting to read about focal lengths and more advanced measurements, but the explanations are not that intuitive to me. I think I'm getting it, and it would help if I could get confirmation that what I think is actually correct:

  • focal length of our eyes is about 50mm (or is it 35mm? I have seen both numbers). So if a lens has a 50mm focal length, it will take a picture roughly of what I can see with my eyes.
  • less than 50mm is considered "wide angle" because it's essentially "zoomed out" of what I can see with my eyes
  • the amount of zoom (in standard point and shoot speak) is the ratio of the longest focal length of a lens to the shortest. Some lenses have only one focal length (fixed) and thus no zoom
  • a telephoto lens has a large focal length (how long though?)
  • the Canon SX510 HS has a 24mm lens, and a 30X optical zoom, so thus its longest focal length is 720mm. This should zoom to about 10X what my eye can see.

Is what I've written above correct?

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50 mm is roughly the 35mm sensor equivalent to our main field of view, though our peripheral vision goes much further and can vary quite a lot from person to person.

Wide-angle lenses start at things at or below 35mm, but vary a bit for zooms. For example, most 24-70 lenses are considered standard zooms despite drifting slightly in to wide angle territory.

You are correct about the definition of the x multiplier for point and shoots.

Telephoto simply means that the lens is shorter than the focal length. For example if you have a lens that is 5mm long and yet it has a focal length of 20mm, it would technically be telephoto. Generally speaking, you only hit this on relatively long focal length lenses.

The 720mm will be a number of times smaller field of view, but doesn't necessarily see more than your eye can see (though it most likely does). It further depends on the resolution of the lens (how much detail it can resolve.) If you had a very low resolution lens, it might fill the photo with an area that is only a small portion of your field of view for your eyes, however that doesn't determine the amount of detail caught within that area.

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    Just to Clarify, the "35mm Sensor" in the first line is another name for the "full frame" sensor that we usually equate focal lengths to (more info here), not a focal length. – damned truths Feb 8 '14 at 22:21

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