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Old Sep 19, 2010, 7:11 PM   #1
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Default focal length

What determines for the manufacturer of digital cam as to what focal length or zoom factor will be.Hope I'm asking correctly. Some begin at 28mm-xxmm and some 24mm-xxmm and several others perhaps? Thanks.
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Old Sep 19, 2010, 7:20 PM   #2
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The zoom (X number) is the ratio of the shortest focal length to the longest, and is not an indication of the actual focal length. Two cameras can have very different focal lengths, even very different 35mm equivalent focal lengths, and yet have identical zoom numbers.
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Old Sep 19, 2010, 10:49 PM   #3
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Default focal length

What I should have asked was what determines where a cam maker will start its wide focal length or ultra focal length etc? Example-24mm or 28mm etc. And to what tele zoom?

Thanks TCav.
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Old Sep 20, 2010, 5:58 AM   #4
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What you're looking for is the 35mm equivalent focal lengths at the widest Wide setting and the longest Tele setting. The 35mm equivalent focal length is the only way to compare the angles of view for two cameras. The zoom ratio (X Number) doesn't do it.
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Old Sep 23, 2010, 4:07 AM   #5
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Where a maker starts is with the size of the sensor. The diagonal of a sensor defines its optically 'normal' focal length (FL). Where they go from there in terms of a zoom's range depends on how much they want to spend on optics. Faster and greater-range zooms are necessarily bigger, heavier, and more expensive. That's not a great problem with very tiny sensors, but such midgets can't give good performance -- lotsa pixels squashed together are noisy.

Let's look at a 1 1/8" sensor, about 5.3 x 7.2 mm, with a diagonal of 8.9mm. That's equivalent to 43mm on FF film, and 30mm on the APS-C sensors used in most dSLRs. For a zoom to have an equivalent of 25mm on the wide end, that sensor demands a 5mm FL. A 10x zoom, equivalent in field-of-view (FOV) to 25-250mm, would thus be 5-50mm. Such zooms aren't terribly difficult or expensive to make, so we see many in that neighborhood. A 6-60mm zoom, equivalent in FOV to 30-300, is even cheaper and easier, so zooms in that neighborhood are more common. (I use 25mm and 30mm here to avoid including pesky decimal points.)

That's the engineering. But camera production is usually controlled by marketing guys, not by engineers. The execs run focus groups, sťances, etc and decide what market they want to target. And that's the answer to your question: blame the sales execs.
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Old Sep 23, 2010, 5:26 AM   #6
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Quote:
Originally Posted by RioRico View Post
... The diagonal of a sensor defines its optically 'normal' focal length (FL). ...
This is a popular notion, but it has no basis.

The concept presumes that a lens of a certain focal length will reproduce an angle of view in a print, that closely matches the angle of view that the observer experiences when not looking at a print. This also presumes that the observer would hold the print at a distance equal to the diagonal of the print. But most people choose to view prints at a distance greater than the diagonal of the print, so the whole premise falls apart. Yet it persists.

But I'll add that its validity or lack thereof has nothing to do with the rest of RioRico's explanation.
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Old Sep 25, 2010, 8:29 PM   #7
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Should I have said, "arbitrarily defines its optically 'normal' FL" ?? Whether normality is valid or not, that's still the starting point for many lens-camera combos. Call it the common wisdom. Or blame the marketing execs.
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Old Sep 25, 2010, 8:39 PM   #8
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Originally Posted by RioRico View Post
Should I have said, "arbitrarily defines its optically 'normal' FL" ?? Whether normality is valid or not, that's still the starting point for many lens-camera combos. Call it the common wisdom. Or blame the marketing execs.
Actually, none of the above. It's a coincidence. Some people don't believe in coincidences, so they attribute some higher meaning to it all. Those same people probably also don't believe that Man walked on the Moon either.
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Old Sep 26, 2010, 8:38 AM   #9
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Actually, none of the above. It's a coincidence. Some people don't believe in coincidences, so they attribute some higher meaning to it all. Those same people probably also don't believe that Man walked on the Moon either.
Or that Evolution isn't true and there's a God.
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Old Sep 26, 2010, 9:16 AM   #10
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Originally Posted by Track View Post
Or that Evolution isn't true and there's a God.

Let's save that discussion for another time and another place.
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