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I'm just getting into the world of photography, stepping up from a point-and-shoot. I've just purchased a Nikon D3100 and it should be here in a couple of days.

I've done lots of research, but there's one thing I don't understand.

Why can a 200 dollar compact point and shoot zoom 10x or greater optically but to get anything over 200mm-300mm for a DSLR is going to cost an arm and a leg?
Is it sensor size that allows that kind of distance? Does something like the SX120IS and other compacts just have that small of a sensor so even zoomed all the way out, its going to be alot more magnified than something like the D3100 (which yes, I know, don't have a full size sensor either)?

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Welcome to photo.SE! –  Reid Oct 30 '10 at 21:07
    
WOW! I was going to ask this EXACT QUESTION! hehe +1! –  BBischof Oct 30 '10 at 21:09
    

2 Answers 2

up vote 10 down vote accepted

It has to do with the sensor size - large sensors require correspondingly larger lenses. For example, a lens on a small sensor with field of view equivalent to 400mm on full-frame is quite small; 66mm real focal length if I did my math right, and needing to cover a much smaller image circle - both factors which lead to a smaller lens. On the other hand, a 400mm lens for use on a full-frame sensor is a giant $5000 beast. Those really big lenses are expensive because they require huge, precisely manufactured chunks of specialty glass along with the machinery & electronics to focus, zoom, and VR them.

http://en.wikipedia.org/wiki/Image_sensor_format is a good comparison of various common sensor sizes. Your Canon SX120 has a 1/2.5" sensor (5.8x4.3mm), while the D3100 has an APS-C sensor (24x16mm).

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The second part of your answer was what I expected the answer to be, that the lenses are just of higher quality. But the first part makes sense on reflection now also. Is this why people always warn you that the lens specs are slightly wrong on a crop body rather than a full frame? –  BBischof Oct 30 '10 at 21:14
    
@BBischof, if you're thinking of the confusion stemming from the use of millimeters to express both field of view and focal length, see photo.stackexchange.com/questions/139/… and photo.stackexchange.com/questions/1592/…. It's not exactly that lens specs are "wrong" on APS-C sensors. –  Reid Oct 30 '10 at 21:21
    
@Reid, thanks for the links, I was talking about things like when a lens says 70-200, but on a crop body its really like 84-220 or something like that. –  BBischof Oct 30 '10 at 21:37
    
@BBischof, smaller sensors aren't truly zoomed in more, they crop out the edges of the focal plane which gives the impression that they are more zoomed in. –  rm999 Oct 30 '10 at 23:02
1  
@rm999 - That depends. An APS-C with the same photosite size and pitch will only crop against a FF sensor, but an APS-C with more density on a sensor, that captures more detail, can magnify since that is what magnification is: detail. Of course, other factors play a role, including the lens, noise, etc. so it's not a cut and dry comparison. –  John Cavan Oct 31 '10 at 3:11

Basically, yes.

The zoom on a compact is started in equivalent-terms. So a 5-50mm lens can become equivalent to a 25-250mm lens on a DSLR (those are not exact numbers, just for illustration).

On a DSLR a 250mm has to have a focal of, well, 250mm. That is 25cm or about 10 inches. It gets big and heavy and expensive to manufacture something that size with optical quality glass.

Comparatively, a compact with a sensor 5 times smaller (most are 5.7X smaller) needs to create a lens that has only 50mm or 2 inches of focal length.

The width is pretty much proportional too, it has to be to keep similar aperture sizes. So they get cost and size saving on all dimensions.

BTW, I just reviewed the Olympus SP-800UZ which reaches 840mm! Do you know how much an 800mm lens cost for a DSLR? The only stabilized one cost over $10,000 USD and weighs 4.5k (10 lbs). The SP-800UZ, camera and lens, weighs 416g (less than 1 lbs).

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Careful; you're conflating focal length and field of view. They're not the same and there's a lot of confusion floating around. See the links I added in comment to the question. –  Reid Oct 30 '10 at 21:24
    
That is standard across the camera industry. People use 35mm focal-lengths to signify 'the field of view obtained by such lens on a 35mm film camera' and by extension on a full-frame DSLR. –  Itai Oct 31 '10 at 0:23
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@John - Focal-length and lens-length correlate but they not exactly related due to many factors. In the case of the pancake lens you mention, the dominant factor is the distance between the bayonet mount and the sensor plane which is about 45mm. 45+26 is almost 70mm. There are modern lens constructions that make the relation particularly complex due to concave elements, ultra-high-refractive glass, difractive optics and more. –  Itai Oct 31 '10 at 2:17
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@Itai, it looks like I did not read your answer carefully enough; you do have an "equivalent" in there. Sorry about that. –  Reid Oct 31 '10 at 2:43
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@John for a really odd case, the Canon 24-70 lens extends as you zoom out... –  Rowland Shaw Oct 31 '10 at 7:52

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