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Old Feb 27, 2005, 7:41 AM   #1
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I did some research online but I'm still confuse with focal lenght. I know that you can use it to determine the optical zoom,for example,28-280mm is equal to 10x. But what is the difference between a 18-180mm and a 28-280mm? And I want to know to why a 300-800mm lens is considered a super telephoto when it is just 3.75x zoom?
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Old Feb 27, 2005, 10:39 AM   #2
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chun8143 wrote:
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I did some research online but I'm still confuse with focal lenght. I know that you can use it to determine the optical zoom,for example,28-280mm is equal to 10x. But what is the difference between a 18-180mm and a 28-280mm? And I want to know to why a 300-800mm lens is considered a super telephoto when it is just 3.75x zoom?
forget about the x factor.
it doesn't tell you anything about how close you can zoom to things.

regarding your questions:

a 28-280 has a better telephoto (you can get closer) but isn' t as wide as a 18-180. (because 28 is less wide than 18mm)

a 300-800 is considered as a super telephoto because it gives you a very very good reach. you can take photos of things which are very far away.

take a look at this graphic. it should help you to understand what the focal length means:



(image provided by klaus from hertz-ladiges.com)
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Old Feb 27, 2005, 10:52 AM   #3
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Forget the 10X or 3X thing... it's a sales gimmick. In general, any lens with an equivalent of 28mm or less is considered wide angle. The word "equivalent" word is important. It is based on 35mm film standard. In point and shoot cameras with their tiny sensors, cheap lenses can produce very long zoom ranges... but suffer a lack of wide angel capability. There's a tremedous difference between a 37mm and a 28mm equivalent in performance. I have never seen an 18-180mm lens.

There's a difference beween super zooms and super telephoto lenses. Just look at the numbers. Super telephotos have big numbers (in fact some super telephot lenses aren't zoom lenses at all). Super zooms have a wide range between the two numbers.

As in anything else, big ranges are invariably a compromise. They leave a lot to be desired at one or both extremes. Some purists refuse to use any but "prime" (non-zoom) lenses. Newer zooms with modest ranges can be very good now and cut down the number of lenses needed to cover most circumstances.

A 300-800mm lens would be impractical for general use. Great for wildlife and astrophotograpy(using a tripod).
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Old Feb 27, 2005, 11:55 AM   #4
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I think i have a better understanding now. I'm going to try to explain in my own word. Lets say,the closest a 28-280mm lens can focus is 10feet,with its 10x multiplier it can extend to 100feet. And lets saythe closest a300-800mm lens can focus is 100feets,but with its 3.75 multiplier it can extend to 375 feets. The x factor only tells you how many times it can multiply,not the distance it can zoom in.
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Old Feb 27, 2005, 12:08 PM   #5
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chun8143 wrote:
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I think i have a better understanding now. I'm going to try to explain in my own word. Lets say,the closest a 28-280mm lens can focus is 10feet,with its 10x multiplier it can extend to 100feet. And lets say the closest a 300-800mm lens can focus is 100feets,but with its 3.75 multiplier it can extend to 375 feets. The x factor only tells you how many times it can multiply,not the distance it can zoom in.
hm. I'm not quite sure if this is right.

The multiplier just tells you how much longer the long end of the zoom lens is compared to the widest angle.

let's take your example:
first of all, the focal length doesn't have anything to do with how "far" you can focus, but how close you can zoom in.
And I also don't like the dimension unit of "feet" you're using in your definition. But in general you're right if you take a look at the picture I posted above and you use the field of view in "°" instead of feet, you're right!

You know, you can't use "feet" because it depends on what you're photographing. When a bird is 10m away from me and I got my Sigma 70-300 attached I can see the bird in the middle of the frame and some trees and stuff around. But I'd like to zoom in to make the bird fill the whole frame. So I go to 300mm, but the bird is actually still 10m from me. You know what I mean?
It's the field of view that changes, not the distance.

But let's imagine we got a 100-300mm lens:
At 100mm the field of view is approx. 23° and at 300mm it is about 1/3: 8°

So I think you got it.

Hope I didn't mix you up even more.
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Old Feb 27, 2005, 12:59 PM   #6
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this is a very informative thread...thaanks for all the posts..

i have some questions at this point...

in P&S cameras, there is a scene mode called "Landscape" where the focus is set to infinity by camera. However when i use it to take a snap, and i see the EXIF information, it shows focal length is some number (like 17mm, 20mm, etc) depending on the zoom. So what does this number represent?

thanks.

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Old Feb 27, 2005, 1:15 PM   #7
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kazaan wrote:
Quote:
this is a very informative thread...thaanks for all the posts..

i have some questions at this point...

in P&S cameras, there is a scene mode called "Landscape" where the focus is set to infinity by camera. However when i use it to take a snap, and i see the EXIF information, it shows focal length is some number (like 17mm, 20mm, etc) depending on the zoom. So what does this number represent?

thanks.


the focal lenght just tells you about the field of view and of course also about the zoom, becuase the wider the focal length, the less you've zoomed in and the wider the view of field is of course.
in your example, 17 or 20mm is a typical wide angle which you normally use for landscape
the focal lenght by the way represents the distance from the center of the lens to the ccd/cmos chip if I remember correctly.

here's a link that explains focal lenght again:
http://www.hertz-ladiges.com/modules...owpage&pid=200
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Old Feb 27, 2005, 6:05 PM   #8
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Do you mean 'the larger the focal length', instead of 'the wider the focal length'? Is it true that the bigger the number...eg 300 mm, gives you narrower field of view, but also some magnification (as compared with 35 mm)?
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Old Feb 28, 2005, 10:48 AM   #9
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Kenny_Leong wrote:
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Is it true that the bigger the number...eg 300 mm, gives you narrower field of view, but also some magnification (as compared with 35 mm)?
Yes. It has to be a magnification if the view is narrower but the image size still the same.
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Old Feb 28, 2005, 1:04 PM   #10
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kazaan wrote:
Quote:
this is a very informative thread...thaanks for all the posts..

i have some questions at this point...

in P&S cameras, there is a scene mode called "Landscape" where the focus is set to infinity by camera. However when i use it to take a snap, and i see the EXIF information, it shows focal length is some number (like 17mm, 20mm, etc) depending on the zoom. So what does this number represent?

thanks.


As kex said, don't confuse focal length (the technical name for the distance from the lens to the focal point inside the lens, which translates to the field ofview captured by the lens and thus how "zoomed in" everything is) with the focus distance (what you've referred to in your post Kazaan).

Here's an example: Look out your window with one eye closed. That field of view is approximately the 35mm-camera equivalent of a 50mm focal length. Now, raise your hand in front of the window scene and look at it. Notice that the field of viewhas not changed (that is, you can still see what you see out of your window) but the focus has changed -- your finger is in focus, while the background is out of focus. So what changed is not the focal length (kind of a confusing term) but the point of focus.

Due to the physical properties of light and optics, the farther away a subject is from your eye (and the camera's lens) the more of the surrounding material is in focus. That's why when you look out your window, if you look at a building 500 feet away, the building next to it (which may be 550 feet away) is also in focus, but when you stick your finger up, 3 feet away, and look at it, the scene out your window is now all blurry. What the "landscape" mode is doing is recognizing that what you're taking is a landscape, and thus you want the focus distance to be maxed out, to "infinity" (infinity, because after a certain distance, all things are equally in focus). It doesn't have to do with the focal length of the lens (how zoomed it is) but how far away the subject is.

Clear as mud?


P.S. Great chartKex! Now, what's difficult to do is conceptualize that whatever degree of view you see,that amount is filling up your frame (that's why images are more "zoomed in").


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