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Old Nov 29, 2004, 9:50 PM   #1
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What is the difference between a telephoto and a zoom lens? I have a zoom lens (75-300mm) (1:4-5.6) and it seems to have such a shallow depth of field that I can't get the shots I really want. Is this a lens for just zooming in from far away? I'm trying to do shots like baby feet up close or flowers up close, but I can't seem to get the entire object in focus like I would like.

Is this an f-stop thing? I just can't seem to figure out this f-stop thing, even though I know that f-22 gives greater depth of field and lets in less light. Is this just the problem with my lens...that the f-stop has to be 4 - 5.6?

Can someone please share ideas/help/anything? I'm shooting with a D-Rebel.

Thanks!
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Old Nov 29, 2004, 9:58 PM   #2
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WisconsinGirl wrote:
Quote:
What is the difference between a telephoto and a zoom lens?
A zoom is a lens with variable focal lenght (ie 75mm, 100mm, 200mm, 300mm... and everything in between).
A fixed telephoto lens is just one focal lenght: a 75mm or a 200mm, they are two different lenses

The difference in telephoto is that they are much more simpler to manufacture and to optimize for performance at 1 focal lenght against a zoom which have to work over many focal lenghts and where trade-offs have to be made.



Quote:
I'm trying to do shots like baby feet up close or flowers up close, but I can't seem to get the entire object in focus like I would like.

Is this an f-stop thing?
Yes!
To get f/22, you have to use Av mode: The aperture is fixed by you and the camera will select a shutter speed to match...




Quote:
Is this just the problem with my lens...that the f-stop has to be 4 - 5.6?
No...
The f-stop varies with your lens because the f-stop is a ratio of a lens diameter with its focal lenght -> since the focal lenght varies when you zoom in/out but the physical barrel of your lens stays constant, its ratio changes from f/4 (short focal lenght tele) to f/5.6 (longer focal lenght tele): http://www.uscoles.com/fstop.htm

Note that some zooms have fixed aperture (usually faster f/2.8 ) and theses are much more expensive as well as their larger barrel size and weighty optics to maintain the above ratio constant, but if you need it... :sad:




A zoom generally performs poorly at close distances (except for ones with macro settings). They are also at their worst when used wide open and need to be stopped down a few stops. Telephoto lenses especially macros are designed to be excellent at close range, and some portrait lenses are optimized to be shot wide open like the EF-85mm f/1.2 (or f/1.8 ) with extremely narrow DOF to blur out the background (this amount of blurring is usually called 'Bokeh'), and bring the attention to the subject(s) :?
http://www.wlcastleman.com/equip/reviews/85mm/
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Old Nov 30, 2004, 10:22 PM   #3
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You have a telephoto zoom.

Telephoto is generally refered to as being a lens that has some amount of magnification. e.g. a 105mm is approx 2x telephoto.

However, the true definition of "telephoto" is NOT what I've just said. A telephoto lens does have some amount of magnification, however, telephoto is the optical construction or design of the lens. You can get a 400mm lens (8x magnification), but it would be about 400mm long. This is a standard design of lens. A telephoto lens optically shortens the lens so that although the effective focal length is 400mm, it may only be 200mm long. The design of the lens optically compresses the lens.... this is called a telephoto lens.

Your lens is a telephoto zoom because it has a variable focal length (zoom) and it magnifies (1.5x - 6x) and the optical construction is telephoto.

You probably need a wide angle zoom, say 28mm - 70mm (about 0.6x - 1.5x), or a fixed wide angle lens.(24mm, 28mm, 35mm etc) This should have a macro mode, to get in close. Wide angle lenses also have larger depth of field.

Wide angle lenses (e.g. 18mm-38mm approx) have what is call "Retrofocal" optical construction, which is the opposite to "telephoto", in that the focal length is optically stretched. This is needed on SLR cameras becsause the film plane is more than 40mm from the back of the lens.


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