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Author Topic: Crop factor & fast lens with IS?  (Read 2445 times)

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« on: February 20, 2007, 16:53 »
0
Hi all,

I stumbled over two more questions during my internet research :-).

1) The Rebel DSLRs have a crop factor of 1,6. With a full size (35 mm) sensor, a 35 mm lens would mean a 1x magnification factor, right? With the rebel a 35 mm lens would mean a 1,6x magnification factor, or? Therefore a 75 mm lens would mean a 3,something magnification factor on the rebel, right?

2) I stumbled over this lens: EF-S 17-55mm f/2.8 IS USM. Why does such a fast lens need image stabilisation?

Thanks,
Michael


« Reply #1 on: February 20, 2007, 17:01 »
0
Lenses don't have a focal length magnifier or "factor".  The camera sensor does.

For a camera with a 1.6 factor, you would multiple the factor (1.6 in this example) times the focal length of the lens (hence the name focal length magnifier).  So a 100mm becomes a 160mm, a 200mm becomes a 320mm, etc.

For a camera with a 1.0 factor (or full-size sensor), there is no magnification.

The focal length magnifier is great for telephoto lenses, since it makes them even longer.  But it is not so good for wide-angle lenses.
« Last Edit: February 20, 2007, 17:04 by GeoPappas »

« Reply #2 on: February 20, 2007, 17:06 »
0
re IS, why do people use tripods?  THey are not just used on slow lens. 

You also dont need to use the lens at its fastest appeture.

« Reply #3 on: February 21, 2007, 02:46 »
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re IS, why do people use tripods?  THey are not just used on slow lens. 

You also dont need to use the lens at its fastest appeture.

Doh...you are right. That one was a real stupid question. One shall not post after working all day and then browsing the net all evening in order to learn about lenses ;-)

« Reply #4 on: February 21, 2007, 04:02 »
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re IS, why do people use tripods?  THey are not just used on slow lens. 

You also dont need to use the lens at its fastest appeture.

Doh...you are right. That one was a real stupid question. One shall not post after working all day and then browsing the net all evening in order to learn about lenses ;-)
Not a stupid question as Canon doesn't put IS on their wide angle lens as much as their long lens.  Obvioulsy at the long end is where it is needed most but I think in future IS will be everywhere.  With stabilisation sensors becoming common canon will need to reply iwth IS across their whole range.  Nikon even put VR on a Macro lens (I thought macro were almost exclusively used on tripods??).


 

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