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I have a Nikon D3100, a standard 18-55mm and Sigma 70-300mm lens without IS.

  1. Is it true that a teleconverter cannot be used with my Sigma lens? Can it be used only with block lenses, or is there any way I can make use of the teleconverter for extended zoom?
  2. At the full 300mm zoom my images seems to degrade in quality (soften), is there any way to fix this problem?
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1  
See also photo.stackexchange.com/questions/439/… –  mattdm Apr 27 '11 at 14:28
    
Just a word of caution: I have a APO 70-200mm F2.8 EX DG / HSM outfitted with Sigma 1.4x TC and the quality of the images are extremely poor (chromatic aberration), especially at 200mm. –  user10336 Jun 25 '12 at 15:49

2 Answers 2

up vote 9 down vote accepted

You can use a teleconverter with some zoom lenses -- but not with your Sigma. In fact, most of Sigma's higher-end zooms are specifically designed to work with a teleconverter (Sigma's), and a couple of them actually perform slightly better in some respects (specifically in terms of chromatic aberration and vignetting) with the teleconverter added than without.

The main problem with your lenses (apart from any other aspect of the lens design) is the maximum aperture. A 1.4x converter increases the effective focal length of the lens by a factor of 1.4, but it also decreases the effective maximum aperture by one full stop. That means your Sigma would become a 98-420mm f/5.6-8 lens, and would not be able to autofocus on your D3100 except at its widest setting. A 2x teleconverter doubles the focal length of the lens, but at a cost of two full stops, so your lens would be a 140-600 f/8-11, and would not autofocus at all on your camera.

That said, the optical design of the lens can complicate all of this a great deal. It's not just about the maximum aperture; the shape of the light path behind the lens has a lot to do with it as well. Not all lenses, whether zoom or prime, have an optical characteristic that plays nicely with teleconverters. Most current telephoto zooms with a maximum aperture of at least f/2.8 work well with converters (some are restricted to 1.4x converters and should not be used with 1.7x or 2x converters -- see the lens manual). "Consumer grade", slow, variable-aperture lenses usually don't work well, or don't work at all, with a teleconverter.

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is there anyway i can quench the needs for satisfying zoom with my lens ? –  pradeep sekar Apr 27 '11 at 16:09
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@pradeep sekar: If you mean "can I make my lens longer", the answer is no. Consider this,though: 300mm is a very long lens already, particularly on a crop-sensor camera (on a D3100, it's equivalent to having a 450mm lens on a 35mm camera). Unless you are shooting things that are actually dangerous to get close to (such as great cats in the wild or space vehicle launches) the right answer is almost always "get closer", not "get a longer lens". Your feet will always be the best zoom you own. –  user2719 Apr 27 '11 at 16:23
    
@pradeep, I don't believe I quite agree with @Stan on the availability of foot-zoom: often the object of interest is simply far away, or there is water/etc. in the way. However, very long lenses get insanely expensive very fast, and they have have a variety of use limitations like high minimum shutter speeds and great mass and bulk. Thus, you might end up happier if you focus on photography that doesn't require such long lenses. –  Reid Apr 27 '11 at 17:30
    
For what it's worth, the longest lens I ever owned (though I rented longer, up to 1600mm, for aerospace photos when I was in the Forces) was a 400mm on a 35mm, and it spent most of its time sitting in front of an extension tube so I could get closer than the 3m minimum-focusing distance when needed. That's over a couple of decades hiking through Algonquin and Kejimkujik National Parks. For water, they've got these neat things called boats and canoes (and hip waders). If you see something that's truly once-in-a-lifetime at a distance, it'll be gone before you can change lenses, so crop. –  user2719 Apr 28 '11 at 1:11
    
there may be off-brand TCs that work with that lens (they existed for my old Sigma 70-210 20+ years ago for which no Sigma TC was offered), but indeed not Sigma TCs. It'll deliver extremely poor results if you can fit one of course. –  jwenting Apr 28 '11 at 7:50

The other answer is fantastic, but as a practical matter you should also know about the Sigma teleconverter compatibility chart:

http://www.sigmaphoto.com/sigma-lens-teleconverter-compatability-chart

If you have a Sigma lens and it is not on there, you cannot use a TC with it. The chart is also helpful to understand if you'll be able to use AF with a converter on a specific lens.

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you can't use a SIGMA TC with it. Other TCs may exist. Sigma, like Nikon, doesn't list 3rd party accessories in their documentation (looking at Nikon's lens compatibility charts for their cameras for example doesn't list Sigma lenses as being compatible). –  jwenting Apr 28 '11 at 7:51
    
I just clarified the list is only about compatibility with Sigma lenses. Other TC's exist of course, but if a Sigma lens is not on that list, I don't think a TC from any maker will work with the lens. –  Kendall Helmstetter Gelner Apr 28 '11 at 14:59

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