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12-20-2011, 04:11 PM   #1
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teleconverter

Hello,
does anyone know if it's possible to use a teleconverter on a K-r Pentax with a 18-55mm lens?
And if yes,which one do you think is the best choice?
Thank you.

12-20-2011, 04:33 PM - 1 Like   #2
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It is possible technically, but it is not a good idea. It is much better to get a telephoto lens. Even a cheap one will be much better than the combination of TC and kit lens.
12-20-2011, 06:16 PM   #3
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Most of the TC's that I have looked into recently (just got a 2x AF myself) specifically state that they are not for lenses <50mm. Since nearly all of the 18-55's range in <50 I doubt it would produce a pleasing result if it worked at all. You can find 28-90 and 28-105 lenses that are well regarded for less than the typical 2x Autofocus TC costs and the IQ will be better in nearly every instance.
12-20-2011, 06:55 PM - 1 Like   #4
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The problem, mostly, is the loss of light. Using a 2x tele-converter will cost you about 2 stops of light. This means at 55mm your lens will be at best f/11. It is doubtful the auto-focus will work at that point because not enough light is getting through. If you put say a 200mm f/2.8 on with the TC you get 400mm f/5.6 which is useable. The other issue is the additional optics in front of the lens which hurts the image quality in direct inverse proportion to the cost of TC. In other words a cheap TC will look really bad, and an expensive TC will look a little bad but definitely worse than the lens alone.

There are also 1.4x TC's that do not cost you as much light and therefore seem to be more useable. Unfortunately they are rare and expensive when you find one.

As noted above just buy a tele zoom and you will be better off. Lots of inexpensive options on ebay. SMC-F 35-105 or 35-135 for example. I just bought a 35-105 for about $80, and I'm sure you can do better if you are careful. Check the lens database here before you buy any used lenses though, will give you a sense of the quality and the going price.

12-20-2011, 07:35 PM   #5
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QuoteOriginally posted by jatrax Quote
The problem, mostly, is the loss of light. Using a 2x tele-converter will cost you about 2 stops of light. This means at 55mm your lens will be at best f/11. It is doubtful the auto-focus will work at that point because not enough light is getting through. If you put say a 200mm f/2.8 on with the TC you get 400mm f/5.6 which is useable. The other issue is the additional optics in front of the lens which hurts the image quality in direct inverse proportion to the cost of TC. In other words a cheap TC will look really bad, and an expensive TC will look a little bad but definitely worse than the lens alone.

There are also 1.4x TC's that do not cost you as much light and therefore seem to be more useable. Unfortunately they are rare and expensive when you find one.

As noted above just buy a tele zoom and you will be better off. Lots of inexpensive options on ebay. SMC-F 35-105 or 35-135 for example. I just bought a 35-105 for about $80, and I'm sure you can do better if you are careful. Check the lens database here before you buy any used lenses though, will give you a sense of the quality and the going price.
I've got three zooms in that range. My Quantaray 28-90 Macro, my Tamron 28-105, and my newest the DA18-135WR. The Q was $30, the Tamron was $35, and the DA18-135 was somewhat more While I like all of them the Tamron has the most range and is pretty nice across its range only slightly stopped down (its the basis for the FA28-105 from Pentax as I understand it). It's definitely a solid purchase for someone wanting to go beyond the 55mm range without breaking the bank. The IQ is decidedly better than the 18-55 with the 2x teleconverter that's for sure. It's only f4-5.6 but its AF is snappy, color is good, and its darn sharp. You could do worse.
12-20-2011, 08:24 PM - 1 Like   #6
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Teleconverters are good in applications where content is more important than image quality, such as surveillance, blackmail, crime planning, etc. If you want images that look like actual photographs, skip the TC. If you need REAL LONG REACH for not much money, think about a cheap mirror lens. If that's too much reach, a number of good zooms have been suggested.
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