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Old Oct 26, 2008, 11:27 PM   #1
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I have a pentax K20D and DA 18-250 lens that I have been happy with but on occasion wish I had a longer lens. The longer lens is not quite in the budget right now so I am considering a teleconverter. I realize that I will pay a penalty in speed and image quality but was wondering if anyone has direct experience with a teleconverter that they were satisfied with. The primary use would be wildlife and motor sports. This is my first DSLR and I have been away from photography for many years, and I must say I love the camera and am having a blast!
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Old Oct 27, 2008, 12:46 PM   #2
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buying a converter for that lens and you will be

wait a while, save some more, and buy a better lens that you will be pleased with.

Rome was not built in a day, so by the mean time, learn to use thatcamera, and with the resolution of the K20D, trim the images, so as to look as ifit was taken with a longer lens

BTW welcome to this forum, and always remember the only silly question, is the one that was not asked





Dave


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Old Oct 27, 2008, 1:28 PM   #3
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Hm, let me add something here. Of course you can add a TC (teleconverter) to your lens, but did you take the f-stop-problem into account?

Your 18-250 lens is "wide open" at f/6.3 for 250mm (and you want to use it in tele mode, ok?). If you use a 2xTC, doubling the focal length to 500mm (yay!) this means also doubling the f-stop, so with a 2xTC you got a 36-500/7-12.6 lens (technically) so we talk about 500mm at f/13 or stopped down f/16 here.

Can you imagine how much light you need to get a reasonable exposure time to "freeze" animals (I guess you are going for moving objects)? Just take your lens for a walk, set it to f/16 and try to get 1/500s exposure time or better (rule of thumb: an exposure time of (1 / focal length) gives a steady shot).

These are things to consider, aside from image quality and stuff. If you browse the forum you'll see the combination TC+lens for high-quality, fast lenses, i.e. 1,4xTC + 200mm/2.8 lens etc.

Oh, and did I mention, that there are AF limitations with TC's? Read the descriptions, but I'd say a reliable AF (I didn't say SNAPPY, FAST, that's another story) you can get with lenses up to f/4, the faster the better.

The advise of dafiryde is good. Stay with that lens and crop for a start. Then choose a fast, long lens and add (if you still need it then) a high quality TC.

Just my 2 cents.
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Old Oct 27, 2008, 3:56 PM   #4
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Hi TTL,

. . . and welcome to the forum!

I use TCs extensively, but only with lenses at least a stop faster than your 18-250 (f 4.5 or faster). The advice given by Dave and Th is good, especially concerning max aperture (lens speed).

The K20 does well at high ISO, so by bumping this up, you can compensate for the speed loss as far as exposure goes, but you'd likely be losing at least some AF capability, even with a 1.4x TC (your lens would have a max aperture of about f8.8 at the long end). The viewfinder image would also be a bit dark to focus accurately manually, so you'd only be able to use it practically in pretty bright light.

Another factor you might consider is that wide range zooms (over about 3x range), though amazing in their IQ (Image Quality) nowadays, suffer a bit more from design compromises to get that range, so aren't always the best candidates for TC use. The general rule is -- the better the lens, the less IQ is degraded by a good TC. TCs will always compromise IQ, but this effect is negligible or at least acceptable with high-end lenses.

Luckily, the K20 has a lot of resolution, so cropping is probably your best bet -- or you can always "zoom with your feet" -- get closer. . .:-)

BTW -- I don't use or recommend any of the 2x TCs with the possible exception of the Vivitar 2x Macro Focusing TC for its special close focusing ability.

The best TCs optically for K-mount lenses are generally conceded to be the Pentax models, but they are Manual Focus only so far (there is one exception, the F 1.7x AFA which allows sometimes limited AF with any mount comaptible lens about f4.5 or faster). There is a new SDM TC that's been tentatively announced that will have AF capability, but there's little info available, other than it's going to be a 1.4x, and both SDM and screw drive compatible, and will probably be pretty expensive.

Tamron (generally accepted as a rebadged Kenkos) markets AF 1.4x and 2x TCs that are not that expensive (a bit over $100 USD for the 1.4x), but very good optically. The Kenkos are listed as 1.5x and 2x respectively.

Sigma makes 1.4x and 2x TCs their EX APO line, but there can be compatibility problems with a lot of lenses because their design requires use of a main lens that has a recessed rear element. The 1.4x currently sells for @200 USD.

Congrats on your new camera -- be sure to post some pics -- we're always interested in the wide range of subjects that our members here like to shoot.

Scott
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Old Oct 27, 2008, 5:27 PM   #5
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welcome to the forum.
i'll agree with everything said here. i don't think i've ever been happy with a 2x + zoom lens.. a good one will cost a little less than a new lens.. i've got a tamron 2x mc4 and i'm happy happy with it on some lenses but never on a zoom. zooms usually are mad of of 14-18 pieces of glass.. you'll be adding 4-7 more pieces when using a TC. .. the loss of aperture is enough alone. ck what you'll see by using the stop down feature of the camera. put the lens on 250mm and set the aperture to f16, hit the stop down button and you'll see the brightest you'll ever get with that lens.

just my 2ยข,
roy
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Old Oct 27, 2008, 6:09 PM   #6
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To all who replied, thanks for the tactful reality check. Sometimes I need to be reminded there really is no free lunch, not even cheap lunch these days (unless you want a bellyache):-)

I will start saving my pennies and focus (small pun intended) on improving my sorely lacking skills.

It looks like posting pics is pretty much organized into different subject matter on different areas of the forum. I assume proper etiquette is to post at those locations rather than in a Q and A forum such as this one?
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Old Oct 27, 2008, 7:25 PM   #7
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thruthelens wrote:
Quote:
To all who replied, thanks for the tactful reality check. Sometimes I need to be reminded there really is no free lunch, not even cheap lunch these days (unless you want a bellyache):-)

I will start saving my pennies and focus (small pun intended) on improving my sorely lacking skills.

It looks like posting pics is pretty much organized into different subject matter on different areas of the forum. I assume proper etiquette is to post at those locations rather than in a Q and A forum such as this one?
Welcome to the forum. I post here to get the feed back I need from those that use the same equipment I do. Nothing wrong with doing that. Here is my latest post....http://forums.steves-digicams.com/fo...mp;forum_id=80
Follow all of the good advice above for the TC. I've a Sigma 1.4 that works well with my Bigma and a Vivitar f/5.6 300mm lens. Won't fit many others though for the above mentioned reason. Save and get a longer lens with a lower aperture no.

Dawg

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Old Oct 27, 2008, 10:50 PM   #8
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Let me add one more welcome thruthelens! If you have a Pentax related question (or even a general photography question) you will likely get the answers you need in either the Pentax Dslr or Pentax lens forum (this one). This is a good group of people, ask questions, post pictures, and jump right in.

Tim
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Old Oct 27, 2008, 11:39 PM   #9
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Again thanks to all for the kind welcome. The internet truly has opened the door to a world of good neighbors. Thanks for the encouragement, and "bigdawg" I really enjoyed your pictures. I have attached a picture taken with the camera and lens referenced in this thread and have a dumb question, how do I post more than one picture?:O


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Old Oct 28, 2008, 1:21 AM   #10
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thruthelens wrote:
Quote:
Again thanks to all for the kind welcome. The internet truly has opened the door to a world of good neighbors. Thanks for the encouragement, and "bigdawg" I really enjoyed your pictures. I have attached a picture taken with the camera and lens referenced in this thread and have a dumb question, how do I post more than one picture?:O

You can use the browse button below the post window and the reply to your photo post and use the browse button again. Keep replying and using the browse button to upload as many as you wish. That is one way. The way to get them all in one window is to use a photo hosting sight such as Photobucket or Image shack and direct link to the post. When doing this you have to put [img before the link and [/img] after the link. Kinda like this....

[img] the direct link url [/img]

by doing this you can have as many photos as you like in the one post window. I hope this helps.

Dawg
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