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Old Nov 16, 2007, 9:38 PM   #1
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I am getting a Pentax 18-250 when they become available. My question is how many of you have used a teleconverter? Are they any good? Are they worth the? Anything I should keep in mind?

I have read on the Sigma site that they recommend only using... well at least there teleconverter with the lenses listed. Is this a must or are they just blowing smoke? Will an AF lens still AF with say the 1.4x one?

Did Pentax ever make one?

Robert
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Old Nov 16, 2007, 11:17 PM   #2
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Pentax made them, but they were not AF and fairly expensive. Then there was the AF 1.7 that Pentax made the would make a manual lens AF, but they seem out of control price wise if you can find them used.

Tom
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Old Nov 17, 2007, 12:24 AM   #3
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Hi Robert,

I use TCs all the time (probably more often than not). The good ones can be very good, and the bad ones -- well, lets just say that there's a lot of 'em weighing down stacks of paper. . . and some members have found that they work better as extension tubes if you remove the optics.

Your AF system is designed to work with lenses that have a max aperture of f5.6 or faster (it'll work with a slower lens, but performance usually suffers -- read very slow, hunting, and maybe not locking reliably), and since you lose a stop with a 1.4x, 2 stops with a 2x, and 1.5 stops with a 1.7x TC, you can pretty much count on none of these working in AF with the 18-250 at 250mm, since the max aperture of the lens is f6.3 at the long end. Even a 1.4x TC will take your max aperture to f8.8, and I doubt that AF will work, even in really bright light, but each lens is a little different, so realize that there is no absolute statement here.

The Tamron F 1.4x TC, which is generally reported to be a rebranded Kenko 1.5x AF TC, is widely regarded as the best of the AF 1.4x TCs for the K mount, and it's pretty reasonably priced at @ $100 USD. Their 7 element 2x TCs are are also considered to be good, but most 2x TCs degrade IQ more than they are worth, plus need very fast, very sharp lenses to work with, so they usually aren't very satisfactory. Both of the Tamron/Kenkos have tested better than the respective Sigmas IIRC.

The Sigma TCs are, IMO overpriced for their optical performance, and both the 1.4x and the 2x have an extended front element which makes them physically incompatible with a lot of lenses since the front element of the TC will actually contact the rear element of the lens when you try to fit it. Unfortunately, Sigma has chosen to only list their own lenses in the compatibility chart, so if you only have non Sigma lenses, it's hard to tell if one will work with their TCs. IMO, this is a bad situation, since trying to fit a non-compatible lens might damage both the lens and TC if the elements touch, especially if there is some abrasive grit in the wrong place. -- and though they might work with what you have now, who knows what you might get in the future. I wouldn't want something in my bag that might trash my virtually impossible to replace long fast teles.

The Pentax TCs are all manual focus except for the P F 1.7x Auto Focusing Adapter, which is my personal favorite. The F 1.7x AFA allows the AF system to focus within the TC, which allows manual focus lenses to auto focus within a certain distance range. It's also has very good optics. The rule of thumb is that the better the lens to start off with, the better the TC will perform. The P F 1.7x AFA gives very little IQ degradation with good lenses, in my experience. Unfortunately, it's been discontinued for some time, and demand for this almost unique item (Nikon made a similar one for a short time) has caused the prices to skyrocket.

The other Pentax TCs are the A Rear Converter 1.4x S (and L) and the A Rear Converter 2x S (and L). The S models can be used on any lens, and the L models have extended front elements (like the Sigmas) and are designed to give better performance on Pentax lenses over 300mm which have recessed rear elements. The Pentax TCs are very good optically, but are only MF. The hope is that Pentax will, sometime in the future offer newly designed AF TCs that will give good AF performance with the new DA* SDM telephotos in the roadmap, and I am hoping to see a resurrection of the AFA in some form, hopefully a 1.4x.

Another oddball model that deserves mention is the Vivitar 2x MFTC (Macro Focusing TC). It's also a manual focusing model, but has a built in helicoid with optics that can give you up to 1:1 macro capability in addition to the 2x magnification on any lens. If one has a 50mm f1.4, then this model will give you a 100mm f2.8 1:1 macro lens that is just as functional (and almost as good optically) as a dedicated 1:1 macro lens. It was made in both "K" and "A" configurations without and with the electrical contact pins respectively, so one needs to watch what version is being offered before buying if exposure automation is desired.

IMO, all the other brands/models haven't made enough of an impression to be considered, but I don't have any experience with any of them, so take that for what it's worth.

Probably more than you really wanted to know. . .

Scott
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Old Nov 17, 2007, 1:38 AM   #4
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Looks like I will have to look for the Tamron then for $100 I can afford to play.

Thanks for all of the information both of you. Very interesting and informative.

Robert
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Old Nov 17, 2007, 5:42 AM   #5
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There is another one I think should be mentioned ... There is the Vivitar 75 - 205mm lens that has a matching 2X TC, I've had the TC for a while, and recently acquired the lens, so far I've only run a few test shots with this combination which came out quite well. I know Roy had this set-up, and I'm pretty sure he rated it highly. ... Jack.
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Old Nov 17, 2007, 10:55 AM   #6
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jachol wrote:
Quote:
There is another one I think should be mentioned ... There is the Vivitar 75 - 205mm lens that has a matching 2X TC, I've had the TC for a while, and recently acquired the lens, so far I've only run a few test shots with this combination which came out quite well. I know Roy had this set-up, and I'm pretty sure he rated it highly. ... Jack.
I also have that lens but did a lil research and found that most did not rate the 2 x matched TC as any better than a normal TC. Do you have any samples to post?
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Old Nov 17, 2007, 11:22 AM   #7
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TC3 wrote:
Quote:
jachol wrote:
Quote:
There is another one I think should be mentioned ... There is the Vivitar 75 - 205mm lens that has a matching 2X TC, I've had the TC for a while, and recently acquired the lens, so far I've only run a few test shots with this combination which came out quite well. I know Roy had this set-up, and I'm pretty sure he rated it highly. ... Jack.
I also have that lens but did a lil research and found that most did not rate the 2 x matched TC as any better than a normal TC. Do you have any samples to post?
Hi,
No I don't have any samples, the test shots I did were just random ones of no great interest so got deleted, I did try a couple earlier today in my garden, but the wind is moving things around too much, so I abandoned that idea.
Lets see if Roy comments, I know he had this combination and I'm pretty sure he liked it. Do you have any links to your research ?. ... Jack.
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Old Nov 17, 2007, 12:43 PM   #8
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yes, i had this setup for a week or two. i got it as a low ball bid on ebay, 16bucks delivered. when i shot examples to sell it i was really surprised at the IQ it produced. with the 2x it produced excellent images that were a little soft but nothing a little USM could not handle.. for 50>, it's a good deal.. i don't know how the TC worked with other lenses..

about TCs.... not sure if i've ever been too satisfied with using a 2x on a zoom. the 1.4x may give you acceptable results.. i've got the tamron mc4 2x and have been fairly happy with it on primes. it's their cheaper version that sells for about 50+ on ebay.. the pentax TCs i'd recommend but they will be at a premium price..

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Old Nov 17, 2007, 3:14 PM   #9
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There is a 2x 7 element on auction i saw and it states that 7 element tc's are meant to be better than 4 element. Can anyone back that up?
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Old Nov 17, 2007, 6:54 PM   #10
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don't really know but if it's cheap it probably is.. with the lens you are planning on using it with i don't think i'd go more than 1.5x.. there's just too much glass in the way and that lens is pushing the envelope already by it's self. i haven't tried the most expensive TCs but i do know i've never been happy with a 2x on a zoom lens..
i'll say this tho, that i am trying a pentax A1.4x-s on my A70-210/4 right now and will post results in the next day or two..
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