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Old Jul 27, 2007, 7:34 AM   #11
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Thanks Phil, thats the info I needed.
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Old Jul 27, 2007, 8:41 PM   #12
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Found out throughout Camera Canada that the 1.4 tele made by Tamron only works with lens less than aperature 4.0. No other lens cuts it either. So far it does not appear that enhancment of the 18-250 is possible. Too bad I did not rechearch further before posting. I was looking for the magic answer.















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Old Jul 28, 2007, 4:35 AM   #13
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dwssas wrote:
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Found out throughout Camera Canada that the 1.4 tele made by Tamron only works with lens less than aperature 4.0. No other lens cuts it either. So far it does not appear that enhancment of the 18-250 is possible. Too bad I did not rechearch further before posting. I was looking for the magic answer.
f4 ?
that spec probably only means that it will only work in AF with a lens faster than f4. it may have nothing to do with the lens if you use it with a slower than f4 lens. if you find out differently, please post the results. i'm not quite sure. i've got an AF tamron 2x and it works with any lens i have.. however it does not AF with the kit lens which is the only AF i have.

roy
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Old Jul 28, 2007, 5:19 AM   #14
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dwssas wrote:
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Found out throughout Camera Canada that the 1.4 tele made by Tamron only works with lens less than aperature 4.0. No other lens cuts it either. So far it does not appear that enhancment of the 18-250 is possible. Too bad I did not rechearch further before posting. I was looking for the magic answer.
Hi dwssas,

The manufactures state a general working aperture for their TCs which is usually slower than what they actually will work with, depending on the lens in question. They are forced to in order to avoid numerous false advertising claims. I'm not saying that it will work with your lens, but neither am I saying that it won't. You really have to try it out, and if it won't AF in good light, then you either have to accept it as a MF converter for that particular lens, keep it for future use with a more suitable lens, or return it for a refund, accepting whatever charges might be appropriate.

A good example is the P F 1.7x AFA. Pentax states that it will only work with lenses with widest apertures of f2.8 or faster, yet I find it works well with the FA*300 f4.5, A 70-210 f4, and a number of other lenses slower than the what the specs allow.

Something that you might consider is a used Kenko 1.5x -- KEH as of this writing has a number of them in stock for good prices. They are a very reputable dealer. It might be worth a shot. . .

Scott

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Old Jul 28, 2007, 6:52 AM   #15
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snowstorm=robar----you are correct. I forgot to mention that they also said this tc could be used but would not autofocus. Only manual focus would be available but at least thats an option.
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Old Jul 28, 2007, 11:34 AM   #16
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Let me jump in here and point out that it is not the lens itself the TC will or will not work with, but the lens&tc combination will or will not work with a particular body.

A 1.4x TC costs 1 stop of light (a f/2.8 lens becomes a f/4, an f4 now passes the same light as a f5.6 lens)
A 2x TC costs 2 stops of lignt (a f/2.8 lens is now down to f/5.6 and a f/4 is only letting through the light of a f/8 lens)

It seems most bodies stop trying to AF when they see a lens with a smaller aperture than around f5.6/f6.3. There are exceptions. For instance the really expensive canon pro 1d bodies will try to AF down to f/8.
So you are left with manually trying to focus a very dim image in the viewfinder.

However you can try to fool them, there was a tape trick being tried a while back. ( I have never tried it )
If you cover the right contacts on the tc the new effective aperture is not reported to the camera and it continues to try to AF. Mostly the results were a slow constant hunting for focus, but in very bright sunlight it sometimes did work. Not sure what this would do to the in camera IS system if it did not know what lens was mounted.
Need to find someone who has tried it and see what their results were :G
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Old Jul 28, 2007, 1:00 PM   #17
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PeterP wrote:
Quote:
Let me jump in here and point out that it is not the lens itself the TC will or will not work with, but the lens&tc combination will or will not work with a particular body.

A 1.4x TC costs 1 stop of light (a f/2.8 lens becomes a f/4, an f4 now passes the same light as a f5.6 lens)
A 2x TC costs 2 stops of lignt (a f/2.8 lens is now down to f/5.6 and a f/4 is only letting through the light of a f/8 lens)

It seems most bodies stop trying to AF when they see a lens with a smaller aperture than around f5.6/f6.3. There are exceptions. For instance the really expensive canon pro 1d bodies will try to AF down to f/8.
So you are left with manually trying to focus a very dim image in the viewfinder.

However you can try to fool them, there was a tape trick being tried a while back. ( I have never tried it )
If you cover the right contacts on the tc the new effective aperture is not reported to the camera and it continues to try to AF. Mostly the results were a slow constant hunting for focus, but in very bright sunlight it sometimes did work. Not sure what this would do to the in camera IS system if it did not know what lens was mounted.
Need to find someone who has tried it and see what their results were :G
Hi Peter,

I don't think that it's a matter of the aperture value (f stop number), but the amount of light the AF sensor receives that determines whether the AF system can work effectively. The contrast and resolution of the lens also comes into play, and the more contrasty and higher resolution the lens wide open, the less light would be needed for the AF sensor to get a lock, and for these lenses with these attributes, a slower max aperture would still AF.

Scott
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Old Jul 28, 2007, 2:50 PM   #18
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Just made a discovery. With the K100d plus a 18-250 tamron, I hand held an Olympus 1.7 tele on front of the 18-250. The auto focus worked perfectly. If I can find a step down 62-55 ring I think I may have a winner especially since I already own the Olympus. I am posting examples of the results.
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Old Jul 28, 2007, 2:52 PM   #19
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Trying again.
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Old Jul 28, 2007, 3:09 PM   #20
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Still trying to post examples. One at 250mm and the other at 250mm + the t-con 1.7
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