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Old Jul 28, 2007, 3:09 PM   #21
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and the other.
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Old Jul 28, 2007, 3:11 PM   #22
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Having trouble posting these.
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Old Jul 28, 2007, 3:12 PM   #23
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And again
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Old Jul 28, 2007, 5:00 PM   #24
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Hi Scott,
Yes, we are talking the same thing :-)
Each adjecent aperture value number either halves or doubles the light the sensor and AF system receives from the lens.

So a lens with a max value of f/4 will transmit twice as much light to the sensor as one with a max value of f/5.6 and half the light that a f/2.8 lens will transmit. :-)

The max value a lens is capable of is what is important as cameras perform their AF operations with the lens wide open, only stopping it down just as the image is taken.

Now I am more familiar with Canon equipment and the AF cutoff at ~f/6.3 for prosumer bodies is built into the cameras firmware. As soon as a lens/TC combination reports a smaller available max f/number the cameras just shut AF off.
Nikon seems to do the same and I suspect similar is built into the Pentax to prevent the camera from doing endless seeks.


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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.

Scott
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Old Jul 28, 2007, 5:06 PM   #25
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Is that 1.7 t-con meant to be used in front of an existing lens?
Having a hard time imaging how a standard teleconverter meant to go between the lens and camera would work held infront of a lens :?

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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, 6:51 PM   #26
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PeterP-----Yes what I did was to max the zoom to 250mm on the Tamron 18-250. I then hand held the Oly 1.7 tele, which I used frequently on my Panny FZ-20. The results are as you see. I am not a technical person so why this worked, I have no idea.
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Old Jul 28, 2007, 9:24 PM   #27
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Hi dwssas,

The Oly Tcon 17 will work on your lens, but you have to watch it if your front lens element rotates when it focuses. If this is the case, you might overstress your focusing motor with the extra weight, it would be quite an additional amount of mass for it to move. If your lens is internal focusing, then, obviously this is not a problem.

Scott
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Old Jul 28, 2007, 9:45 PM   #28
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PeterP wrote:
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Now I am more familiar with Canon equipment and the AF cutoff at ~f/6.3 for prosumer bodies is built into the cameras firmware. As soon as a lens/TC combination reports a smaller available max f/number the cameras just shut AF off.
Nikon seems to do the same and I suspect similar is built into the Pentax to prevent the camera from doing endless seeks.


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Hi Peter,

AFAIK, and in my experience, this hardware/firmware cutoff doesn't happen with Pentax DSLRs, and in most cases, the TCs just pass the aperture values sent by the lens without any multipliers to the camera anyway. The Pentax F 1.7x AFA is the only one that I've used that does the conversion. Both my DS and K10 retain AF even when I use the FA*300/4.5 and the P F 1.7x AFA which reports a max aperture of f8 to the camera -- and I'm glad it does!:-), but I can see the reasoning behind this limitation, even if I wouldn't want it implemented in future Pentax bodies.

I didn't realize that the Canons and Nikons did this as I've never owned an AF body from either, otherwise I wouldn't have made the distinction between amount of light and aperture value. I guess we both learned something today. . .

Scott


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Old Jul 28, 2007, 10:37 PM   #29
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Yes that is indeed good to know :!:

I apparently wrongly assumed most dslr would have implemented similar cutoff mechanism, and would have not even tried to mix and match :G



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Old Jul 29, 2007, 5:54 AM   #30
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snostorm wrote:
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The Tamron F 1.4x AF TC is usually considered a better alternative for three reasons. It's usually considered better optically, it doesn't have the lens compatibility problems, and it's less expensive
Had never heard of the the SIgma TC compat issues.... glad I never bought one.....

But I will second the vote for the Tamron 1.4x.... usued it on many a manufacturers lens and era typs (M, A,, F, FA, DA, third party lenses including Sigma) with no problems
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