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05-02-2013, 08:08 AM   #1
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Is autofucos works, if stack two Pentax 1.7X adapter to Pentax DA* 300mm F4?

Need your opinion, regarding stacking up two Pentax AF 1.7X adapter to Pentax DA* 300mm F4. Does the autofocus works? The quality of pictures?
Need to clarify prior to purchasing another Pentax AF 1.7X adapter.
Thanks a lot.

05-02-2013, 08:16 AM   #2
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No idea if anyone has ever tried that before.

The Pentax AF 1.7x auto-focus adapter does not pass through the screw drive auto-focus so at best only the first adapter will function. I've no idea if there is enough focus range in the adapter to focus correctly with a second one in place. Theoretically, manually focusing the DA*300 to close and then allowing the AF adapter to complete the focus would work. But in practice, I've no idea.

Not sure the image quality would be worth the trouble either.

If you do get it to work, please post back and let us know.
05-02-2013, 08:25 AM   #3
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QuoteOriginally posted by kulas3VII Quote
The quality of pictures
It's my understanding that IQ starts to suffer a little with one adaptor, let alone two.

If you have a requirement for extra reach, I would put my money into longer glass, for use without adaptors.
05-02-2013, 08:25 AM   #4
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QuoteOriginally posted by kulas3VII Quote
Need your opinion, regarding stacking up two Pentax AF 1.7X adapter to Pentax DA* 300mm F4. Does the autofocus works? The quality of pictures?
Need to clarify prior to purchasing another Pentax AF 1.7X adapter.
Thanks a lot.
i don't have 2 1.7 but i could try the tamron 1.4 and the pentax 1.7 but my guess is it won't autofocus...i have used the 2 on the q which was manual focus and there was some iq loss but was better than i expected
did additional testing with the q..first 2 had the sigma 500mm plus pentax 1.7 adapter 850mm equiv, last 2 had the 1.7 plus tamron 1.4 1190 equiv..increased iso to 500 to have enough light | Flickr - Photo Sharing!

05-02-2013, 09:31 AM   #5
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QuoteOriginally posted by kulas3VII Quote
Need your opinion, regarding stacking up two Pentax AF 1.7X adapter to Pentax DA* 300mm F4. Does the autofocus works? The quality of pictures?
Need to clarify prior to purchasing another Pentax AF 1.7X adapter.
Thanks a lot.
There won't be enough light for AF to work, for starters. You're going to be shooting at F9.5, which is far too dark.

Adam
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05-02-2013, 11:48 AM   #6
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i am amazed anyone is considering two of these at the price,

as to AF, if you read the instruction book, they do not recommend slower than F2.8 but there are a ton of forum members (self included ) who have reliably used it on F4 lenses with no issue. going to F4.5 lenses is where it becomes really challenged, and F5.6 is pretty much an absolute no.

virtually all AF cameras need a native aperture of lens plus TC of F6.9 or lower,
05-02-2013, 12:15 PM   #7
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The second 1.7x will essentially be in a random position since it has a floating element and it won't be moving. You could probably get away with body -> 1.7x AF -> fixed 1.4x (or 1.5x) TC -> lens (2.8 or faster) and still have it focus under bright light. Probably look like crap though (if resolution is suffering too much it won't focus just for that reason, let alone the light loss).
05-04-2013, 04:06 AM   #8
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Thanks a lot everybody. I really do appreciate your input/opinion. Have a great weekend.

05-04-2013, 09:05 PM   #9
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Hi kulas3VII,

I have done this, but not with a DA* 300 because I don't own one. I tried this a few years ago with my FA* 300 f2.8, and the images were okay, but the AF was touchy enough to not warrant further work with this setup. vonBaloney is correct in that normally the focus position would be random, but this can be worked around by first mounting one of the AFA s with a lens, then using AF to focus the AFA to infinity, then mount the second AFA to the body, the infinity focused AFA on top of it, and finally the lens.
Those who stated that the AF sensor would not get enough light are correct, but the actual effective max aperture for an f4 lens rig would be something like f11.6, and that's way too dark.

With a late model body (K-7 or newer), AF would work with an f2.8 lens and stacked 1.7xs (effective max aperture would be @ f8.1), but the lighting would have to be pretty good with a reasonably contrasty subject. I found that stacking a good 1.4x and the AFA on a 300 f2.8 lens gives a lot better overall performance since the resulting f6.7 effective max aperture is reasonably within any Pentax DSLR's AF sensitivity range for most daylight conditions. Sketchy AF was the main reason I didn't pursue this further.

"brandrx" on DPR also experimented with stacked AFAs a while back and came to the same conclusion.

For those who might be curious. . . I currently have 3 F1.7x AFAs, that I picked up before they became outrageously pricey as they are now -- the average price I paid was @ $150 USD. I regularly use them with my FA* 300 f4.5, Sigma EX 180 f3.5 APO DG Macro, Sigma EX 300 f2.8 APO, and FA^ 300 f2.8. The Two Sigmas do not report correct f stop values, so I had to modify two of the AFAs by taping contacts, and it's more convenient for me to just leave two of them taped for use with those lenses.

Scott
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