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06-26-2018, 12:47 AM   #16
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QuoteOriginally posted by Fenwoodian Quote
.
I just tried my Pentax-M 300mm f/4 lens on this Pentax 1.7x AF adapter. I then put this combo on my Pentax D-70 APS-C body.

That's the full frame equivalent of a 765mm f/5.6 lens!

I took about 20 test photos, and on all but one, they were unusable and pretty blurry with low contrast.

Below is the only photo that turned out half way acceptable. I focused on the telephone pole in front of this McDonalds drive-through (maybe 80 yards from me).

In this photo you could make out the telephone pole number (2/3rds the way up on the pole).

Sure, having a 765mm lens would be fun, but the quality of this slow lens set up is wholely unacceptable to me, so I doubt that I will ever again put the 1.7x AF adapter on my Pentax 300M lens.

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Yeah lots of PF... was it wide open?

06-26-2018, 09:40 AM   #17
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QuoteOriginally posted by LensBeginner Quote
Yeah lots of PF... was it wide open?
Yes it was...
06-26-2018, 11:14 AM   #18
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QuoteOriginally posted by Fenwoodian Quote
Yes it was...
That's why.
Multiplying lenses entails a sort of coffin-corner phenomenon, just like the one in aviation.
The greater the wide open f-number of the lens, and the more you have to stop down to get good results, the narrower is the acceptable range between "not sharp enough" and "softened by diffraction".

My M 50/1.7 can be used - with a 2X - from 2.8 (5.6) to 5.6 (11.2), while my 80-200/4.5 can be used almost exclusively at f/8 (16). More open, you get PF (ok you can get away with f/6.7 if there are no high contrast edges); more closed, massive diffraction penalty.
07-26-2018, 04:36 PM   #19
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QuoteOriginally posted by Fenwoodian Quote
You CAN use the Pentax AF adapter on "adapted" Zeiss and Leica lenses!
Do your Nikon adapted lenses have the mechanical aperture lever that the Pentax AF adapter is said to need?

07-26-2018, 06:25 PM   #20
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QuoteOriginally posted by BobSpot Quote
Do your Nikon adapted lenses have the mechanical aperture lever that the Pentax AF adapter is said to need?
It isn't the lever it needs, but a conductive metal base.
07-27-2018, 02:23 PM   #21
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QuoteOriginally posted by vonBaloney Quote
Originally posted by BobSpot QuoteDo your Nikon adapted lenses have the mechanical aperture lever that the Pentax AF adapter is said to need?It isn't the lever it needs, but a conductive metal base.
Thanks.
So any lens any brand like Zeiss or Takumar with M42 adapter will both focus and adjust the aperture or just record the aperture that is set manually ? What is causing the sigma lens problem have they a plastic non-conductive base?
07-27-2018, 04:53 PM - 1 Like   #22
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QuoteOriginally posted by BobSpot Quote
Thanks.
So any lens any brand like Zeiss or Takumar with M42 adapter will both focus and adjust the aperture or just record the aperture that is set manually ? What is causing the sigma lens problem have they a plastic non-conductive base?
I haven't tried it much with m42 lenses -- pretty sure that you'd have to use foil or copper tape (or maybe the right adapter) to create the conductivity and get it to work. I can think of one case where I know I had to do that (Tair 300mm f/4.5 m42 with adapter), but have generally only used it with K lenses. Any K mount Pentax-branded lens should work fine, but there are a few oddball third-party lenses with non-conductive mounts.

And it won't adjust aperture (automatically) on any lens except K mount lenses with 'A' contacts. (So any lens will work essentially the same way it does without it in terms of setting aperture.)

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