PDAF with stacked teleconverters

kamisu · 09-17-2016, 04:18 PM

Hi Pentaxians,

I have two Kenko pz af 1.5x SHQ teleconverters and both work fine with 60-250 on K5.

The problem is when I stack them, where K5 can not focus using viewfinder (PDAF) unless the subject is far (~100 m at 250 mm). But using the Live View (CDAF) AF works well at any distance.

Is this normal?

Thanks.

MadMathMind · 09-17-2016, 04:39 PM

PDAF generally doesn't work with f/8, except with special sensors. Canon makes a big deal out of that in their marketing for the 5D Mark IV. It's not a matter of light. It's a matter of aperture doing I forget what but the sensors just don't work.

CDAF just moves the lens around until it finds a good spot so it doesn't care about anything except the amount of light.

Adam · 09-17-2016, 05:11 PM

Yup, this is normal. Even without TC's, a F8 lens would really struggle to focus.

09-17-2016, 04:18 PM	#1
kamisu Senior Member Join Date: Feb 2012 Location: Iowa Posts: 130	PDAF with stacked teleconverters Hi Pentaxians, I have two Kenko pz af 1.5x SHQ teleconverters and both work fine with 60-250 on K5. The problem is when I stack them, where K5 can not focus using viewfinder (PDAF) unless the subject is far (~100 m at 250 mm). But using the Live View (CDAF) AF works well at any distance. Is this normal? Thanks.

09-17-2016, 05:11 PM	#3
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,597	Yup, this is normal. Even without TC's, a F8 lens would really struggle to focus. Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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09-17-2016, 04:39 PM	#2
MadMathMind Veteran Member Join Date: Dec 2013 Location: Houston, TX Photos: Gallery Posts: 1,717	PDAF generally doesn't work with f/8, except with special sensors. Canon makes a big deal out of that in their marketing for the 5D Mark IV. It's not a matter of light. It's a matter of aperture doing I forget what but the sensors just don't work. CDAF just moves the lens around until it finds a good spot so it doesn't care about anything except the amount of light.