Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
10-16-2016, 11:40 AM   #16
Senior Moderator
Loyal Site Supporter
Loyal Site Supporter
BigMackCam's Avatar

Join Date: Mar 2010
Location: North-East of England
Posts: 13,774
Just an FYI - even Sigma's own 1.4x TC shows as MF-only on their compatibility chart. Whether that means it won't ever work in AF, or that it needs a brightly-lit, high contrast subject for AF to work, I don't know - but if this is the case with Sigma's own model, I wouldn't hold out a lot of hope with the Pentax TC...

10-16-2016, 12:29 PM   #17
Senior Member




Join Date: Oct 2016
Location: New York City
Posts: 100
QuoteOriginally posted by Sterby Quote
This is what I am thinking too. If it was a slower-than-F4 problem then it would lock on in liveview but the AF just drives from one end to another, only locks on when I pull back the focus by hand at an appropriate moment (kind of manually altering the PID loop of the AF drive). In OVF phase detect the focus is just too quick to step in like that.
My theory is the TC doesn't slow down the AF which it should and does on all my other lenses. All my other lenses are screwdrive though. Again, natively it works just fine, quick and snappy both in OVF and liveview.

I bet if the lens communication was okay the TC would step in and slow the AF accordingly.
It sounds pretty much like a delay between communication cause by the TC.
Assuming there will always be a delay between lens and body when the TC is put on. Some lens can work with the delay and AF properly. Some are not or too fast.
But I really have no idea why the delay doesn't apply on lens doesn't work. Maybe a bypass?
10-16-2016, 01:24 PM   #18
Pentaxian
normhead's Avatar

Join Date: Jun 2007
Location: Near Algonquin Park
Photos: Gallery | Albums
Posts: 33,658
QuoteOriginally posted by Sterby Quote
I wouldn't dismiss it so easily, today's AF sensors can work in -3EV, f/11 shouldn't be a problem (though reduced to only center point) let alone near f/8. Also the Sigma 17-50 was reported above having the same problem and that is an f/2.8 lens.
I have never seen a modern AF sensor work on a lens with a wide open aperture of ƒ11, have you? I not a big fan of what "should" be true.

I'm going with the "some lenses just aren't designed to support a TC theory,

Just leave it at "the TC doesn't work with some lenses" and "TCs are not really designed to be use with wide angle lenses." Many manufacturers even tell you which lenses their TCs can be used with, I know Sigma does. The Sigma 1.5 and 2x TCs work with a very specific list.

What I'm saying is, if a specific TC doesn't work with a specific lens, accept it and get on with your life. There most likely isn't a fix.
10-16-2016, 07:01 PM   #19
Loyal Site Supporter
Loyal Site Supporter
clackers's Avatar

Join Date: Jul 2013
Location: Melbourne
Photos: Albums
Posts: 13,480
QuoteOriginally posted by Sterby Quote
I wouldn't dismiss it so easily, today's AF sensors can work in -3EV, f/11 shouldn't be a problem (though reduced to only center point) let alone near f/8.

I'm not sure you understand the issue, Sterby.


Light level is nothing to do with it, geometry is.


The light has to be split wide enough to register on two different sensor rows in the AF module. f5.6 is the normal rating for this capability, so this is why even the slowest consumer lenses are capable of this.


When you shoot at f16, remember that to focus first of all the lens opens up to wide open, then closes down again, otherwise the beams are so parallel the module will struggle.

10-17-2016, 06:22 AM   #20
Site Supporter
Site Supporter




Join Date: Apr 2012
Photos: Gallery
Posts: 65
teleconver on 50-500

i have non 50-500 lens. i have tamron and kenko teleconver. i manual focus to infinite. there is a thread
where a person uses a teleconver on non os 50-500
10-17-2016, 06:28 AM   #21
Pentaxian
normhead's Avatar

Join Date: Jun 2007
Location: Near Algonquin Park
Photos: Gallery | Albums
Posts: 33,658
I can MF any TC on any lens I own, I think this discussion is about getting the camera's AF system to work with it.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
1.4x, 1.4x tc focus, af, bigma, focus, focus issue, hd da 1.4x, k-mount, lens, pentax, pentax lens, sigma, slr lens, tc, teleconverter
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
For Sale - Sold: Sigma 50-500 DG OS HSM UserAccessDenied Sold Items 11 09-15-2015 05:35 PM
Part 2 - (K-3) Compare Sigma 50-500 OS to DA*300/HD-TC crewl1 Pentax SLR Lens Discussion 41 04-20-2015 09:46 PM
Comparing Sigma 50-500 OS to DA*300 w HD-TC crewl1 Pentax SLR Lens Discussion 19 06-23-2014 03:21 PM
Pentax DA*300 2.8 w/TC Tamron AF 1.4x or Sigma "Bigma" 50-500 OS HSM? RTogog Pentax SLR Lens Discussion 16 09-29-2010 09:00 PM
For Sale - Sold: FS: Bigma (Sigma 50-500) and 1.4x TC PaulAndAPentax Sold Items 20 02-25-2008 08:50 PM



All times are GMT -7. The time now is 08:26 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top