[mattv]

Just a question, but why does the Sigma 150-500 HSM not work with the Sigma 2X TC? is it the HSM or is it an issue with the mount? Just curious to see if I want to keep the zoom seeing as I also have the Sigma 300mm 2.8 and the TC.....is it worth the overlap. And do I really need a MF 1000mm F13 lens????

[cmohr]

The Sigma TC's don't have the SDM contacts in them to power the HSM in the Sigma lens. Only screwdrive.

You could try a Kenko or similar 1.5x PZ version, I know mine works with my DA SDM zooms and my 8-16 HSM, so would imagine it would work on the Bigma.

[Mehlsack]

But it could become to dark for the AF

[Lowell Goudge]

there are three issues with respect to sigma TCs.

one issue is mechanical, one electrical, the other is optical.

the mechanical issue is that sigma TCs have a somewhat proud or forward extending front element, that requires a circular recess in the rear of the lens roughly 35mm in diameter, and a minimum of 12mm deep.

Many lenses do not have this type of recessed rear element or rear barrel inner diameter, and the TC simply does not fit.

Other lenses , like the BIGMA (i;e sigma 50-500) have a focal length restriction, as at 50mm the rear element hits the TC and therefore there is a restricted focal length on the bigma.

the electrical issue is in 2 parts, first, sigma TCs for pentax are not HSM compatible, so if your lens is HSM forget it, you have MF only. Second, the TC feeds lens data unmodified, and therefore your aperture is not correctly reported, which is OK but can lead to the need for exposure compensation, and it also messes up P-TTL flash

the optical issue is that on any camera for the AF to work reliably you need a maximum aperture of F6.7 or larger, and therefore the AF does not work with either the 1.4x or your 2x TC (Or any other TC for that matter)

I am not sure whether the TC can be mounted to your lens, but if it can you are limited to a dark viewfinder to focus with manually

[SOldBear]

In terms of autofocus, the issues of TC for SDM lenses are much more than just electrical contacts and aperture. I don't think anyone has figured it out yet. Here are the symptoms I observe:

1. There is NO 2X TC that works with SDM lenses. Example: the Tamron 2X TC has electrical contacts, yet is does not work at all with any SDM/HSM lenses. For DA* 50-135 F/2.8, the effective aperture with the TC is F/5.6, still it doesn't work.

2. Even for 1.4X TC, autofocus is iffy at the long end (there is no reason to use a TC at the short end). On half-press of the shutter release, the autofocus keeps going back and forth as if it can't decide. If you release the half press and half press again, it MAY lock in focus. This is not due to aperture not large enough because the lenses works fine with Pentax 1.7X AFA. I've heard that the Tamron 1.4X TC works with DA 16-45 F/4, but I don't have the lens so can't confirm that myself.