Originally posted by jpzk Can you stack:
1. TC's of different brands, i.e; A Sigma-for-Pentax TC 1.4x plus a Tamron 1.4x
2. Different types of TC's, i.e: a 1.4x Tamron plus a Pentax A FA 1.7x adapter
JP
Hi Jacques,
I do all the time. In AF TCs, I have a Tamron F 1.4x AF PZ MC4, a Sigma EX 1.4x APO, and a couple of Pentax F 1.7x AFAs.
I usually carry them all, and combine them if it's to my advantage. Order is sometimes important. The Sigma has a protruding front element, so you cannot it must be the one that attaches to the lens in any combination (considering that the lens has a recessed rear element). Any additional TCs (except for another Sigma) can be attached to the Sigma. With 2 1.4x TCs, you get the equivalent of a 2x in both magnification and light loss (2 stops). With a 1.7x + a 1.4x, you get the equivalent of 2.38x with @ 2.4 stops of light loss.
A few other thoughts about TCs:
I bought the Sigma because of build quality. The Tamron is probably a little better optically, but the Sigma feels more solid with its aluminum barrel, and I just trust it more when using a heavy lens like a 300/2.8. The Tamron works as well, it just doesn't allow as much confidence for me.
I like two stacked 1.4x TCs better than any of the 2x TCs I've tried, but I haven't tried the two best for the K mount, the Sigma EX 2x APO or the Pentax A Rear Converter-L which is MF, but probably easily the best optic available for the K mount of this type. Remember that you can't stack 2 Sigma TCs of any type because of the protruding front element of the TCs. The only 2x TCs I have are all MF -- the Vivitar "A" 2x MFTC, Pentax A 2x Rear Converter -S, and a Tamron SP 01F Adaptall 2, all considered very good, but IMO, they all fall a little short in optical performance, even with premium lenses.
Two P 1.7x AFAs can be stacked, but you must first focus the AFA attached to the lens to its infinity setting, then mount the second AFA. The AF function of the AFA can be disabled by shorting the Data pin (farthest away from the red dot on the mount) if one wants to use it with Focus Trap. The same is true of any AF lens. Aperture value is still transmitted, and full AE functionality is retained. You must short the pin, not insulate it, so tape is not an option if you want to retain AE capability.
SDM can definitely a (?) factor in TC use. I've found that my DA*50-135 just hunts and will not lock focus with my Tamron PZ compatible 1.4x. Others have said that this can work, so I'll take their word for it, but in my mind this only raises the compatibility rating to questionable. It's probably sample variation in the lens, but there's no way for me to be sure, so I'd still have to question whether stacked PZ TCs would work with SDM on the DA*300/4, even if a single TC works fine. AFAIK, Sigma claimed to have an HSM TC in the works, but I haven't seen it, and the APO and APO DG TCs don't have SDM/HSM contacts, so they can't AF with an SDM lens.
All the existing AF TCs, with the exception of the AFA, still transmit lens ID info to the body, so when used with AF lenses, you cannot correct the SR setting for the increased effective FL. With a 1.4x TC, this might not make enough of a difference to effect usability, but with 2x (stacked 1.4xs) and the resultant 2stop light gathering difference, this could cause some significant challenges with handholdability.
This is a significant difference from the functionality of the AFA, IMO. It does not relay lens ID information, so you can enter an appropriate FL when using it. Add that to its very good optical quality, Quick Shift and focus limiting functionality, and focusing speed, and IMO, you get the best TC to use for the K mount, for just about any lens. All of my SR bodies remember the AFA as a distinct lens, so focus corrections and SR FL information, once set, are remembered and used by the body -- in other words, if you set either focus correction or SR FL for the AFA and one lens, then change the lens, it will continue to use the previously set info which will probably be inappropriate for the second lens.
The last is a caveat -- Sample variation in lenses and TCs can all contribute to actual usability, either in AF operation or IQ. The combos that I use and like may not work as well with different samples. I will say that all 4 AFAs that I've had worked identically, AFAIC, so my conclusion is that there's very little sample variation in these. All of my AFAs have been older ones.
Scott