Pentax 300/2,8 and teleconverters

PePe · 02-10-2008, 03:59 AM

I am looking for a replacement to my old faithful Tamron MF 300/2,8, which served me well for a decade or so.
The Pentax FA* 300/2,8 was discontinued some time ago. However, there are some available every now and then. As discussed elsewhere on this forum, the current market prices seem to be ... err.. horrendous! To say it least!
The other concern I have is teleconverters. Possibility of using TC´s without sacrificing AF is one of the fundamental benefits of this lens concept. To my knowledge Pentax only offered MF teleconverters, but no dedicated AF teleconverters for these lenses. There are third-party TC´s available from a couple of suppliers, Sigma, Tamron,Kenco etc. Some other manufacturers advertise their teleconverters to be dedicated to certain lenses or lense families only.
I have no idea about how these third-party AF TC´s work with this Pentax lens. I quess the mount probably works OK, but my concern is really about optical performance. There might be some people on the forum who have been able to check this. I would really appreciate your comments before going into something I might have to regret !

rvannatta · 02-10-2008, 06:04 PM

Originally Posted by PePe

I am looking for a replacement to my old faithful Tamron MF 300/2,8, which served me well for a decade or so.
The Pentax FA* 300/2,8 was discontinued some time ago. However, there are some available every now and then. As discussed elsewhere on this forum, the current market prices seem to be ... err.. horrendous! To say it least!
The other concern I have is teleconverters. Possibility of using TC´s without sacrificing AF is one of the fundamental benefits of this lens concept. To my knowledge Pentax only offered MF teleconverters, but no dedicated AF teleconverters for these lenses. There are third-party TC´s available from a couple of suppliers, Sigma, Tamron,Kenco etc. Some other manufacturers advertise their teleconverters to be dedicated to certain lenses or lense families only.
I have no idea about how these third-party AF TC´s work with this Pentax lens. I quess the mount probably works OK, but my concern is really about optical performance. There might be some people on the forum who have been able to check this. I would really appreciate your comments before going into something I might have to regret !

Mechanically there is no issue with an AF converter.---You just need something with a Cam that connects through. None of the Pentax ones do that. Pentax FA* f2.8's are very scarce even compared to A* f2.8s of which they seem to be quite a few.

One possibility: the currrent pentax roadmap includes an SDM compatible converter it it may also have a drive shaft for non SDM lenses. I"ll doubt however if you will ever get a converter
for those long lenses better than the dedicated converters made only for them which is effectively what the 1.4x-L and 2x-L converters are. they are ONLY useful witih a small number of extreme telephoto pentax lenses because of their optical configuration.

IF you note the optics that stick up the back of the mentioned lenses you will understand why they don't work on other lenses and one must assume they made them that way to gain optical advantage, and not just to be different.

GaryML · 02-10-2008, 07:00 PM

Originally Posted by rvannatta

Mechanically there is no issue with an AF converter.---You just need something with a Cam that connects through. None of the Pentax ones do that.

...

One possibility: the currrent pentax roadmap includes an SDM compatible converter it it may also have a drive shaft for non SDM lenses. I"ll doubt however if you will ever get a converter
for those long lenses better than the dedicated converters made only for them which is effectively what the 1.4x-L and 2x-L converters are.

There is a mechanical issue with teleconverters using the screw drive system. By adding two more couplings and additional bearings to support the shaft, there is a lot more mechanical play and friction in the system. This makes the AF hunt back and forth a lot more, and makes the AF slower and noisier. I had a Tamron 1.4X AF converter for a short while, and I used it mainly with a Pentax FA 135mm f2.8 lens. It did work and I could get correct AF, but the AF performance was very slow and noisy. Also, it added so much drag that manual focusing was almost impossible (but that isn't an issue with QuickShift lenses).

I hope that the upcoming SDM converter does not have the screwdriver shaft. That would give terrible performance. One benefit of SDM and the electrical contacts for AF is that there is no loss of AF performance with a tele-converter (other than the loss of light for the AF sensor).

And I agree that dedicated converters generally have much lkess impact on image quality than generic converters designed tyo work with any lens.