Link Posted 25/04/2007 - 23:30
I have just received my new lens, and could not wait to use it. But have had a few problems. I hope someone could explain what I may be going wrong.
I fitted it on the camera and then my pentax 50-200 zoom lens on manual it works fine.

Switch to Auto and then the problems start, focuses fine at say 50 to 100 but when I use it at full zoom the focus does not seem to lock on the the subject just keeps trying to re-focus again. Am I doing something wrong??

One other question I would like to buy a SMC Pentax - F 1.7x AF adapter. Any Idea's what I would have to pay for one of these please.


Link Posted 25/04/2007 - 23:42
The Pentax AF system is rated to work with lenses with a maximum aperture of F5.6 or faster.

The 2x converter doubles the focal length, and therefore the F number (since the focal ratio is just focal length / aperture diameter).

This means that the combination of your new converter and the 50-200 is a 100-400 F8-11

It's already outside the advertised capabilities of the AF system at 50 (x2=100)mm, but unlike some companies the Pentax AF system is good for more than it says it is and works anyway. F11 is just too much to ask though.


Link Posted 25/04/2007 - 23:44
I'm not sure I understand the problem. Do you have a new 50-200mm lens, or a new Tamron converter?

What is fitted to the camera?

Peter E Smith

My flickr Photostream


Link Posted 26/04/2007 - 09:07
Thanks Mongoose that makes it a lot clearer now. So will have to stick to manual on those long zoom shots then.

I'm not sure I understand the problem. Do you have a new 50-200mm lens, or a new Tamron converter?

What is fitted to the camera?

I have a new Tamron Converter and the converter and the 50-200 lens is all fitted to the camera.


Link Posted 26/04/2007 - 16:48
Yes Mongoose is right, at F11 you'll need an extremely bright and contrasty subject for the AF system to get its teeth into.
Also, as you approach the extremes of the focal range of zoom lenses the image quality tends to drop off a bit and adding a converter makes it even worse. Even with good quality lenses and converters such as yours, the image might be just a little too soft for the camera to focus on - you might manage a shot of a zebra on a bright sunny day though!

I have exactly the same situation with my AF 1.7x converter and F 35-135 3.5/4.5 (nice but not one of Pentax's sharpest lenses), with average lighting/contrast conditions, AF is a bit 'hit and miss' at 135mm.

When I bought my adapter new over fifteen years ago it was about 80 but they're hard to find nowadays and likely to cost a fair bit more.
When fitted to the Pentax AF 1.7x converter, you focus the lens manually (you normally just set it to infinity) and the auto-focusing is done within the converter - so it's real purpose is to provide AF capabilities to MF lenses!
Die my dear doctor, that's the last thing I shall do!


Link Posted 26/04/2007 - 18:29
I was taking moonshots through a telescope at 2000mm F10 the other night and was surprised when the focus assist started working. Still it is a pretty good telescope and it was a very clear night. That's the highest F stop I've seen K10D's AF system consent to work.
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