[Mikeyotaku44]

Hi
If anyone can help here it's appreciated.
I have a Pentax pz-20 film camera. The lens mount is type KAF 2. I'm wondering if it's possible to buy a teleconverter (such as a 2x magnification) that would allow me to effectively double the focal length of my lens.
If such a teleconverter exists, it would be the cheaper option rather than buying a new lens with double the focal length.

Thanks
Mike

[BigMackCam]

Hi Mike, welcome to the forums

There are various tele-converters that exist, usually in 1.4x (or sometimes 1.5x) and 2x magnification. However, most 2x converters - even the good ones - tend to soften detail and reduce contrast / micro-contrast so much that they're of fairly limited use. Tele-converters also result in a loss of light. A 1.4x model will lose one stop of light, while a 2x model will lose two stops. So, if you're shooting with a fast f/2.8 constant aperture zoom, its light gathering ability will be equivalent to an f/4 lens if fitted to a 1.4x converter, and an f/5.6 lens on a 2x converter. Slower lenses can become quite impractical on a 2x converter, for obvious reasons...

Which lens or lenses were you hoping to use with a tele-converter?

[Mikeyotaku44]

Hi Mac

Thanks for your reply.
The lens I am using is: tamron af aspherical xr ld 28-300mm 3.5-6.3 macro. (If you copy and paste into google you'll see my lens)
So, this is a lens that ranges in focal length from 28mm to 300mm, and as it is adjusted towards the 300mm zoom position the aperture closes down (from initially being 3.5, at 28mm, to 6.3, at 300mm).

I have half stops on my Pentax. Between 5.6 and 8 is 6.7, so I assume the 6.3 is a one third stop? So a 2x teleconverter would push the aperture to just smaller than f11?

In regards to light, usually I am shooting this type of shot in daylight with reasonably and/or with high speed film so light is probably okay.

The issue I hadn't considered was the effect on clarity / contrast. Thanks for pointing this out. I was initially wondering if such teleconverter betters existed. It seems they do but that the next consideration would be which is best in reproducing clarity / contrast.
Please let me know what you think from the lens I am using.
Thanks, Mike.

[Kerrowdown]

Welcome to the forum, I tend to crop in rather than use an extender these days.

[BigMackCam]

The lens I am using is: tamron af aspherical xr ld 28-300mm 3.5-6.3 macro. (If you copy and paste into google you'll see my lens)

OK, I have the Minolta AF-mount version of this lens, that I mainly use on an old Minolta film camera, but occasionally on my Sony digital bodies too. It's a useful and convenient lens, but - like many so-called "superzoom" designs - not exactly the sharpest bit of glass... especially at the long end. My first concern, then, would be adding a tele-converter to what is already a lens of optical compromises. I suspect it would produce quite sub-par image quality.

I assume the 6.3 is a one third stop? So a 2x teleconverter would push the aperture to just smaller than f11?

Not quite; two stops down from f/6.3 is f/13. In addition, because the focal length would be doubled from 300 to 600mm, you'd need a shutter speed of at least 1/600s (maybe even a little higher, for good measure) for hand-held shots, to avoid the effects of camera shake. Shooting 1/600s @ f/13 is going to require fast film and/or extremely good light. You'd be limited to the sunniest of days, I think.

If you're set on trying a tele-converter, I'd strongly recommend getting a good quality 1.4x or 1.5x model, and moving a bit closer to your subject. The degradation of IQ won't be so pronounced, you'll only lose one stop of light instead of two, and you'll be able to use a slightly lower shutter speed.

Have a read through some of the reviews here: Miscellaneous Lenses for Pentax: Teleconverters (All Brands) - Pentax Lens Review Database

I've heard very good things about the Tamron 1.4x Pz-AF and fairly good reports on the Kenko 1.5x Pz-AF converters, so you might want to look into those. I don't have personal experience of them, but I'm sure there will be other members here who can advise.

I don't know of any 2x converters that have good reputations. I own two old Russian M42 2x converters, purely because they're relevant to my lens collection, but I never actively use them

[Mikeyotaku44]

Hi again Mac

Thanks very much for a very comprehensive and helpful reply on the original query. It's a good welcome for me to the forum.

I'll follow up based on this useful advice.
Michael

[BigMackCam]

Thanks very much for a very comprehensive and helpful reply on the original query. It's a good welcome for me to the forum.

I'll follow up based on this useful advice.

You're very welcome

I've moved this thread into our "Pentax SLR Lens Discussion" forum, as it'll have more visibility there and is a more appropriate forum.

Let us know how you get on?