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08-01-2013, 07:12 AM   #1
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Lens choice questions

I have a Pentax K-x digital. Pentax lenses 18-55, 50-200, and a Sigma 70-300. I need a longer lens for some nature/wildlife shots but am not able to get a 400 or 500mm. I was thinking of a 2x converter. how well do they work, do they really double the focal length ? Is there any problems with picture quality, auto focus, compatibility, etc.? Thanks for any info or suggestions.

08-01-2013, 08:11 AM   #2
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I have both a 400mm (quantaray) and 500mm (5-star) fully manual lenses, and both are easy to use on my KX. Manual settings are not that difficult to estimate, and if you shoot RAW you will find a wide latitude of acceptable exposures. You will want to use a shutter speed of at least 1/500, ISO of 400 and f8. Take a test shot and review on the LCD and go from there. Use a tripod. The 400mm Quantaray cost $9.95 on ebay, plus $6.00 postage, and came with a Pentax k-mount adapter. The attached sample is from the Quantaray
08-01-2013, 08:16 AM   #3
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Yes, yes, yes, and no (with some caveats).

They really do double the effective focal length (if you're using a "2x" converter--they also make 1.4x, 1.5x, 1.7x, and 3x converters). Beware they also reduce the amount of light you get. A 2x teleconverter loses you 2 stops. A 300mm f/5.6 becomes a 600mm f/11!

The image quality goes down. Even with a really nice expensive teleconverter. Cheap ones are worse. That said, the images are still usable if you're not a pro getting paid tons of money.

Whether or not AF will work depends on the exact teleconverter/lens combination. First, if you want AF, you'll need an AF compatible teleconverter (they're more expensive). Second, you'll need to get enough light to the AF sensors. Most cameras stop working with AF somewhere around F/8. This means that you can use an F/4 lens with a teleconverter less than 2x (like a 1.4x or 1.7x), but you'll need an expensive f/2.8 lens to use AF with a 2x (or more) teleconveter.

Since you're using a Pentax camera, you can use any K-mount teleconverter. If it was originally designed for Ricoh, you may want to watch out for the infamous Ricoh pin (if teleconverters would even have it). Other than that, you should be good. I use an old Sears one (strictly mechanical, no electrical contacts). Mechanical 2x TCs can be had for about $10-20. AF ones are a LOT more expensive, around $100 unless you catch a good deal. These are all "poor quality" ones, optically. You'll need to spend even more if you want a "good" one. Oh, you'll also need to watch out for TCs that are made for a specific lens or a specific series of lenses. They won't work with just anything. Usually these are expensive, so if you're on a budget, they probably won't be a problem.

To read a more professional opinion on TC's, try this article on LensRentals: - Teleconverters 101

Since you're going to lose autofocus anyway, why not get an old manual focus 500mm lens? You can even get a mechanical TC as well, for 1000mm equivalent.

For example, KEH charges $70 for a 500mm manual lens (manual focus and manual aperture).

KEH TC's for Pentax K-mount (manual focus).
Pentax Manual Focus Lens Converters -

KEH TC's for Pentax K-mount (autofocus)
Pentax Autofocus Lens Converters -
08-01-2013, 08:20 AM   #4
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Hey there. When going above 300mm you will either have a cheap, but very bad, lens, or a good, but very expensive lens.
TCs are usually very bad, unless you get a very high quality one and use it on a high quality lens. 2x TCs are an extreme and are the worst (well, something higher than 2x would be even worse). People often use 1.4X or 1.7X TC instead. The problem with TC is that they also make the image darker, they effectively multiply the aperture. So if you have a dark lens (like, f5.6 wide open at 300mm), it will get even darker, which means that AF won't work and you will need to use very high ISO. Keep in mind that at that focal length, you also need to use a fast shutter speed to avoid motion blur, so the image is even darker. Of course, you can use a tripod, but wildlife generally still demands some speed, because animals twitch and move quickly. At that point, its better to shoot at 300mm and crop the photos - you can literally get better quality.
I would instead recommend trying to find a good tele prime, something faster than f5. And then maybe a TC if you still need it. Or a Sigma zoom that goes above 300mm, these can be quite good.
You can even try one of those mirror lenses or the cheapo 500mm lenses (that look like telescopes, Samayang makes some), but these are generally pretty bad. Putting on a tight hood and being very patient can help, though.

08-01-2013, 09:49 AM   #5
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In almost any circumstance I would rather crop a photo from a good lens than use a teleconverter. If you want longer, find a used MF 400 as the results most likely will be better than a TC with less hassle.
08-01-2013, 10:30 AM   #6
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A good 1.4x TC will lose very little IQ. I also had good luck with the Promaster 1.7x TC and the DA55-300. If you get the Pentax 1.7x TC and a manual prime, it will give you some Autofocus Capability. Many people have used quality manual Primes and that TC with success. What is your budget? Sigma and Tokina made 400mm 5.6 primes for Pentax, with AF. They are pretty rare in the mount, however, and that makes them hard to find and pricey. You could also buy an older version of the Bigma (Sigma XX-500mm).
08-02-2013, 09:05 AM   #7
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thanks to all for the info. A lot to sift thru. I think I will look for a longer prime lens to use, and stay away from the teleconverter if I can. Along with my 300 zoom I should do ok for a while.
08-05-2013, 02:59 PM   #8
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Someone is selling a Tokina 400mm/5.6, the autofocus version, just in case you are interested.


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