Originally posted by daacon Hi all
I was wondering what type of converter I could get to increase the range on 300mm ? What do this convertors cost and I assume they are Lens specific (meaning I need to look for one that will fir the Sigma?) - Can I order them online (very few Pentax dealers where I am - got all my supplies at London Drugs so far)
Also for sports photography (Son football) are there any downsides to using these convertors ? I have seen some pretty awsome short in the Gallery of the moon and I was wondering if these convertors are better suited for stills with a tri-pod ?
1. There is no free lunch: all teleconverters will reduce the quality of the image. The difference can range from slight to severe. It depends on the specific lens and the specific converter. The best results are with very high quality lenses and a high quality converter specifically matched to the lens. The worst is with lower quality lenses and lower quality converters designed to work with any lens. The higher the magnification, the greater the loss of image quality. With marginal combinations, you need to stop down several stops from the maximum for decent sharpness.
2. All converters cause a loss of light. A 1.4X will cause the loss of one stop of light, a 1.7X one and 1/2 stops, and a 2X will lose two stops. This will give a darker viewfinder image and slower, less reliable auto-focus. Generally, auto-focus is unreliable at slower than f5.6. Since your long lens is already at f5.6, even with a 1.4X converter you will lose reliable auto-focus, since the effective aperture will be f8. It might still AF in bright light, but it will be slow and will hunt. For sports action use where AF speed and a fast shutter speed are essential, it will be useless.
3. Sigma makes high quality, matched teleconverters (the 1.4X and 2X EX DG converters) but they are not compatible with your lens. Your only option will be a generic converter. There are many of these available new and used in both 1.4X and 2X models. There are no current Pentax converters, but the Converter A 1.4X-S and Converter A 2X-S are the best generic ones (the Pentax "L" converters are matched to certain lenses and will not work with your Sigma). The Pentax converters are auto-exposure, but manual focus only.
The only 1.7X converter that I am aware of for the Pentax mount is the (discontinued) Pentax F auto-focus adapter, which allows auto-focus of manual lenses and magnifies the image 1.7 times. These can be found used but the recent prices are quite high (over $300). It does work with auto-focus lenses, but your f5.6 lens will not give reliable AF. Pentax recommends f2.8 or faster.
4. Your Sigma is a decent consumer zoom but it is not a top quality lens and it is quite slow at f5.6. I would not have high expectations of good results with a teleconverter using that lens. Feel free to give it a try if you like. But I wouldn't want to shoot sports photos at f8 or f11, since the background will not be blurred enough and the shutter speed will be too slow.
If it was me, I would save my money and put it towards a longer lens. Something like a Sigma 135-400, 170-500, or 50-500 EX (the "Bigma") will be a more satisfactory solution.