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02-26-2013, 09:34 PM   #1
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How to use a teleconverter

I have just seen a tele converter at a good price, so my question is, is it possible to use one with a K30 kit lens and if so how would I set the aperture

Cheers Andrew

02-26-2013, 10:18 PM   #2
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Andrew, that is not an entirely simple question. The short answer is "don't bother".

A TC uses up light, a 2x TC uses 2 full stops and a 1.4x at least one stop. On a fast (f/2.8) lens this is tolerable. On the kit lens it simply is not. The kit lens at 55mm is f/5.6 take away two stops of light and you get f/11. Even wide open it will be close to f/8 and that means the AF will not work reliably (or at all).

The other issue is the degradation in image quality. Many tests posted on this forum show that there is little difference in the quality of an image shot with a TC versus one shot without and then cropped to make the subject the same size. Adding any additional glass to the light path reduces the image quality, if there is a benefit to that then it can be worth it but in this case it is not likely to do so.

If you have a fast telephoto (a 300mm f/2.8) then a TC can definitely have value. With the kit lens, please don't waste your money.

All of this assumes the TC is even compatible which in Pentax land is harder to say than it should be. Not all TC's work with all lenses. Some have protruding lens elements that can hit the rear element of your lens, some only work with screwdrive lenses and not SDM lenses. Many of the cheap ones listed are actually M42 mount and will not work with your camera without an adapter.

Your question re setting the aperture cannot be answered without more details about the TC. If it is manual then you need to set the aperture on the lens, unfortunately your lens does not have an aperture ring so you cannot. If the TC has contacts that pass through the lens info then the aperture is set on the camera.

Post a link to the one you are interested in and I can tell more. But IMHO, it's not worth the effort with the kit lens.
03-02-2013, 07:36 PM   #3
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In my film days I used my teleconverter for only "gotta have" pictures as they made the image quite dark. But maybe this dimming could be compensated post-production? We'll see.

I am going on an upcoming trip to England, and to keep weight down, I'm taking the 18-35 kit WR lens, and an old favourite, a 135mm Pentax-M. And, a 2x Teleconverter. So if I "gotta have" that shot to see something really zoomy, like a bird perched on Big Ben or something, I will be able to have that memory. The 135 and the 2x Teleconverter will (kind of) give me all the telephoto I should need, hopefully.

But in my film days experience, Teleconverters are a compromise...usually there's a bit of a drop in quality and exposure...but I'm taking it along as it's light and a good experiment, and if it doesn't work, or the opportunity doesn't present itself that I have to have that much "zoom", so be it, but I have it just in case.

Anybody use a 2x teleconverter and fixed the image and the light loss post-production?
03-03-2013, 09:55 AM   #4
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QuoteOriginally posted by Reliant K1000 Quote
Anybody use a 2x teleconverter and fixed the image and the light loss post-production?
You should not have 'light loss' in the image if you adjust the exposure to be correct. But the correct exposure will be about two stops different than the same image without the TC. Depending on the ability of the TC (whether it is all manual or has auto-exposure contacts) you might have to adjust manually or use the green button to get a good meter reading. But you should be able to get the correct exposure.

The problem is that you are now shooting two stops slower in shutter speed, or have to open the aperture 2 stops, or have raise the ISO two stops. As you note the image quality degradation inherent in using a TC is immaterial if all you are looking for is a 'memory' shot.

If the image does turn out too dark, by shooting in RAW you can easily recover 2 stops in Lightroom without noticeable degradation. You can go as high as 4 stops but you will be able to see artifacts at that extreme.

03-04-2013, 01:01 PM   #5
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OK - thanks jatrax. Good to know.
03-04-2013, 01:20 PM   #6
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QuoteOriginally posted by morgun Quote
I have just seen a tele converter at a good price, so my question is, is it possible to use one with a K30 kit lens and if so how would I set the aperture

Cheers Andrew
The Teleconverter will need to have the pass through contacts to work with a modern (DA) kit lens to be able to set the aperture.
If it does not have these (the row of buttons on the base of the lens) then you cannot use the TC with your kit lens.

Actual performance may be less than desired as explained by jatrax.
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