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06-26-2008, 03:44 AM   #1
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2x Tele and auto focus.........HELP!... now got it to work...

I have bought a Jessops 2x teleconverter for my K100D....

But I cant use it because the auto focus goes nuts!
yes its an auto focus tele!

but the cam is set to auto on the wheel on top and it sometimes asks for the focal length.

What should I do?
What settings SHOULD I be using?

I am a complete novice here so please not too complex....

Many thanks guys/gals, I know you will sort this....


Last edited by PentaxPZ; 06-30-2008 at 06:17 AM.
06-26-2008, 04:13 AM   #2
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Hi,

Which lens are you using the TC with ?

simon
06-26-2008, 04:58 AM   #3
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Keep total apature bigger than f6.7

my experience with pentax AF systems is that for autofocus to work, the total maximum apature needs to be larger than f6.7

This means that a 2x TC simply won't work on any lens F4 or smaller. Some woill report success on F4 but it requires bright daylight.

I use a 2x TC quite successfully, but on a Sigma 70-200 F2.8. I tried once coupling both my 1.4x and 2x together, (resulting in maximum apature of F8) and my K10D struggled.
06-26-2008, 05:15 AM   #4
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2x TC decreases light by 2 stops, it means that AF is receiving 2 stops less light, what in turn means that in other than very good lighting condition AF may not work.
Solution? - use "brighter" lens (with lower F-stop - number), or focus manually.

06-26-2008, 06:30 AM   #5
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QuoteOriginally posted by piotro Quote
2x TC decreases light by 2 stops, it means that AF is receiving 2 stops less light, what in turn means that in other than very good lighting condition AF may not work.
Solution? - use "brighter" lens (with lower F-stop - number), or focus manually.
Even in bright light it won't necessarily work - If you read up on autofocus systems, it is not a function of the available light, but of other effects of the lens's F number (similarly to how DOF changes with F number). It's the same phenomenon that causes split prism focusing screens to go VERY dark with small apertures, far darker than the rest of the screen.
06-26-2008, 07:16 AM   #6
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I just tried to give simple answer to a simple query , there the simplifications come from.
But what you mentioned is another, quite interesting thingy.
06-26-2008, 09:36 AM   #7
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I have a 2x conveter that i've tried with the tamron 70-300,AF works up to 200mm above that it will hunt.The missus has a sigma 50-200 that if i remember rightly AF's fine both these examples are of course in good lighting.
06-27-2008, 04:56 AM   #8
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Original Poster
thanks guys.....

Well I bought a 70-300mm KA (not auto focus) that works great!
Plus it works on the Mrs pentax ME too.....

Is there a teleconverter that works on 300mm or more?
Or do we need to buy a bigger lens?

That question is mainly on my brothers behalf......
He loves to take pictures or birds, you see.
A tele is cheaper than a 600mm zoom lens.
300mm is good for me, I mean I do the odd wildlife photo but its mainly fairly close beach shots (kite surfing, etc) and motor cars (classic car shows)...

06-27-2008, 06:02 AM   #9
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The problem with teleconverters is that they eat stops of light
Think about it - A 300mm f/5.6 more or less has a front element of 54mm in diameter. Slap a 2x teleconvert in between the lens and body and you have a 600mm lens! yay! Great, right?
Well, wait......The front element didn't get bigger, it's still 54mm Doing the math, you end up with a 600mm f/11 lens. That's REALLY slow. A 1.4x teleconverter only eats ONE stop of light, so you could get a 420mm f/8 lens, a little better but still quite dim.

I had a manual 2x teleconverter I used to use with my Pentax FA 100-300mm f/4-5.6 and it turned the lens into a piece of cr*p! No autofocus and the quality just went to mush, not to mention having to deal with the f/11 @ 600mm.

Now if you had a 300mm f/2.8, you could use a 2x to get 600mm f/5.6, but we're talking thousands of dollars here.
06-27-2008, 07:31 AM   #10
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So the answer is just buy bigger lens?

My head hurts now

if its so bad, who the hell invented the teleconverters?
they need shooting!
06-27-2008, 10:55 AM   #11
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QuoteOriginally posted by PentaxPZ Quote
So the answer is just buy bigger lens?

My head hurts now

if its so bad, who the hell invented the teleconverters?
they need shooting!
The answer is to buy a faster lens (which will be bigger).

In general, the rule is that above f/6.3 (most say 5.6, but there are f/6.3 lenses with reliable AF), autofocus becomes unreliable. Sometimes AF will sort-of work at f/8, but not reliably and only in bright light. At f/11, AF is almost guaranteed to be very unreliable if it works at all.

A teleconverter will take a lens's aperture and multiply that value by the value of the teleconverter. So a f/5.6 lens with a 1.4x TC will become an f/8 lens, and with a 2x TC it becomes an f/11 lens.

The end result is that officially, 2x TCs are only intended to be used with very fast (f/2.8 or lower) lenses. Anything slower and manufacturers won't officially support it. 1.4x TCs are only intended to be used with moderately fast (f/4 or lower) lenses.

TCs are really intended for photographers who spend MOST of their time shooting at lower focal lengths with a fast lens, and occasionally want to go to longer FLs at the expense of speed. For example, owners of a 70-200 f/2.8 zoom might occasionally want it to be a 140-400 f/5.6 zoom with a 2x TC, or owners of a 100-300 f/4 zoom might want a 140-440ish f/5.6 zoom with a 1.4x TC.

Unfortunately all of the 70-300 lenses out there are f/5.6 at the long end - they're almost at the practical limits of AF systems already. In bright light you can push them to f/8 with a 1.4x TC occasionally, but 600mm f/11 just isn't going to work with AF.
06-27-2008, 01:13 PM   #12
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Thanks entropy, thats much clearer!

I see your point now yes, so ditch the TC and go buy a bigger, faster lens....

300 is good for me but my brother wants a bit bigger, he does barn owl photos.
but they are at a distance 300 zoom just about enough, so 400/500 is what he wants....
he dont care about the bottom end too much... so 200-400? 200-500? if such exist!

many thanks for helping me understand guys...
06-27-2008, 01:55 PM   #13
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Of course the other (read less costly)solution is to focus by eye when using the x2 extender on a slower lens. Takes a bit of practice to get used to going this way but the results are much more consistent than trying to trust the AF to do something other than wind in and out hunting for the shot.
KennJ
06-29-2008, 01:38 PM   #14
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QuoteOriginally posted by PentaxPZ Quote
Thanks entropy, thats much clearer!

I see your point now yes, so ditch the TC and go buy a bigger, faster lens....

300 is good for me but my brother wants a bit bigger, he does barn owl photos.
but they are at a distance 300 zoom just about enough, so 400/500 is what he wants....
he dont care about the bottom end too much... so 200-400? 200-500? if such exist!

many thanks for helping me understand guys...
Unfortunately, once you get beyond 300mm, the options get much heavier and more expensive.

Sigma 100-300 f/4 + a 1.4x TC will still AF properly, and it is a good lens, BUT:
The Sigma 50-500 f/4-6.3 is the same price and will give you some extra range without needing a TC. The "Bigma", as it is known, is quite popular.

Unfortunately, while they are about the same price, that price is high - both are around $100.

I think Sigma has a 135-400 for Pentax mount too, but it is not very popular.
06-29-2008, 01:59 PM   #15
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Well I bought a 70-300mm manual focus lens...
I got that to work quite well with a TC...
Look...

First shot taken at 300mm, f8.9 and 1/180
Second shot taken at 300mm + 2x TC, f2.4 and 1/90
Third taken at 300mm + 2x TC + 2x TC, f2.4 and 1/90
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