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06-12-2008, 03:28 AM   #16
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QuoteOriginally posted by axl Quote
OK.
Any tips for GOOD 2x AF teleconverters?
I mean not just brand, but which ones specificaly are the good ones?
Thanx
AF converters for Pentax are rare, especially in the 2x variety. Lowel already mentioned the Sigma 2x Apo, which is a dedicated converter for some of the fast Sigma lenses (I sometimes - but rarely - use it with my Sigma 70-200/2.8 and it gives good results. You will find, that the Sigma TCs have a protruding front element, which prohibits the use on lenses with a rear element, that right on the K-mount plane. Otherwise most of the good Kenos (Pro...) are not available for PAF and the Pentax TCs are all without AF - though the Pentax converters are superb (apart from the old K6T).

Generally the TCs with 7 lens elements perform better than 4-element TCs. But I wouldn't bet on a good performance of all the cheap TCs you currently find on ebay. May be (but I can't confirm that) a Kenko 2x TC with just 4 elements is a better choice, than a no-name 7-elements model. - But I just saw, that BH lists a Kenko 7-element for Pentax for a moderate 134 USD (SLR Teleconverters | B&H Photo Video), which is not more expensive than the nonames. I would perhaps give it a try - or wait until Pentax releases its new TC sometime in the future (at least its anounced on the roadmap).

Ben

06-12-2008, 08:19 AM   #17
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Interesting thread & some very useful information...

having just purchased the new DA 55-300 I'm also wondering about a TC for those occassional times when I need the extra reach. I'm not really considering any current TCs but waiting further information on the new Pentax one, hopefully it made include "stabilisation info" too

simon
06-12-2008, 08:32 AM   #18
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QuoteOriginally posted by simonkit Quote
Interesting thread & some very useful information...

having just purchased the new DA 55-300 I'm also wondering about a TC for those occassional times when I need the extra reach. I'm not really considering any current TCs but waiting further information on the new Pentax one, hopefully it made include "stabilisation info" too

simon
You may find that there is no benefit with the DA55-300, and adding a TC. The reason is that it may be too slow especially with a 2X to autofocus properly.

My experience is that AF only seems to work on combined apatures of about f6.7 or less, otherwise it hunts excessively or does not focus at all except in the brightest of conditions. Some people may have different experience, and I invite them to comment.
06-12-2008, 09:08 AM   #19
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Just in the interest of probing the limits, there are many situations where even an ideal TC wouldn't outperform cropping in terms of sharpness. For a given shooting configuration, the wavefront coming out the back of the lens is capable of reproducing a limited number of resolvable spots (lens tests usually try to quantify this by measuring Modulation Transfer Function [MTF] or occasionally Point Spread Function [PSF]). There are fixed formulae for thresholding these, though imapct on image quality is somewhat subjective - how much can adjacent points overlap before they are 'blurred together'? Anyway, at best the teleconverter can only spread these out over a larger sensor area. So if the resolution of your lens in a particular configuration isn't better than your sensor resolution, then adding a teleconverter to that same lens configuration can't give you more resolution than skilled cropping and resizing.

06-12-2008, 09:25 AM   #20
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QuoteOriginally posted by nanobaud Quote
Just in the interest of probing the limits, there are many situations where even an ideal TC wouldn't outperform cropping in terms of sharpness. For a given shooting configuration, the wavefront coming out the back of the lens is capable of reproducing a limited number of resolvable spots (lens tests usually try to quantify this by measuring Modulation Transfer Function [MTF] or occasionally Point Spread Function [PSF]). There are fixed formulae for thresholding these, though imapct on image quality is somewhat subjective - how much can adjacent points overlap before they are 'blurred together'? Anyway, at best the teleconverter can only spread these out over a larger sensor area. So if the resolution of your lens in a particular configuration isn't better than your sensor resolution, then adding a teleconverter to that same lens configuration can't give you more resolution than skilled cropping and resizing.
I am not sure we have reached that point yet, at least I haven't with my sigma 70-200 F2.8 and 2X TC. I might have with my celestron C90 1000mm F11 however. I will play this summer and let you know.
06-12-2008, 12:21 PM   #21
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QuoteOriginally posted by Lowell Goudge Quote
You may find that there is no benefit with the DA55-300, and adding a TC. The reason is that it may be too slow especially with a 2X to autofocus properly.
Thanks, I'm in full agreement - I'm hoping the Pentax one is a 1.4x which I may just get away with ??

simon
06-13-2008, 03:57 PM   #22
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which one is better?
and would they work with Sigma 105 and DA40?
a)
Kenko Teleplus MC-7 DG 2x AF Teleconverter for Pentax Best Price in UK
b)
Sigma EX Apo TeleConverter 2X for Pentax best price sigma teleconverter uk
06-13-2008, 04:02 PM   #23
axl
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another candidate
Kenko Teleplus DG 1.5X MC Teleconverter For Pentax uk 84

PS: correct me if I'm wrong: with 2x TC coupled with Sigma 105, the min. focusing distance stays the same, but because of the TC the combo wouldn't produce 1:1 but bigger than lifesize magnification (maybe 2:1)?
right?!?

06-14-2008, 01:24 AM   #24
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Thanks everyone who took the time to weigh in.

I really didn't mean to “post & run”. For whatever reason, the forum software unsubscripted me from receiving email notifications on this one.

QuoteOriginally posted by Wheatfield Quote
Expanding slightly, getting into focal lengths past 250mm benefit greatly from using a tripod. Shake reduction does not mean shake elimination, so some sort of support should be used, especially with the apertures he would be working at
That seriously sucks to hear.
Losing SR @ higher focal lengths seems a pretty obvious consequence now that you point it out.
A tripod is a definite no-go for me.


QuoteOriginally posted by Abbazz:
There have been numerous comparisons between unsized images vs. images taken with teleconverters. When done seriously (same subject, stable tripod), these comparisons are usually in favour of the teleconverter
Not sure why i didn't think to google “Teleconverter Crop Comparison” before I posted.
Thanks 4 the link.

QuoteOriginally posted by Richards64 :
Umm, no. The resolution is determined by the sensor
...snip....
Nope, you are talking about the resolution of the image coming from the sensor, not the resolving power of the lens. So you cannot say that there is any connection between that and the use of a TC.
This is how I got myself under the impression a TC would cost resolution...
Wikipedia : “The use of a 2 teleconverter gives the effect of using lens with twice the focal length. It also decreases the intensity of the light reaching the film by the factor of 4, as well as the resolution the factor of 2

FWIW, i was aware the RAW image in my camera would indeed be recorded @ 10mp, however the above statement left me under the impression,that i would essentially be photographing a magnified version of the image @ the end of the 18-200mm lens and therefore entropy w/ respect to available light would result in direct proportion to the magnification level of the TC.
While the laws of thermodynamics are in agreement with the statement I quoted above , it wasn't obvious to me until I delved a bit further into what specifically, a TC does to overcome the incumbent information loss.

QuoteQuote:
that is because you are magnifying it twice.
*slaps forehead* in other words I'd essentially be re-magnifying a magnified image that is already hard-pressed to not look clunky/grainy. This is making sense now.

The gist seems to be, performing a “Digital Zoom” creates pixelation, while the sacrifices made while using a TC in place of a big lens due to diffraction are less obtrusive, and decidedly less apperent when enlarging the final print. The end result being sharper details, and more pleasing contrast(?).

Oh, and enough teasing -lets see the damselfly s already !

QuoteOriginally posted by Lowell Goudge:
Pentax correct this in software, but the TC does not modify the aperture value, and with Fast lenses, introduces an exposure error.
Thanks for the heads-up.
Thats probably the deal breaker right there for me as I still cling to the auto settings and don't venture into manual very often.
It probably follows that I should get the basics down first w/ the lens I have, and worry about “getting a bit more” when I'm more confident about over-ruling the picture and scene modes.

QuoteOriginally posted by 713alan:
You say you like the super zoom so you don't have to change lenses. You will have to take off the lens to use a TC.
Apologies, I implied a contradiction of sorts.
To clarify, what i found myself doing (when I had the two over-lapping lenses) was I was simply getting lazy by not changing out lenses because....
“oh, thats not worth the trouble”, or
“I'll come back for that some other time” or,
“I don't wanna risk getting snow in my camera”, etc..
I never really needed to carry a spare memory card before I bought that lens.
Honestly, if Pentax didn't bring out that lens when they did, I would have lost twice, as I was going to jump to Nikon. Pentax did such a crummy job marketing this thing, I didn't even know they released an 18-250, until I happened upon that tid-bit while gathering prices on Nikon s equivalent VR lens.
But i Degress....
What i meant to infer was that while swapping lenses is arduous while on the move, having more reach in the rarer times when such opportunities present themselves might not suck -given the amount of pack space/weight a TC requires.

QuoteOriginally posted by Ben_Edict:
Pentax TCs are all without AF
Its not so much the losing AF that bugs me, its the firmware in the camera not being able to recognize and compensate accordingly and automagically make pretty pictures in the Auto mode(s).
Because the K*00D's do this right out of the box, is exactly the reason I upgraded to the K200, instead of the K20, or even K10.
Thats again, another point I didn't even take into consideration before starting this thread, Thanks

QuoteQuote:
or wait until Pentax releases its new TC sometime in the future (at least its anounced on the roadmap)
Anyone have any beta on this? Should I hold my breath?

QuoteOriginally posted by Nanobaud:
Just in the interest of probing the limits,..
Well, I figure if I need either an MTF-orometer or a PSF-alizer to see the difference, chances are I can't see'em for jack anyway, but an informative post none the less. Clearly this isn't the cut & dried issue i thought it was.


Great discussion here, and thanks everyone 4 all your expertise.
I've decied @ the very least, I'm just going to hold off on a TC for now.
From the sounds of things, you've pro'lly saved me from wasting a few bucks.

Hope everyone has a nice weekend.
Thanks again,

Teek
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