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12-14-2018, 07:48 PM   #1
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which teleconverter to use?

Hi

I was wondering if anyone can shed some light on which teleconverter I should try using. I have a K-70 camera and a sigma 18-300mm lens which I love but would like to add some extra focal length without using a longer heavier lens. I am disappointed to find that sigma do not make a teleconverter with a k mount so am wondering if the pentax A2X-S would work. I have tried to find some information but seem to have come to a dead end. Anyone with any information or ideas?
Thanks Margaret Jukes Australia

12-14-2018, 08:17 PM - 2 Likes   #2
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Generally teleconverter use is best with a fast aperture high quality lens. Primes are best - some very high end zooms will be well suited as well. The problem with other uses is that the quality suffers and simply cropping is likely to outperform combinations that are not of the utmost quality.

Additionally the lower the teleconverter multiplier factor the more likely the quality will be higher all other things held similar. So a 1.4x is preferred over a 2x in many cases.

There are also some lenses that have dedicated teleconverters.

BTW the teleconverter you mentioned is not autofocus if that matters to you.

Finally - light weight, cheap, and high quality in long telephoto lenses is really a tall order.
12-14-2018, 08:18 PM   #3
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This would be manual focus only and losing 2 stops of light would put you at 1/3 stop below f/11 which would be pretty dark and difficult to focus not to mention the drop in image quality if it even works at all. This is why teleconverters are best suited for high quality lenses with a large aperture.
12-14-2018, 09:35 PM   #4
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I just bought a KAX MC4 2X teleconverter and it works just fine. Mine does not have the autofocus so must focus manually - not a big deal.


The pic below was taken with an older 2x manual converter on the 55-300


All the best



12-15-2018, 12:54 AM   #5
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Yes, teleconverters are supposed to be best on fast primes but as your pic shows, for normal viewing they can do the job. High contrast might be a problem but I can live with that if I have to. I'm with you - we can't all buy "top drawer".
12-15-2018, 01:36 AM   #6
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It's not worth it to get a teleconverter for your lens. Cropping will always yield better quality. Even if there are more pixels on your subject, the pixel-level quality will be much worse and will negate the reach of the TC. Not only that but the autofocus will suffer and the lower-light capability will be worse.
12-15-2018, 06:16 AM - 1 Like   #7
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QuoteOriginally posted by FreeSpirit9 Quote
I just bought a KAX MC4 2X teleconverter and it works just fine. Mine does not have the autofocus so must focus manually - not a big deal.


The pic below was taken with an older 2x manual converter on the 55-300


All the best
The real question is would that image have been as good or better cropped from the lens without the converter. I'm not trying to be a snob about the equipment, I'm completely aware that budgets exist. But cropping is free, and often better than using a teleconverter unless you are using very high quality glass as your starting point. Also you are giving up a lot of light which will potentially negatively impact quality or force exposure compromises that you don't want.

I own the HD DA 1.4x and the f1.7x. I previously owned the 1.4x A series converter as well. I rarely use them except as follows: on my 50-135 for slightly more reach (great to fit in a small bag); macro work with the DFA 100; telephoto use of the 60-250 where reach is needed AND zoom due to unpredictable distances, and finally for long range details such as on moon shots using the FA* 300.

Previously I owned the 72mm front element Sigma 400mm f5.6*, that lens is soft and not great but it is light and fun. I recommend it as a cute fun toy - but cropping a DA 55-300 is sharper and more versatile.

(* The nicer 400mm 5.6 from Sigma has a 77mm front element and while I've never shot it, the images from it I have seen appear much better than those from the older lighter lens. The cost is in line with this as well.)

12-15-2018, 06:39 AM   #8
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Even though the 18-300mm seems pretty reasonable for an ultrazoom, it has faults which any TC will amplify such as a disturbing amount of chromatic aberration. I would not think the combo with a TC worthwhile, even if you use the Pentax HD DA 1.4x converter - there's just too much borderline stuff to let you down.

Also the long end is already f6.3 wide-open so you'd generally stop it down to f11 to allow the lens to perform at its peak. Taking 2 stops of light away from that and you're at f22-equivalent which, when combined to the faster shutterspeeds required by longer focal lengths will push your ISO through the roof. Now the K-70 does really well all the way up to ISO6400 and even a bit beyond but everything is working against you here.

Generally, zooms which straddle the "normal" FOV and go from wide to tele are already hard to properly correct optically in the design stage, you may have more luck with a 55-300mm on a 1.4 TC.
12-15-2018, 01:23 PM   #9
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The photo of the flower with the 55-300 and the converter look good and all but why not just move closer and get a better photo without the converter? For me the need for a longer focal length would be for photographing things that can't be approached like wildlife or sports. Manual focus and a dark viewfinder at F/13 doesn't seem like the best option here to me? Sure you can find a use for that combo but it certainly won't be as simple as doubling your focal length, the drawbacks exceed the positives.
12-15-2018, 01:50 PM   #10
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To answer your question directly, there are several teleconverters that you could use, including fully manual and fully automatic types, Pentax and non-Pentax varieties. Have a look through the reviews here for the Pentax DA teleconverter, and here for the other K-mount teleconverters. Most are 1.4x magnification, and I wouldn’t consider using anything longer than a 1.7x converter under the best circumstances, although the Pentax 2x teleconverters made to go with some older F* and FA* lenses are considered good for those uses.

That said, the previous posters’ advice is worth considering, too. One of the problems with going very long is that, for distant objects, atmospherics, as well as lens aberrations, can interfere with the final image quality. Heat haze shimmer is a particular problem in many cases, although not in all climates, and unless you use an appropriate filter, dust and water vapour can drastically reduce image contrast. None of that applies, of course, if your intent is to simply magnify closer objects.
12-15-2018, 02:35 PM   #11
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Hi Margaret.

One point that hasn't yet been made is that there aren't many teleconverters (TCs) that would, even theoretically, allow for autofocus (AF) with the Sigma 18-300, because it doesn't have screw-drive AF but instead uses an in-lens motor for AF. The Pentax DA 1.4x TC might work, but it's quite expensive. There are Tamron and Kenko TCs with "power zoom" contacts (they have the designation "pz" in their name) that might work, but they are not easy to find. Expect to pay something like $A150-200 if you find one, and you would be wise to test it on the lens first, as there are no guarantees it would.

But the main take-home point is what everyone else has said, that there are too many downsides to using a TC with a lens like the 18-300 to make it worthwhile. Particularly when you have a 24 megapixel camera, you are better off cropping. Using a 1.4x TC effectively restricts the aperture by 1 stop; a 2x TC by two stops. So even using the lens wide open at 300 f6.3, with a 1.4x TC, while the field of view would be roughly equivalent to 420mm the aperture would be equivalent to f9. And that effective f9 would affect the ability to focus, either manually or (if available) with AF.

There's no particularly easy or cheap way to get more reach than 300mm with high quality optics in a Pentax system. Even the older manual focus 400mm primes (or 300mm primes + TC) are quite pricey for what they offer. The long AF consumer zooms like the Sigma 50-500 (known as the Bigma) or 150-500 generally go for upwards of $A600 on Gumtree, and they are 2kg beasts that take some getting used to. The gold star solution is a DFA 150-450 - a very fine lens but as heavy as a Bigma and it costs in the region of $A3000, even when on sale. A Pentax DA*300 f4 with the matching DA 1.4x TC is a good option but will cost in the region of $A1500 even second hand (if you can find them second hand) or $2000 new.

Last edited by Des; 12-16-2018 at 12:18 PM.
12-15-2018, 03:19 PM   #12
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I have Tamron 1.4x TC, which is great, and works with in-lens AF motors. Having said that, I really doubt if it would work well with 18-300. The reason is simple: TCs were meant to be used with tele lens, i.e. ones with FL > 50mm (on full frame). Zooms which reach wide angle area (on APS-C < ca. 35mm) are not best candidate for coupling with TCs.
12-15-2018, 04:20 PM - 1 Like   #13
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QuoteOriginally posted by UncleVanya Quote
The real question is would that image have been as good or better cropped from the lens without the converter. I'm not trying to be a snob about the equipment, I'm completely aware that budgets exist. But cropping is free, and often better than using a teleconverter unless you are using very high quality glass as your starting point. Also you are giving up a lot of light which will potentially negatively impact quality or force exposure compromises that you don't want.

I own the HD DA 1.4x and the f1.7x. I previously owned the 1.4x A series converter as well. I rarely use them except as follows: on my 50-135 for slightly more reach (great to fit in a small bag); macro work with the DFA 100; telephoto use of the 60-250 where reach is needed AND zoom due to unpredictable distances, and finally for long range details such as on moon shots using the FA* 300.

Previously I owned the 72mm front element Sigma 400mm f5.6*, that lens is soft and not great but it is light and fun. I recommend it as a cute fun toy - but cropping a DA 55-300 is sharper and more versatile.

(* The nicer 400mm 5.6 from Sigma has a 77mm front element and while I've never shot it, the images from it I have seen appear much better than those from the older lighter lens. The cost is in line with this as well.)
Just for referance, my A* 400 has a 145 mm filter thread, so many things change with the availlable light that a f2.8 lens allows.
12-15-2018, 05:36 PM   #14
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I own the AF 1.4x, the 1.4 xl, the 2XL and the 1.7 af converter, all Pentax.. You need a superb len to use any of these adaptorrs, the 1.4xl and 2xl adaptors only fit certain lens. If you have a fast lens that is compatible, these adaptor are a great addition to what lens you have...otherwise I suggest getting a better lens.
12-16-2018, 12:34 AM - 1 Like   #15
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QuoteOriginally posted by UncleVanya Quote
Generally teleconverter use is best with a fast aperture high quality lens. Primes are best - some very high end zooms will be well suited as well. The problem with other uses is that the quality suffers and simply cropping is likely to outperform combinations that are not of the utmost quality.

Additionally the lower the teleconverter multiplier factor the more likely the quality will be higher all other things held similar. So a 1.4x is preferred over a 2x in many cases.

There are also some lenses that have dedicated teleconverters.

BTW the teleconverter you mentioned is not autofocus if that matters to you.

Finally - light weight, cheap, and high quality in long telephoto lenses is really a tall order.
Thank you for all your information and suggestions, looks like I just stay with my 18 - 300 and enjoy the shots I get

---------- Post added 12-16-18 at 12:35 AM ----------

QuoteOriginally posted by bschriver11 Quote
This would be manual focus only and losing 2 stops of light would put you at 1/3 stop below f/11 which would be pretty dark and difficult to focus not to mention the drop in image quality if it even works at all. This is why teleconverters are best suited for high quality lenses with a large aperture.
Thanks for all your info

---------- Post added 12-16-18 at 12:36 AM ----------

QuoteOriginally posted by FreeSpirit9 Quote
I just bought a KAX MC4 2X teleconverter and it works just fine. Mine does not have the autofocus so must focus manually - not a big deal.


The pic below was taken with an older 2x manual converter on the 55-300


All the best
Nice photo, and thanks for your input

---------- Post added 12-16-18 at 12:37 AM ----------

QuoteOriginally posted by PJ1 Quote
Yes, teleconverters are supposed to be best on fast primes but as your pic shows, for normal viewing they can do the job. High contrast might be a problem but I can live with that if I have to. I'm with you - we can't all buy "top drawer".
Thanks for that and you are right about having a budget

---------- Post added 12-16-18 at 12:39 AM ----------

QuoteOriginally posted by automorphism Quote
It's not worth it to get a teleconverter for your lens. Cropping will always yield better quality. Even if there are more pixels on your subject, the pixel-level quality will be much worse and will negate the reach of the TC. Not only that but the autofocus will suffer and the lower-light capability will be worse.
Thanks for your input

---------- Post added 12-16-18 at 12:41 AM ----------

QuoteOriginally posted by newmikey Quote
Even though the 18-300mm seems pretty reasonable for an ultrazoom, it has faults which any TC will amplify such as a disturbing amount of chromatic aberration. I would not think the combo with a TC worthwhile, even if you use the Pentax HD DA 1.4x converter - there's just too much borderline stuff to let you down.

Also the long end is already f6.3 wide-open so you'd generally stop it down to f11 to allow the lens to perform at its peak. Taking 2 stops of light away from that and you're at f22-equivalent which, when combined to the faster shutterspeeds required by longer focal lengths will push your ISO through the roof. Now the K-70 does really well all the way up to ISO6400 and even a bit beyond but everything is working against you here.

Generally, zooms which straddle the "normal" FOV and go from wide to tele are already hard to properly correct optically in the design stage, you may have more luck with a 55-300mm on a 1.4 TC.
thanks for that, I do use a high ISO when using the zoom at it's full extent and have reasonable results

---------- Post added 12-16-18 at 12:43 AM ----------

QuoteOriginally posted by RobA_Oz Quote
To answer your question directly, there are several teleconverters that you could use, including fully manual and fully automatic types, Pentax and non-Pentax varieties. Have a look through the reviews here for the Pentax DA teleconverter, and here for the other K-mount teleconverters. Most are 1.4x magnification, and I wouldn’t consider using anything longer than a 1.7x converter under the best circumstances, although the Pentax 2x teleconverters made to go with some older F* and FA* lenses are considered good for those uses.

That said, the previous posters’ advice is worth considering, too. One of the problems with going very long is that, for distant objects, atmospherics, as well as lens aberrations, can interfere with the final image quality. Heat haze shimmer is a particular problem in many cases, although not in all climates, and unless you use an appropriate filter, dust and water vapour can drastically reduce image contrast. None of that applies, of course, if your intent is to simply magnify closer objects.
thank you for all your information on other TC's I am just thinking to stick with the lens I have and be happy with the reach it has

---------- Post added 12-16-18 at 12:45 AM ----------

QuoteOriginally posted by Des Quote
Hi Margaret.

One point that hasn't yet been made is that there aren't many teleconverters (TCs) that would, even theoretically, allow for autofocus (AF) with the Sigma 18-300, because it doesn't have screw-drive AF but instead uses an in-lens motor for AF. The Pentax DA 1.4x TC might work, but it's quite expensive. There are Tamron and Kenko TCs with "power zoom" contacts (they have the designation "pz" in their name) that might work, but they are not easy to find. Expect to pay something like $A150-200 if you find one, and you would be wise to test it on the lens first, as there are no guarantees it would.

But the main take-home point is what everyone else has said, that there are too many downsides to using a TC with a lens like the 18-300 to make it worthwhile. Particularly when you have a 24 megapixel camera, you are better off cropping. Using a 1.4x TC effectively restricts the aperture by 1 stop; a 2x TC by two stops. So even using the lens wide open at 300 f6.3, with a 1.4x TC, while the field of view would be roughly equivalent to 420mm the aperture would be equivalent to f8. And that effective f8 would affect the ability to focus, either manually or (if available) with AF.

There's no particularly easy or cheap way to get more reach than 300mm with high quality optics in a Pentax system. Even the older manual focus 400mm primes (or 300mm primes + TC) are quite pricey for what they offer. The long AF consumer zooms like the Sigma 50-500 (known as the Bigma) or 150-500 generally go for upwards of $A600 on Gumtree, and they are 2kg beasts that take some getting used to. The gold star solution is a DFA 150-450 - a very fine lens but as heavy as a Bigma and it costs in the region of $A3000, even when on sale. A Pentax DA*300 f4 with the matching DA 1.4x TC is a good option but will cost in the region of $A1500 even second hand (if you can find them second hand) or $2000 new.
Thanks for your input, I find the camera and lens is enough weight for me to carry so didn't want to look for anything larger and just thought a TC would do the job, obviously I missed lots whilst reading about them. Looks like I will just stick with my lens and be happy with the reach it has.

---------- Post added 12-16-18 at 12:46 AM ----------

QuoteOriginally posted by pentageek Quote
I have Tamron 1.4x TC, which is great, and works with in-lens AF motors. Having said that, I really doubt if it would work well with 18-300. The reason is simple: TCs were meant to be used with tele lens, i.e. ones with FL > 50mm (on full frame). Zooms which reach wide angle area (on APS-C < ca. 35mm) are not best candidate for coupling with TCs.
thanks for your info

---------- Post added 12-16-18 at 12:47 AM ----------

QuoteOriginally posted by stihlmania Quote
I own the AF 1.4x, the 1.4 xl, the 2XL and the 1.7 af converter, all Pentax.. You need a superb len to use any of these adaptorrs, the 1.4xl and 2xl adaptors only fit certain lens. If you have a fast lens that is compatible, these adaptor are a great addition to what lens you have...otherwise I suggest getting a better lens.
thanks for that, but think I will just be content with the lens I have
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