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04-20-2008, 12:19 PM   #1
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70-300 and a 2x teleconverter

Having 450mm is not limiting me however, has anyone used a 2x tele-converter with their 70-300? In particular the Sigma 70-300 APO and 2X EX DG APO tele-converter. Just curious about IQ at 900mm and what the f-stop would be on the long end. TIA.

04-20-2008, 12:37 PM   #2
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With a 2x TC, you'll lose two stops of light, so on the Sigma for a total of 600mm (900mm is only the field of view, not the zoom), you'll be at f/11. I don't know what it is you shoot, but this setup will pretty much only work outdoors in very bright light, AF speed will suffer, and it will hunt a lot have a very tough time achieving focus if you aren't already close to being focused on the subject. And I get all of that just using a 1.4x TC with that lens. A 2x will only be worse. On a bright sunny day at shutter speed of 1/1000, I need an ISO of 1600. With the extra stop lost between a 1.4x and 2x, that will give you 1/500 at ISO 1600, which is probably slower than you want to shoot at 600mm, at least handheld. Yes you have SR, but with the TC attached, it's still dialed in at 300mm.
04-20-2008, 12:38 PM   #3
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I use one with my 70-200F2.8, and it works well, see the following 2 posts.

https://www.pentaxforums.com/forums/187646-post19.html
https://www.pentaxforums.com/forums/208249-post1.html

Not sure if your 70-300 is fast enough to allow the autofocus to work reliably.

you can check on the sigma site to see if it is recommended for that lens
Sigma - Lenses - Teleconverters
04-20-2008, 12:55 PM   #4
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Thanks for the info both of you. I was just curious as to what people thought about the use of a tele-converter was on a lens like the 70-300 class. By the sounds of it its not worth it unless you are at f/2.8 before the tele-converter is added on.

04-20-2008, 01:48 PM   #5
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Like I said, it depends on what and where you are shooting. For me, casually shooting youth soccer, a 1.4x works well enough. There is a noticable difference in focusing with it, but it's tolerable. I just need to be a little steadier and keep them on the focus point at all times. But once I have them, it's fast enough to keep up using AF-C. I could get away with (and have using a MF TC) the slower shutter speed a 2x would give, but I don't know about the focusing ability. A couple of things you could try are getting a hold of a cheap MF 2x TC to play with, or look into a 1.4x instead. For the record, Sigma doesn't recommend any TC on the 70-300, but that doesn't mean it won't work.
04-20-2008, 05:49 PM   #6
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The Tamron 70-300 is tough enough to use with a 1.4x TC (slow, lots of hunting)- I would never want to use it with a 2x.
04-20-2008, 06:25 PM   #7
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I'd have to agree with Entropy, you'd prolly get frustrated with a 2x unles you were doing manual focus, and the hit on the IQ may not be worth it.
04-20-2008, 07:55 PM   #8
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Thanks again everyone, the tele-converter idea is over. I don't need the extra range.

04-21-2008, 12:47 AM   #9
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I have been experimenting with a Sigma 70-300 DL Macro and a Kenko MC7 2x TC. As others have mentioned TCs are not really recommended for use with zooms, but it is possible to get good results as Lowell has. If you go up to 2x you are best to go for a 7 element not 4 model. I don't know which the Sigma is. With the Sigma at 300mm I have to use the Kenko in manual focus as F5.6 isn't enough to allow autofocus. The upside of this is that the SR menu comes up when I attach the combination and I am able to dial in 600mm. A lot of my shots have not been very sharp but I have mainly been attempting hand held and monopod shots. I think it is possible to get sharp shots with the right support/technique but a tripod is going to be best. I intend to persevere as I need the reach for some shots. It is a combination for using on bright days. My main aim is to develop technique with a view to using this TC with a 300mm prime when I get one.

Paul

Last edited by channeler; 04-21-2008 at 01:05 AM.
04-21-2008, 07:51 AM   #10
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QuoteOriginally posted by channeler Quote
I have been experimenting with a Sigma 70-300 DL Macro and a Kenko MC7 2x TC. As others have mentioned TCs are not really recommended for use with zooms, but it is possible to get good results as Lowell has. If you go up to 2x you are best to go for a 7 element not 4 model. I don't know which the Sigma is. With the Sigma at 300mm I have to use the Kenko in manual focus as F5.6 isn't enough to allow autofocus. The upside of this is that the SR menu comes up when I attach the combination and I am able to dial in 600mm. A lot of my shots have not been very sharp but I have mainly been attempting hand held and monopod shots. I think it is possible to get sharp shots with the right support/technique but a tripod is going to be best. I intend to persevere as I need the reach for some shots. It is a combination for using on bright days. My main aim is to develop technique with a view to using this TC with a 300mm prime when I get one.

Paul
Thanks for that Paul.

BTW, the lens construction for the 2 Sigma tele-converters are;

1.4X 5 elements in 3 groups

2X 6 elements in 5 groups

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