tele-extenders and sweet spots

Fitzcarraldo · 03-05-2021, 10:21 AM

If one of my lenses has a "sweet spot" at f/8, and I add a 1.4x tele-extender, can I expect the "sweet spot" to remain at f/8? Or will it be shifted to f/11, or is it totally unpredictable?

**PJ1** · 03-05-2021, 01:59 PM

All tele-extenders must have some effect on image quality, even if it is not very obvious. That is a consequence of adding more glass to the optical path. If a 1.4 tele-extender had absolutely no impact on the image you could probably expect the sweet spot to move from f8 to f11. But since no one knows how the refractions of the two sets of elements (those in the lens and those in the tele-extender) will interact, it is impossible to say - and as you said "totally unpredictable". You can only try the combination and see how it works.

**MossyRocks** · 03-05-2021, 02:43 PM

Having 2 good converters, the 1.4X-L and 2X-L, and one lens to use them with I find that things are not all what they may seem. With the 1.4X-L and SMC A* 400mm f/2.8 I find that the converter improves things for astro shooting. So instead of having to stop the lens down 2/3 or .5 stops (I run the 400 by it self at f/3.5 for astro) I can run it wide open (f/4 effective with the lens at f/2.8) and things are as good as they can get with that setup. This may be because of how good the 1.4X-L is coupled with that lens basically using the best part of the image cropped to the best part of that on my K-3 or K-3ii. With the 2X-L I've found that I really need to be running at between f/8 and f/11 (lens at f/4 to f/5.6) to clean up the CA that it introduces. I haven't done a good detailed test to nail down the sweet spot with that setup but want to do it one night soon.

**Lowell Goudge** · 03-05-2021, 03:11 PM

I use the sigma APO 1.4x and 2x EX teleconverters, which , like the Pentax 1.4x-L and 2x-L converters were designed with specific lenses in mind, and to some extent the lenses were designed with the TCs in mind, work exceptionally well.

But the sigma TCs like the Pentax ones can only mount on a few specific lenses due to protruding front elements on the TCs.

Fitzcarraldo · 03-05-2021, 07:14 PM

Thank you everyone for your input! I'm cautiously optimistic but, as you say, the thing is to test it out.

03-05-2021, 10:21 AM	#1
Fitzcarraldo New Member Join Date: Oct 2020 Posts: 8	tele-extenders and sweet spots If one of my lenses has a "sweet spot" at f/8, and I add a 1.4x tele-extender, can I expect the "sweet spot" to remain at f/8? Or will it be shifted to f/11, or is it totally unpredictable?

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03-05-2021, 01:59 PM	#2
PJ1 Loyal Site Supporter Join Date: Jul 2012 Location: Toowoomba, Australia Photos: Gallery \| Albums Posts: 5,486	All tele-extenders must have some effect on image quality, even if it is not very obvious. That is a consequence of adding more glass to the optical path. If a 1.4 tele-extender had absolutely no impact on the image you could probably expect the sweet spot to move from f8 to f11. But since no one knows how the refractions of the two sets of elements (those in the lens and those in the tele-extender) will interact, it is impossible to say - and as you said "totally unpredictable". You can only try the combination and see how it works.

03-05-2021, 02:43 PM	#3
MossyRocks Loyal Site Supporter Join Date: Nov 2017 Location: Minnesota Photos: Gallery \| Albums Posts: 2,982	Having 2 good converters, the 1.4X-L and 2X-L, and one lens to use them with I find that things are not all what they may seem. With the 1.4X-L and SMC A* 400mm f/2.8 I find that the converter improves things for astro shooting. So instead of having to stop the lens down 2/3 or .5 stops (I run the 400 by it self at f/3.5 for astro) I can run it wide open (f/4 effective with the lens at f/2.8) and things are as good as they can get with that setup. This may be because of how good the 1.4X-L is coupled with that lens basically using the best part of the image cropped to the best part of that on my K-3 or K-3ii. With the 2X-L I've found that I really need to be running at between f/8 and f/11 (lens at f/4 to f/5.6) to clean up the CA that it introduces. I haven't done a good detailed test to nail down the sweet spot with that setup but want to do it one night soon.

03-05-2021, 03:11 PM	#4
Lowell Goudge Site Supporter Join Date: Jan 2007 Location: Toronto Photos: Gallery \| Albums Posts: 17,891	I use the sigma APO 1.4x and 2x EX teleconverters, which , like the Pentax 1.4x-L and 2x-L converters were designed with specific lenses in mind, and to some extent the lenses were designed with the TCs in mind, work exceptionally well. But the sigma TCs like the Pentax ones can only mount on a few specific lenses due to protruding front elements on the TCs.

03-05-2021, 07:14 PM	#5
Fitzcarraldo New Member Join Date: Oct 2020 Posts: 8 Original Poster	Thank you everyone for your input! I'm cautiously optimistic but, as you say, the thing is to test it out.