Stacking TCs

[orbmansony]

I'm well aware of the loss of IQ when stacking teleconverters, but need to explore this as a possible option, and would appreciate your opinions.

I am doing planetary imaging with my Canon 7DII camera body, and the optimal performance would be with an f/20 system.

I was using a 102mmMAK telescope (1300mm, f/12.7) with a Canon 2x II TC yielding 2600mm, f/25.4 and got okay results.



I am now using a loaner Celestron-8 telescope (2000m, f/10) with a Canon 2x II TC yielding 4000mm, f/20 with much better results.



However, in a couple of years I will have to return the C-8, and may not be able to keep the MAK (limited storage availability), so I am exploring other options.

I currently have a Canon 2x TC (II) as well as a Kenko 1.4x Teleplus Pro 300.

I have both a Canon 400mm, f/5.6 prime lens and a Canon 100-400mm II f/4-5.6 lens, and the optics on the zoom lens are much better.

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

So my question is if I buy another Canon 2x TC, will it stack with my Canon 2x II TC yielding a 1600mm, f/22.4 system? Would any of the Canon 2x TCs (I , II, or III) be better than the others, or required, or worth the extra cost?

 

[Abbott Schindler]

I currently have a Canon 2x TC (II) as well as a Kenko 1.4x Teleplus Pro 300.

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

So my question is if I buy another Canon 2x TC, will it stack with my Canon 2x II TC yielding a 1600mm, f/22.4 system? Would any of the Canon 2x TCs (I , II, or III) be better than the others, or required, or worth the extra cost?

There are 2 ways I know of to stack Canon TCs, and I don't think either will give you what you're after.

The common way is to insert a short extension tube, like 12mm, between the extenders. That produces magnification, but you may not be able to focus at infinity—hence useless for astro.

The other way I've seen reported is to grind one of the TCs to make it fit on the other. I haven't tried this method and, frankly, I'd never attempt it.

I've had both the II and III TC's and didn't see a difference with my EF 100-400 (II) or macro lenses on 5DIV and 5Ds R cameras. I've seen similar assessments in a number of forum threads.

 

[rsn]

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

I won't answer your question, it's very specific and I'm guessing few can, but I see you are aware of IQ downgrade with stacked TC's. You would probably be better off asking this in the astrophotography forum here.

But I think your math is wrong. I will personally illustrate with my equipment. So I have the Canon 7D mk 2 with 150 - 600 Sigma lens, I will just talk about the long end. With the Canon 1.6 crop my 600 becomes a 960mm lens. Throw on the TC 1.4 and it becomes 1344 mm lens, let's be lazy and say 1345mm.

So your camera with 1.6 crop with 400 lens, is 640mm. Add on a 2 TC and it becomes 1280 mm lens. Now throw on the 1.4 TC and you have a 1792mm lens, let's be lazy and say 1795 mm lens.

 

[orbmansony]

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

I won't answer your question, it's very specific and I'm guessing few can, but I see you are aware of IQ downgrade with stacked TC's. You would probably be better off asking this in the astrophotography forum here.

But I think your math is wrong. I will personally illustrate with my equipment. So I have the Canon 7D mk 2 with 150 - 600 Sigma lens, I will just talk about the long end. With the Canon 1.6 crop my 600 becomes a 960mm lens. Throw on the TC 1.4 and it becomes 1344 mm lens, let's be lazy and say 1345mm.

So your camera with 1.6 crop with 400 lens, is 640mm. Add on a 2 TC and it becomes 1280 mm lens. Now throw on the 1.4 TC and you have a 1792mm lens, let's be lazy and say 1795 mm lens.

You are confusing effective focal length (EFL) with actual focal length. EFL is the actual focal length times the crop factor. It is not a measure of magnification. It is simply what focal length of a lens used with a full frame camera would be needed to cover the same image area of the crop factor camera.

If your logic were correct, I could use a 6x crop factor camera with a 400mm lens, and instead of 8x magnification (the eye sees 50mm), I would then have 48x magnification. If what you said were true, we should all be using 6x crop factor point and shoot cameras for astrophotography and other telephoto applications.

 

[Sittatunga]

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

I won't answer your question, it's very specific and I'm guessing few can, but I see you are aware of IQ downgrade with stacked TC's. You would probably be better off asking this in the astrophotography forum here.

But I think your math is wrong. I will personally illustrate with my equipment. So I have the Canon 7D mk 2 with 150 - 600 Sigma lens, I will just talk about the long end. With the Canon 1.6 crop my 600 becomes a 960mm lens. Throw on the TC 1.4 and it becomes 1344 mm lens, let's be lazy and say 1345mm.

So your camera with 1.6 crop with 400 lens, is 640mm. Add on a 2 TC and it becomes 1280 mm lens. Now throw on the 1.4 TC and you have a 1792mm lens, let's be lazy and say 1795 mm lens.

You are confusing effective focal length (EFL) with actual focal length. EFL is the actual focal length times the crop factor. It is not a measure of magnification. It is simply what focal length of a lens used with a full frame camera would be needed to cover the same image area of the crop factor camera.

If your logic were correct, I could use a 6x crop factor camera with a 400mm lens, and instead of 8x magnification (the eye sees 50mm), I would then have 48x magnification. If what you said were true, we should all be using 6x crop factor point and shoot cameras for astrophotography and other telephoto applications.

And that's why the size of an astronomical telescope is defined in terms of its diameter, not its focal length.

 

[Andy01]

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

I won't answer your question, it's very specific and I'm guessing few can, but I see you are aware of IQ downgrade with stacked TC's. You would probably be better off asking this in the astrophotography forum here.

But I think your math is wrong. I will personally illustrate with my equipment. So I have the Canon 7D mk 2 with 150 - 600 Sigma lens, I will just talk about the long end. With the Canon 1.6 crop my 600 becomes a 960mm lens. Throw on the TC 1.4 and it becomes 1344 mm lens, let's be lazy and say 1345mm.

So your camera with 1.6 crop with 400 lens, is 640mm. Add on a 2 TC and it becomes 1280 mm lens. Now throw on the 1.4 TC and you have a 1792mm lens, let's be lazy and say 1795 mm lens.

You are confusing effective focal length (EFL) with actual focal length. EFL is the actual focal length times the crop factor. It is not a measure of magnification. It is simply what focal length of a lens used with a full frame camera would be needed to cover the same image area of the crop factor camera.

If your logic were correct, I could use a 6x crop factor camera with a 400mm lens, and instead of 8x magnification (the eye sees 50mm), I would then have 48x magnification. If what you said were true, we should all be using 6x crop factor point and shoot cameras for astrophotography and other telephoto applications.

This is probably an interesting subject because there appears to be differing views.

I consider what my FoV is when looking through the viewfinder is and equating that to magnification - because that is relative to how much of the subject is going to be visible in the final image - think pixels per duck (or whatever your subject is - in your case a planet).

So, I agree with rsn - using a 400mm lens on a APS-C camera does produce an image that is as "magnified" as if it was taken with a FF camera at 640mm.

Using a lens (or telescope) with higher magnification achieves the same thing as using a (smaller) higher density sensor - it gives you more pixels on subject.

So, using a 2x + 1.4x TC on a 400mm lens would provide the same number of pixels per planet on a 24Mp APS-C camera is it would if a 1792mm lens was used on a 24Mp FF camera. At least that is correct to my knowledge, but as always, I am happy to be corrected.

And yes, solely from a reach point of view, astrophotographers probably would use tiny 1" or smaller sensors to make their lenses (or telescopes) reach further, and some do. There are people in the astro forum who use compact bridge cameras with effective lens "lengths" of 2000mm or longer.

The flipside to using a tiny sensor is that they almost always display more noise than a FF or even APS-C sensor, and since light is very limited when shooting stars (or planets) at night, it is a fine line between getting the reach and sacrificing IQ (via noise). Then, of course, there is the whole "equivalence" discussion about equivalent apertures etc etc - which tends to get too much for my brain.

 

[John Crowe]

Sorry to be quite late.

I have, or had, the EF 1.4x, EF 2x, EF 2x II, EF 1.4x III, and Kenko 300 Pro DG 1.4x.

It is not worth stacking the Kenko with anything. It is not nearly as sharp as even the original EF 1.4x.

The Canon EF 2x II is the only Canon teleconverter that can be stacked. The rear lens opening is large enough to take any of the six EF 1.4x or EF 2x versions behind it.

You could actually stack EF 2x II units continuously to infinity!

I have stacked the EF 1.4x III and EF 2x II with the original EF 300/2.8 L and EF 500/4.5 L. Image quality held up well. Autofocus with the 300/2.8 L combo is virtually useless, so I have not been using that combo at all. The 500mm will not AF. I have started using manual focus with both big lenses more often, so I will revisit the possibility of using the stacked combos again.

 

[mclewis]

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

I won't answer your question, it's very specific and I'm guessing few can, but I see you are aware of IQ downgrade with stacked TC's. You would probably be better off asking this in the astrophotography forum here.

But I think your math is wrong. I will personally illustrate with my equipment. So I have the Canon 7D mk 2 with 150 - 600 Sigma lens, I will just talk about the long end. With the Canon 1.6 crop my 600 becomes a 960mm lens. Throw on the TC 1.4 and it becomes 1344 mm lens, let's be lazy and say 1345mm.

So your camera with 1.6 crop with 400 lens, is 640mm. Add on a 2 TC and it becomes 1280 mm lens. Now throw on the 1.4 TC and you have a 1792mm lens, let's be lazy and say 1795 mm lens.

Sensor size does not change focal length. The 'crop factor' is just an easy way to compare the field of view differences between different size sensors when used with the same focal length. If you aren't comparing cameras with different size sensors for a particular application there is zero need to even think about it. In your example above it should not be used at all.

You have a 600mm focal length lens. Add a 1.4 TC you have a 840mm focal length lens.

A 400mm focal length lens with 2x TC is becomes a 800mm focal length lens. Add a 1.4 TC you have 1120mm of focal length as originally stated.

 

[Abbott Schindler]

The Canon EF 2x II is the only Canon teleconverter that can be stacked. The rear lens opening is large enough to take any of the six EF 1.4x or EF 2x versions behind it.

Just pointing out that John specified the EF 2X II; the EF2X III can't be stacked. Just to be sure, I just tried stacking both series III Extenders, and it doesn't work.

 

[Victor Engel]

I'm well aware of the loss of IQ when stacking teleconverters, but need to explore this as a possible option, and would appreciate your opinions.

I am doing planetary imaging with my Canon 7DII camera body, and the optimal performance would be with an f/20 system.

I was using a 102mmMAK telescope (1300mm, f/12.7) with a Canon 2x II TC yielding 2600mm, f/25.4 and got okay results.



I am now using a loaner Celestron-8 telescope (2000m, f/10) with a Canon 2x II TC yielding 4000mm, f/20 with much better results.



However, in a couple of years I will have to return the C-8, and may not be able to keep the MAK (limited storage availability), so I am exploring other options.

I currently have a Canon 2x TC (II) as well as a Kenko 1.4x Teleplus Pro 300.

I have both a Canon 400mm, f/5.6 prime lens and a Canon 100-400mm II f/4-5.6 lens, and the optics on the zoom lens are much better.

So I can use the 2x TC for an 800mm, f/11.2 or stack both 2x TC and the 1.4x TC for 1120mm, f/15.6. Using both TCs, I need to attach the Kenko to the camera body first and then attach the Canon 2x and lens. Reversing the order of the teleconverters will not stack.

So my question is if I buy another Canon 2x TC, will it stack with my Canon 2x II TC yielding a 1600mm, f/22.4 system? Would any of the Canon 2x TCs (I , II, or III) be better than the others, or required, or worth the extra cost?

I have tried the Kenko and Canon Mark II and Mark III TCs. The Canon Mark III TCs are significantly better than the others. That said, I didn't find an advantage to using any TC with the EF 100-400 II lens. I do get a significant benefit with my 600mm f/4 III lens, though.

P.S. I don't stack TCs.

--
Victor Engel