Canon doesn't offer a 1.5X TC.... it's a 1.4X Extender.
They can be stacked, but you need to put a 12mm macro extension tube between them. Due to their protruding front elements, you cannot directly attach either Extender to the rear of the other. On some longer tele lenses, this doesn't prevent the lens from focusing to infinity.
But the image quality really drops off using them both.
This was shot using both EF 2X II and 1.4X II on an EF 500/4 IS lens, all in turn on a 10D camera... and all on a very sturdy tripod!
That deer was about a quarter mile away. If you do the math, this set up makes for an effective 1400mm f11 lens (autofocus no longer works).... and if you factor in the 1.6X of the camera used, it's the equivalent to using a 2240mm lens on a full frame (or film) camera. So even an IS lens struggles to get a steady shot, a rock solid tripod is needed.
Image quality okay for Internet resolutions and smaller print sizes, but not good enough for any significant enlargement. The old 6MP 10D camera might actually have been more forgiving than the 18MP cameras I use now! The higher resolution cameras tend to pick up more on any lens weakness.
You didn't mention what lens you want to use the teleconverters on. Be aware that Canon Extenders can only be used on select lenses, due to those protruding front elements. Mostly they work best on primes instead of zooms. Canon states they are designed for use with 135mm f2L and longer primes, and with the 70-200mm zooms. They don't work well or even fit on other zooms. In my opinion, the 1.4X II is okay on 70-200/2.8 IS and 70-200/4 IS, but I try not to use it. I won't use the 2X II on either of the zooms, due to too much loss of IQ. According to various reviews, the 70-200/2.8 IS Mark II and EF 2X III work pretty well together.
I use the 1.4X II a lot on 300/4 IS, and occasionally on 135/2L. I use both 1.4X II and 2X II fairly often on 300/2.8 IS and 500/4 IS lenses.
"I use it with the EF 70-200/2.8 IS on a 6D"
I have never tried stacking 2X and 1.4X on a 70-200, so I can't say if the lens will still focus to infinity properly (the way a 500mm does, even tho there's a 12mm extension tube between the two TCs).
1.4X II or III alone will work pretty well with 70-200/2.8 IS. There's some loss of IQ, but not too bad.
I won't use 2X II on the 1st version 70-200/2.8 IS... there's just too much loss of IQ.
The 70-200/2.8 IS Mark II and 2X III seem to work pretty well together. Some people are happy with it, tho it makes for a fairly expensive combo. I am still using the older versions of each, so haven't got any personal experience with the combination of the newer lens and TC.
Honestly, you might consider a second body - perhaps a 60D - a cropper that will give you "free 1.6X" teleconverter effect. I know, I know... this is technically not a real teleconverter effect... but the result is a lot "more pixels on target" than you get from cropping the FF image down to the same size, and there's no optical loss or loss of light. Besides, it's always nice to have a backup, and an APS-C crop camera nicely complements a full frame.
"... but the result is a lot "more pixels on target" than you get from cropping the FF image down to the same size,
and there's no optical loss ..."
This could start a whole new thread all on it's own!
Thanks for the response. Have 5D M III & using 300, 2.8 IS also 70 - 200. 2.8 IS. Have Extender 2X II & was considering adding Extender 1.4x III to get more. Am not sure & can't get any answers as to whether the II & III series are compatible. Was not aware or using the extender between them but makes sence.
2XII works quite well on the 300/2.8 IS, doesn't it?
Either 1.4X II or III will work fine, too. By itself.
Stacking both TCx on that lens.... Well I'm pretty sure you will lose quite a bit of image quality... not to mention 3 stops of light. You'll have an effective 840mm f8 lens, which will still focus on some cameras (1D series, 5DIII... center AF points only). Even on those, AF will slow a lot due to the loss of light and the extra connections.
The 300mm + 2X for an effective 600mm f5.6 isn't long enough for you?
I'd go for the 1.4X II, used, rather than the pricier 1.4X III. The Mark III version seems optimized for the newer, Mark II lenses. Most reports I've seen, it's not all that different from the 1.4X II, optically.
Alternatively, if your camera is full frame you might consider picking up a cropper to complement it, instead. You could pick up a used 50D or 60D for little more than the cost of the 1.4X III.
I have 300/2.8 IS and both the Mark II TCs. I've used each TC individually with the lens (and they work well), but have never tried stacking both on that lens. So I can't really say if it would be useful. Simply haven't the need to stack them, since I can get an effective 800mm f4 using my 500mm on an APS-C camera.
I'd use either 1.4X II or III on 70-200/2.8 (though you and I have little reason to do so, having a 300mm lens avail.). I won't use 2X II on the original 70-200/2.8 IS.... there's too much loss of IQ. So I certainly wouldn't use the zoom with the two TC stacked on it.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.