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I want to buy a 1.4 or a 2,0 extender for my 70D but i heard my camera will not auto focus then


I want to buy a 1.4 or 2.0 extender for my 70D but I was told my camera would not autococus using the extenders and I would have to manually focus. Is this correct?




@irish626 wrote:

I want to buy a 1.4 or 2.0 extender for my 70D but I was told my camera would not autococus using the extenders and I would have to manually focus. Is this correct?


For most lenses, this would be correct, most especially for super telephoto lenses.


When your camera measures the amount of light for a given exposure, it typically does this with aperture opened as wide as possible, which allows for the most light, and most accurate reading.  The AF points on the AF sensor need a lot of light for the most accurate measurements.


Not all AF points are created equal, however.  Some are sensitive to vertical lines, some to horizontal lines, and some are sensitive to both directions.  Some are even designed to also be sensitive in the diagonal directions.  Naturally, the more sensitive the AF point is in different directions, the more it costs.


Likewise, AF points are given an aperture threshold, which reflects the narrowest aperture that the AF point can accurately and efficiently read and measure light.  In many of Canon's DSLRs, the center AF point is the most sensitive and accurate AF sensor of them all. 


Remember, the camera typcially measures exposure with an AF lens at maximum aperture.  Some AF points become extra accurate with lenses that have f/2.8 apertures, or wider.  At the other end of the scale, some AF points cannot accurately and consistenly measure light when the maximum aperture is narrower than a certain amount, typcially f/5.6 in Canon DSLRs.  Canon's top-of-the-line DSLR have one [center] or more AF points that are rated to f/.8. 


The maximum aperture of your 70D is limited in firmware to f/5.6.  This is done is firmware, because Canon engineers know that the AF system will be degraded with aperture sizes smaller than f/5.6.  So, the camera is designed to not auto focus under those conditions.  Many third party get around this limitation by reporting false data to the camera, which fools it into thinking it is using a faster lens than it actually it using.


Why is all of this about AF point sensitivity important?  If you look at the descriptions and specifications of your extenders, you will see that they are also rated in terms of the negative impact that they have on the apparent aperture of a lens.  An extender reduces the size of the effective maximum aperture.  This impact is typically described in terms of f/stop. 


An EF 1.4x extender will add one stop to the effective maximum aperture of a lens, while the EF 2.0x extender will add two stops of light.  This means that when you use the 1.4x with an f/4 lens, the effective maximum aperture is reduced one full stop to f/5.6, while the 2.0x will reduce effective maximum aperture by two full stops to f/8.  With an f/2.8 lens, the effective apertures would be reduced to f/4 and f/5.6, respectively.

"The right mouse button is your friend."

Add to the above information that Tele converters DO NOT fit all lenses. Many lenses have elements that interfere with the front of the different brands of TC.

"A skill is developed through constant practice with a passion to improve, not bought."

  • Canon EF 100-400 f/4-5.6L with 2x extender 3 on 60D. I tried to make this work. however best f stop is 9.0 so autofocus is disabled. I tried manual focus and even with a tripod a cropped photo without the extender is better quality. I should have joined this forum before purchasing it. I read a review where one could. spot focus or manually focus to help the autofocus. Not on the case with me,


As a simple rule the notion that you cannot use an extender with your camera is not correct.  It really depends on the lens.


Your camera will have reliable auto-focus available as long as the lens focal ratio is f/5.6 or lower. 


However, when you use a teleconverter (aka 'extender' or focal-length multiplier) you not only have to multiply the focal LENGTH of the lens by the teleconverter factor... you also have to multiply the focal RATIO of the lens by that same factor.


Suppose you have an f/2.8 lens such as the EF 70-200mm f/2.8L IS USM II.  


If you use this lens with a 2x teleconverter then you multiple the focal length by 2 so now the 70-200 becomes a 140-400mm... but you ALSO multiply the focal ratio by 2... so the f/2.8 lens becomes an f/5.6 lens.    


Since your camera can auto-focus reliably at f/5.6 that's no problem and the 2x extender works fine WITH THAT PARTICULAR LENS.


However, let's suppose you don't own the 70-200 f/2.8 and instead own the 70-200 f/4L version.  NOW when you do the math, you still get the 140-200mm focal length, but the f/4 focal ratio doubles and it becomes an f/8 lens.  Your camera will not focus reliably at f/8 so in this case it will not work.


BUT... if you owned an f/4 lens and instead of using the 2x converter, you use the 1.4x convert, then 70-200 becomes 98-280 and the f/4 x 1.4 becomes f/5.6.  So the 1.4x teleconverter WILL work with this lens on your camera.


So the simple thing to remember is that you can use the 1.4x extender with any f/4 or faster lens and you can use the 2x extender with any f/2.8 or faster lens.


Trouble happens when you try to use it with a variable focal ratio lens... for exmaple suppose you have a Canon EF 70-300mm f/4-5.6 lens.  Well that's f/4 IF you are at the 70mm end ... and at that end the 1.4x would take you to f/5.6.  The problem is that the focal ratio increases the moment you start to zoom and most zooms hit the high end of the focal ratio when they're merely about 1/2 way through the zoom range.  So the moment you begin to zoom in (even a little) the focal ratio goes beyond what the camera can handle.


BTW, the limit isn't arbitrary.  To use phase detect auto-focus, the phase-detect sensor puts light through a beam-splitter (prism) and when the aperture stops down the opening is no longer wide enough for the sensor to get clear light through both sides of the phase and the auto-focus stops working (it's a physical challenge.)  Even cameras that support f/8 are mostly limited to the center AF points -- the focus points around the sides of the image wont work.


Tim Campbell
5D III, 5D IV, 60Da

"As a simple rule the notion that you cannot use an extender with your camera is not correct.  It really depends on the lens.


However, let's suppose you don't own the 70-200 f/2.8 and instead own the 70-200 f/4L version.  NOW when you do the math, you still get the 140-200mm focal length, but the f/4 focal ratio doubles and it becomes an f/8 lens.  Your camera will not focus reliably at f/8 so in this case it will not work."


Yes, it does depend the lens. 


But, Canon firmware also allows a number of exceptions, particularly with a few "L" Llenses that have constant apertures.  One of those exceptions is the 70-200mm f/4 lens.  You should still be able to autofocus with this lens on certain camera bodies, and I believe your 70D should still auto focus with that combination.  


Another lens that works with the Canon EF 2x III is the 100-400mm f/4-5.6L  Each of Canon's Extenders have a list of compatible lenses.



"The right mouse button is your friend."


And to further confuse or defuse the issue....

Canon extenders (both the 1.4x and the 2x) do not fit on all Canon lenses. They simply won't mount. Canon disables the AF because they can not guarantee accurate reliable AF. 


Most third party extenders like the Kenko 1.4x and the Tamron 1.4x will mount, but you will lose AF, or more correctly the AF will become unreliable and/or slow.

Some third party extenders don't tell the body that they're there.  The camera thinks that only the lens is mounted. In this case it will try to AF.   It is also possible to tape off certain lens contacts to fool the AF.


I however do not recommend this nor do I recommend extender use at all.  I do admit there are a few examples where the 1.4x does work well.  One is the ef 70-200mm f2.8L with the 1.4x as an example.  But to each his own.

EOS 1DX and 1D Mk IV and less lenses then before!


 Thanks to each and everyone of you for providing informative answers to my question. I appreciate it.




Independent of the autofocus issue, the Canon teleconverters only fit certain lenses because of physical interferences. From Canon site:


"Note: This lens is only compatible with fixed focal length L-series lenses 135mm and over, as well as the EF 70-200/2.8L, EF 70-200/2.8L IS, EF 70-200/4L, and EF 100-400/4.5-5.6L."


Third party units generally don't have the physical problem.

John Hoffman
Conway, NH

1D X Mark III, Many lenses, Pixma PRO-100, Pixma TR8620a, LR Classic


If you have a very expensive F/2.8 lens that happens to fit the extender, then it will likely auto focus.

I do not know about the EOS 70D, but my EOS 80D will auto focus with the extender in live view only and only in bright sunlight with something high contrast in the center of the image if the aperture was smaller than F/8. It may be that as others have written, the threshold for the EOS 70D is F/5.6 instead of F/8.0.

The extender will increase the F number (decrease the aperture). Depending upon the spacing of pixels on the sensor and the F number there will be diffraction blur. Canon DPP software can correct some diffraction blur. There will also be a slight loss of contrast when using a 2x extender.

Here is an extreme example from playing with extenders that illustrates the loss of contrast and blur. I stacked two extenders, a Canon 1.4x and a Kenko 2x. The resulting focal length was 1120mm. This was made with a newer camera body. The focus distances are as reported by the camera and I would not have considered anything with 5 miles to be in focus, much less 89 meters. The Kenko extender will fit on many lenses including some EF-S lenses, but in most cases manual focus will be required. Since the extender results in only the center part of the lens being used and the center is usually the best part it does not degrade the image so much as some might think. The Kenko extender tells a lie to the camera about the F Number, so the camera will still attempt to auto focus and only succeed in auto focus in bright sunlight with high contrast.



F Number16.0
Shutter Speed Value1/21
Focal Length1120.0 mm
Focus Distance Upperinf
Focus Distance Lower81.91 m
Lens ModelEF100-400mm f/4.5-5.6L IS II USM +1.4x III



Moon seen from Norman, Oklahoma, September 14, 2022Moon seen from Norman, Oklahoma, September 14, 2022

At this link is a photo I made with two stacked extenders and an EF-S 55-250 lens: and one might see more blur but I think it does not look too bad when downsized.

Here is an example of autofocus on an EOS 80D with an extender: Canon EF-S 55-250mm f/4-5.6 IS,Kenko Teleplus HD C-AF 2X DGX


F Number6.3
Shutter Speed Value1/6317
Focal Length500.0 mm
Camera Temperature32 C
Focus Distance Upper15.11 m
Focus Distance Lower12.51 m
Lens IDCanon EF-S 55-250mm f/4-5.6 IS
Field Of View2.7 deg
Hyperfocal Distance2052.78 m

The reason that the focal length is listed as 500mm for a 250mm lens is that a Kenko Teleplus HD C-AF 2X DGX teleconverter was attached. This photo was made hand held through a window from inside the house. F/11 is wide open for this combination of lens and teleconverter. The F6.3 listed is incorrect and two stops should be added. Auto focus was used. The combination of lens and telephoto converter confused the metering with a resulting very high ISO.