My setup is a Canon EOS 600D with a Canon EF 70-200mm f/4 L (non-IS), and I really want to buy a 2× teleconverter (1.4× is too short). I have done a lot of research, but I can't wrap my head around this.

Now, I understand that I will lose AF after f/5.6(?) with this body, but does that mean that I need to use my hand for manual focusing? I normally never use all the AF points. I use the central AF point, half-press the shutter button, then move my camera to where I want, keeping my focus locked and taking the picture. Will this way of focusing still be possible, or does this too count as AF?

  • Note: You can still go into live view and use contrast detection AF – Nayuki Sep 12 '16 at 17:21
up vote 10 down vote accepted

You will have to manually focus by moving the focusing ring on the lens.

Using the center focus point to focus and then recompose is still considered autofocus. Even if you have manually selected which autofocus point the camera uses, the camera is still focusing the lens, not you.

You'll also lose more image quality with a 2X than with a 1.4X. This will be most evident in terms of CA and flare when shooting into bright lighting. You'll see much less image degradation when the brightest light source, such as the sun, is behind you.

In general 2X teleconverters and extenders are only recommended for f/2.8 or faster lenses. Some bodies (most of the 1D series, the 5D Mark III, and the 7D Mark II) can use the center AF point with f/8 lens/extender combinations. But the narrower PDAF baseline of the APS-C cameras makes this much more difficult than for their full frame counterparts. The FF 1D X Mark II and 5D Mark IV can even use many of their AF points with an f/8 lens/extender combo, but even those high end models don't perform AF as fast or as well as they do with a faster lens.

For more on how f/8 combinations may or may not work with third party TCs with Canon EOS cameras, please see this answer to Will the Canon 5D MK II with 100-400 1:4.5-5.6 work properly with Kenko 1.4 converter?

For more on the variety of TCs please see this answer to How do I choose a teleconverter for my Tamron 70-200 f2.8 and Canon camera?

  • 1
    There are a lot of different 1.4X teleconverters of various quality out there. They're not all the same. – Michael Clark Sep 12 '16 at 10:49
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    @Noldor130884 The rule of thumb applies in cases where the 1.4X and 2X are from the same maker and based on the same design. Almost all of the third parties who sell TCs for EOS cameras/lenses use the same design that is likely produced in the same Chinese factory. There are several versions of both the 1.4X and 2X varieties. Even Canon has produced three series of 1.4X and 2X extenders. The III versions are significantly better than the II versions, which were marginally better than the original EF TCs. – Michael Clark Sep 12 '16 at 10:57
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    For more on the variety of TCs please see this answer to another question: photo.stackexchange.com/a/69660/15871 – Michael Clark Sep 12 '16 at 11:02
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    One other note is that zooms, from my experience don't fare as well in sharpness than teleprimes when adding a teleconverter. Also note that not all lens work well with a teleconverter. The new Tamron 70-200/2.8 is softest at 200mm so your mileage will vary and is more a case-per-case in terms of usability. – unsignedzero Sep 12 '16 at 16:30
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    It all depends on the zoom. The EF 70-200mm f/2.8 L IS II takes a 2X as well as any lens. – Michael Clark Sep 12 '16 at 16:33

No - that's still using AF (auto-focus). It will no longer work because your lens will become a 140-400 F8, and that's just too dark for the AF sensor to function.

So you'll have to use MF i.e manual-focus by switching the lens to MF from AF and then manually focussing via the focus ring.

  • Doesn't that depend on what you're shooting? – nbubis Sep 12 '16 at 12:57
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    I don't think so - the camera can't AF at those apertures. Pretty sure it won't regain the ability if the subject is right. – Steve Ives Sep 12 '16 at 13:42
  • @SteveIves I assume the comment is suggesting that it might depend on the amount of light in the scene. f/8 being too dark logically would depend on whether you were shooting indoors or in full sun – MikeW Sep 16 '16 at 21:47

I understand that I will lose AF after f/5.6(?) with this body,

The maximum aperture of the lens is what your camera uses when it's autofocusing. Increase the focal length without increasing the aperture opening, and the f-number increases (i.e., f-number = focal length / aperture diameter). Double the focal length, add two stops. So your 70-200 becomes a 140-400 f/8 lens.

At f/8, the camera doesn't have enough light to "see" by to focus.

but does that mean that I need to use my hand for manual focusing?

Mostly yes, and possibly no.

If you get a non-reporting teleconverter (i.e., a non-Canon 3rd-party that doesn't show up in the EXIF), the camera will still think it's attached to a 70-200/4 and will attempt to autofocus, vs. knowing it's attached to a 14-400 f/8 and not even try.

But, it still probably doesn't have enough light to properly autofocus by, so it may hunt, chatter, or have a really hard time locking on target. I used a 3rd-party 1.4x converter with my 400/5.6L USM on a 350D, and ran into this about one time in five (I live in Southern California where sunshine is copious). So, no matter what you do, AF is, at best, compromised.

There are reasons to save up the pennies and just get a 400mm lens. Using a TC never really equates to having the longer lens.

I normally never use all the AF points. I use the central AF point, half-press the shutter button, then move my camera to where I want, keeping my focus locked and taking the picture. Will this way of focusing still be possible, or does this too count as AF?

Unless you're turning the focus ring on the lens, it's AF.

If you use the Canon brand 2x converters, you will loose autofocus. Including the technique you described. It is my understanding that some third party teleconverters (Kenko?) usable on Canon do not turn AF off.

As someone who used both TC 1.4x and TC 2x, I would suggest using the 1.4x. The image quality is better and AF faster (on appropriate lenses). If you truly need 400mm, get a lens like the 100-400. Unfortunately, with long lenses, there is no perfect cheap solution...

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