jeep_daddy Loc: Prescott AZ

Here's what you are missing.
1. If you are using the EF 100-400 (original version) neither one of the Teleconverters will allow you to use autofocus. This is a pain in the ass.
2. If you are using the new EF 100-400 Mark II, and using it on a newer body than the 70D; such as the 80D, 7D Mark II, 5D III, 5D IV, it will work mostly only with the 1.4x and center point focus. The 2x will not work with that lens.
3. When using the 1.4x you lose one stop of light. This means that when you open your aperture wide open on the 100-400 lens at say 400mm, your normal aperture is f/5.6 but with the 1.4x it will now be f/8 and if you put the 2x on you will not only lose autofocus but your f/stop will be f/11
4. Almost forgot to tell you that in most cases, you also lose some quality. Simply put, the images don't quite look as sharp with Teleconverters. If you use the 1.4x on the most modern fast lens and most modern camera body, the quality is very good, and in most cases you can't tell much difference. But there is a slight difference. On older lenses and camera bodies, you can see quite a bit of image quality difference. The quality degrades more with the magnification of the 2x Teleconverter.

Retired fat guy with a camera Loc: Colorado

If you want to keep your images sharp, do not buy a extender. I had both, a 1.4 and a 2. The 2 was awful and the 1.4 was nearly as bad.
Save your money, buy a good pair of comfortable shoes, and walk closer to your subject. You will save yourself a whole lot of anguish.
I used mine with a Canon 1Ds Mark lll, a older camera, but still a pro body, and a 300 mm non IS L . Which according to all that is holy sharper than the IS. A pretty good combination for a tack sharp image. And, without the extenders it is. With them Trash.
There is no way to recover a bad image, photoshop can do only so much. I know I have tired.

amfoto1 Loc: San Jose, Calif. USA

With the 1.4X your 100-400mm will become a 140-560mm with an f/8 aperture at the longer focal lengths. Depending upon what camera you have, it may or may not be able to autofocus, or may be limited in the AF points that can be used. For example, original 7D and 70D cannot autofocus an f/8 combo at all... but 7DII can (center AF point only) and 80D can (at up to 27 of it's 45 AF points).

With a 2X on your 100-400mm it will become and effective 200-800mm with f/11 aperture at the longer focal lengths (and at least f/9 at the shortest). This combo will not be able to autofocus on any Canon camera.

There also is greater degradation of image quality with a stronger 2X teleconverter.

You didn't mention what camera you're using. If it's a full frame, you might be better served putting the money you were going to spend on the TC into an APS-C/crop sensor camera instead (some of which can be bought for not much more than the cost of the Canon III series teleconverters). An APS-C camera acts like a "free 1.6X teleconverter". By "free" I mean there's no loss of light the way there is with an actual teleconverter. With your 100-400mm on a Canon APS-C camera, compared to full frame you would get the effect of a 160-640mm lens with the same f/4.5-5.6 aperture range you have now. Also, the image quality with the 100-400mm without TC on a recent 20 to 24MP APS-C camera will be better than the 100-400mm with a TC on full frame.

Finally, you didn't mention which 100-400mm you have. I haven't used both and compared them myself, but reportedly the "II" works a lot better with teleconverters, than the original lens does.

I do use both 1.4X and 2X Canon teleconverters (mine are series II). They are very high quality, but IMO the 2X works best with super telephoto primes such as the 300/2.8, 400/2.8, 500/4. I haven't been happy with it on any zooms. The 1.4X works best on primes, too... but IMO is also marginally useful on a few of the highest quality zooms such as the 70-200mm Canon models. I have yet to test the 1.4X thoroughly on 100-400 II, simply because I rarely need that much focal length using the lens (on APS-C cameras, mostly).

The Kenko 1.4X MC-4 and Pro 300 rival the image quality of their Canon equivalents. The cheaper 1.4X MC-4 is sharpest in the center, but less so toward the corners and edges.... that's not typically a problem when using super long focal lengths, but this TC might be best for use on an APS-C camera that only uses the central portion of the image anyway. The slightly more expensive 1.4X Pro 300 is a tiny bit less sharp in the center, but has better corner sharpness and so might be a better choice for use on a full frame camera.

Note: the Kenko "HD" 1.4X is their latest model and is a bit more expensive than either of the other two Kenko models (but less than half the price of the Canon TCs)... It's primary differences are that it's the only TC that can be used with Canon EF-S lenses and it uses fewer elements, a simpler optical formula, which Kenko claims makes for better resolution than either of their other two TCs.

Canon TCs certainly aren't "TRASH" with the right lenses....

EF 1.4X II on EF 500mm f/4L IS USM:







EF 1.4X II on EF 300mm f/4L IS USM:

robertjerl Loc: Corona, California

Beg to differ on your #1 - I used a 100-400 mk I with a Canon 1.4x III for over a year before I got a 100-400 mk II and it did AF on my 7DII just fine. Note, later in the thread he states he has both a 7DII and T3i.

CHG_CANON Loc: the Windy City

As noted already, this is incorrect. Any EOS body, including specifically your 7DII, that maintains AF at f/8 works with the old 100-400L and a 1.4x, same as the vII of the 100-400L.

Architect1776 Loc: In my mind

Architect1776 Loc: In my mind

CHG_CANON Loc: the Windy City

Everytime the discussion wanders into a loss of sharpness with a x1.4, I feel compelled to post the following (5dIII, 100-400II, 1.4III, 1/500, ISO-2000, 560MM). Feel free to click the URL and zoom a bit further into the details from the Flickr host .... I have hundreds more images in my Flickr profile with both the 1.4x and 2.0x and various compatible lenses in different applicable situations.

Chicago area Dragonflies by Paul Sager, on Flickrhttp://farm8.staticflickr.com/7659/27971305555_3ea4be7d03_h.jpg

Architect1776 Loc: In my mind

They are jealous of the 100-400 MII by Canon and how well it works and how incredibly sharp it is with TCs, nothing else.

Retired fat guy with a camera Loc: Colorado

What these posts show are the good shots. What you do not see, is all the bad ones. Which in my experience, out weigh the keepers.

imagemeister Loc: mid east Florida

Then, YOU are doing something wrong .....

Szalajj Loc: Salem, NH

Which version of the 100-400 lens do you have?

Matching the version of the lens to correct version of the teleconverter is IMPORTANT.

Make sure that you don't end up with a mismatch.

chikid68 Loc: Tennesse USA

I shoot my rebel xt1 at f8 and beyond frequently and have no problem with autofocus

CHG_CANON Loc: the Windy City

In the context of this discussion, no you don't retain autofocus at f/8 ...

The issue is the maximum aperture of the lens / lens + extender combo. A Rebel body and most EOS cameras historically outside the 1D series, their AF can autofocus a lens up to f/5.6 as a maximum aperture of the lens. This is different than capturing an image at f/8, f/11, f/22 and so forth. When focusing the lens aperture is opened to it's widest point. When the shutter is released, only then does the camera close the aperture to the value selected.

When a 1.4 extender is added to a f/5.6 lens, the max aperture becomes f/8 due to the 1-stop loss of light. A camera, such as your XTi that is sensitive only to f/5.6, cannot autofocus the f/8 combo.

kdogg Loc: Gallipolis Ferry WV

Not enough in the budget for L glass but I have a Canon 100-300mm 5.6 usm mounted with a tamron 1.4 ext on my Rebel T5 and the auto focus works fairly well. I use BBF and a mono pod for BIF. Granted the fails outweigh the keepers but the combination was better than the 70-300mm kit lens. Will be using this combo till I can squirrel away the price of some really good Canon L glass. GAS has always been with me. LOL