I have a Canon EOS Rebel T7 which I am very happy with. I bought at the beginning of the year a Sigma C 150 - 600 mm lens for photographing birds.
I have been thinking of getting a teleconverter (1.4 x vs 2 x) to go with the current set up that I have mentioned for greater reach. From research I have read that there are problems with using a teleconverter in that they slow down the autofocus or you have to use manual focus.
I am looking for input from those in the Canon Community that have used them and their thoughts on this subject, the good and the bad experinces that you might have.
Thank you so much for your time in getting back to me.
A teleconverter will always reduce the image quality of a lens (sharpness, contrast, etc.) and the 2X is particularly bad. With that lens, I seriously doubt if you will be happy with the results. And even the 1 series bodies won't autofocus with a lens where the combined teleconverter plus base lens aperture is narrower than f8 and with your T7 you will lose a lot of AF range with the 1.4X.
I have Canon 1.4X and 2X converters and several of their top telephoto primes. The 1.4X does a very decent job with the primes and my EF 70-200 f2.8 but even with the best glass, I find the 2X results aren't great. I would try to borrow a 1.4X for a test before buying one for that lens because I don't think you are going to find it to be a useful combination with the reduction in quality and the problems with the focusing system.
While I have a different camera (6D2), I have the 150~600 C with the TC-1401 (1.4x) teleconverter.
One thing you'll definitely want to do is update the lenses FW to v2.0.0 or later.
1.0.3 brougt significant improvements to operating noise of the apeture blades. (honestly it never bothered me)
But v2.0.0 and later brings improvements to AF speed and enhancements to image stabilization modes.
Current FW is 2.0.2
If you don't have a Sigma USB dock, $59.. order one right away. FYI, they are mount specific.
While you can shoot with this lens handheld, I recommend a tripod. The moment you put a teleconverter on it, a micron of movement can cause blur. Not to mention your subject no longer being centered. Even a monopod is better than nothing.
You can only use AF to f8. I don't feel you need anything more than 1.4x. If you do, you need to get closer.
One thing to consider. You only have 9AF point on the T7. You may find yourself using manual focus quite a bit.
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"Thank you so much for your time in getting back to me."
I have done about as much experimenting with tele converters as anyone. In a word they are not worth it, so no don't buy one. As mentioned above a much better and far easier approach is to simply crop the photo in post edit. The second and even better approach is to get closer to your subject. The closer the better.
If you still have the hots to get one get the Sigma TC-1401 1.4x Teleconverter for Sigma SA for use on the big "C" super zoom.
I will doubt AF on your camera will work but Sigma has a way to fool the camera to make it think it has a faster lens than it does.
Now for some explanation of why this is going to be a bad idea even if AF works satisfactorily. On a Rebel T7 and a 600mm lens you are looking at an equivalent FL of 960mm. Conventional wisdom says you should never go below the reciprocal of the lens in SS. That would be 1/1000 as your slowest possible SS. In reality, 1/2000 is probably the actual limit. Keep in mind this is the slowest SS which means your ISO has to be in the upper range and perhaps limits of the T7. Or, just use it on very bright sunny days.
Now the additional problem with the equivalent FL. You are starting at 960mm but now adding a 1.4x tel-con you are approaching a FL of 1350mm. Man, this is some serious FL. Almost no one can handhold a lens in that FL range and get a decent IQ picture. So you add a tripod to the mix along with the bright sunny day and high ISO numbers.. Are you beginning to see why I don't recommend a tel-con? However, to find out for yourself get the Sigma TC-1401 1.4x Teleconverter for Sigma SA and give it a go.
"The single focus point of the T7 has it's limits but I have been able to get some good shots with it."
I have a 1DX and the Sigma 150-600mm f/5-6.3 DG OS HSM Sports Lens. I almost always use just the center focus point for birds. Although my 1DX will use more I prefer just the single center focus point.
The majority of the time I use a single focus point with my 1DX series bodies, either the center or the one above or below depending upon the sport I am shooting.
For sports, the only time I have run into an issue with a single point is with the very rare uniform that is so plain that it doesn't have any contrast for the AF to work with and then I will go with single plus expansion to four diagonal points. For wildlife, it is very rare when I come off of a single point to avoid the camera grabbing focus where I don't want it. But no matter which AF setup you use, it won't be perfect for every rapidly changing situation so you select based upon the most probable and accept that you aren't going to perfectly capture every opportunity.
Ernie's points on focal length are perfectly on target. Even with image stabilization, it is very tough to hand hold a really long focal length on target. I have shot a VERY FEW handheld photos with my EF 800 f5.6 plus 1.4x converter for a total focal length of 1,120 mm and although the length and weight of this combination makes things difficult, the greatest difficulty is keeping the target within the very small field of view. Once you get past 600mm, you are in the territory where 90% (or more) of the time you really need to be using a tripod.