dreklaw wrote:
I have a similar inquiry and can’t seem to find info. I have a Sigma 150-600C lens with my Canon 70D crop camera and am thinking of getting a 1.4x teleconverter, and need to know how to tell which one, if any, would work, please. Any thoughts? Thanks so much, in advance.
Canon 70D is an "f/5.6 limited" camera. This means it's AF system requires at least as much light as an f/5.6 maximum aperture lens provides to be able to autofocus with the phase detection system seen in the camera's viewfinder. The Sigma 150-600mm is an f/5-6.3 lens. It "fools" the camera into autofocus f/6.3, since that's only 1/3 stop less light than f/5.6.
If you add a 1.4X TC to that Sigma it will "become a 210-840mm f/7.1-f/9" that will be unable to focus on your camera using the .
It also will make manual focus difficult because your viewfinder will be darkened.
And adding any teleconverter will have some effect on image quality. With a third party zoom like the 150-600mm there's good chance the IQ will be so poor that it's simply not worth the effort.
If you switch to Live View the camera uses a different AF system called "dual pixel CMOS". Even with slower lenses and lens/teleconverter combos, this will be able to focus to a limited extent. However, when the light levels are low expect it to be slower focusing and possibly to fail to focus at all at times. In addition to the max aperture of the lens/teleconverter combo, how well it focuses varies depending upon lighting conditions, subject contrast and more.
Some Canon DSLRs are "f/8 capable", meaning they can at least partially focus lens/teleconverter combos that have smaller max apertures. The 7D Mark II is able to focus f/8 at the center focus point only (out of 65 points). The 80D can do the same, but also can focus f/8 with as many as 27 of it's 45-point AF system with certain lens/teleconverter combinations.
Canon mirrorless R-series cameras can focus f/11 and even in some cases f/16 lens/teleconverter equivalents. One reason for this is the camera's have no mirror. In the DSLRs the light has to pass through a semi-transparent mirror to reach the AF sensors. A significant portion of the light entering the DSLR is "lost" to that mirror (reflected into the viewfinder for you to see). There's no such "loss" with mirrorless cameras.
But, the whole point of lots of telephoto "reach" with a long lens plus a teleconverter is to shoot very distant subjects. Regardless of all the other factors, you will ALWAYS be shooting through a lot of atmosphere, which will often have a negative effect on image quality. In addition, such extreme focal lengths will be very difficult to hold steady to avoid any camera shake blur in images, even with optical image stabilization built into the lens. The longer focal length will magnify any movement, plus you will need to use slower shutter speeds to compensate for less light passing through the lens/teleconverter. You're almost certain to need a tripod or at least a monopod much of the time. Alternatively, you could instead raise ISO to compensate for the lost light, but that risks adding digital noise to images, while reducing dynamic range and resolution to some extent.
There's "no free lunch". 600mm on a Canon APS-C camera such as the 70D is equivalent 960mm on a full frame camera. That's a WHOLE LOTTA LENS even without any teleconverter! The BEST thing you could do is simply get closer to your subjects. That will avoid all the above problems and negative effects. Depending upon what you're shooting, sometimes you can draw subjects closer... such as using calls or food to bring wildlife closer. But there will ALWAYS be subjects just out of reach. If you can't get closer or get the subjects to come to you, sometimes all you can do is stop shooting, sit back and enjoy the show.