I have a Canon T6i and mainly use a Sigma 18-250 lens. I photograph lots of stuff (landscapes, my kids, sports, architecture, wildlife, etc). I have a good wide angle lens for the landscapes. I want to upgrade my lens, and am considering the Canon EF 70-200mm f/2.8L IS III USM. But then I got to thinking perhaps my body will limit the lens capability as my body is not full frame. I could consider upgrading the body. What I'm after: exact focusing even at a distance, bright colors, zoom capability for surfing photos and wildlife. Any thoughts on this lens? Should I upgrade the body, and if so, what should I consider? I'm okay with spending money, but don't want to be stupid about it. I'm not a professional, and I take very good photos with the equipment I have. I know the photos could be even better with better equipment. thank you
I recommend that you upgrade neither lens nor body until you have a clear reason to do so. As it is, I see no reason for you to upgrade at this time. Perhaps there is additional information you can add to your question to make your case.
I have a Canon T6i and mainly use a Sigma 18-250 lens... I want to upgrade my lens...
Why? What about your current lens do you feel is holding you back? Do you have any sample images that you feel could be improved with a different lens?
... considering the Canon EF 70-200mm f/2.8L IS III USM
That's an expensive lens that can be used to take amazing pictures, but it can also be used to take mediocre pictures. What problem do you think this lens solves that your current lens doesn't?
... exact focusing even at a distance...
Have you tried stopping down to increase depth of field when zoomed in?
... bright colors...
Have you tried changing and customizing the color profiles on your camera?
... zoom capability for surfing photos and wildlife...
Does your current 18-250 lens have sufficient reach for surfing and wildlife? If not, the 70-200 won't either.
I know the photos could be even better with better equipment.
How do you know this? You have not clearly identified how your current equipment is deficient. It's not clear to me that "better" equipment necessarily results in "better" photos. For instance, these were taken with broken lenses:
But then I got to thinking perhaps my body will limit the lens capability as my body is not full frame.
I use both full frame and APS-C cameras. For 99% of the time, the EF 70-200mm f/2.8 L IS II is attached to an APS-C EOS 7D Mark II. I use the FF bodies for wider angle lenses and for shooting in very low light, when I use fast primes ranging from a 35mm f/2 to a 135mm f/2. The only time I really consider pulling the 70-200mm off the 7DII and putting it on the 5DIII is when I want to be able to shoot with one body and have the wider angle of view that 70mm gives on a FF camera compared to 70mm on an APS-C camera (110mm FF equivalent) and don't need the extra reach that 200mm on an APS-C body (320mm FF equivalent) provides.
What I'm after: exact focusing even at a distance, bright colors, zoom capability for surfing photos and wildlife.
Focusing tends to be slightly better on FF DSLR cameras with wider baselines for comparing the light coming from each edge of the lens than for APS-C DSLRs with narrower sensors and mirrors, but the AF system of many APS-C cameras is still better than the skill and ability of many of those using it. Until you've mastered the AF system of your camera, which is decent enough for many shooters to get good sports shots, it's not the camera holding you back. The Rebels do miss the option of doing AFMA (Autofocus Microadjustment), but the 50D, 70D, 80D, 7D, and 7D Mark II are APS-C cameras from Canon that do have that capability along with the full frame 1-series and 5-series models since 2007. Again, until you're experiencing AF performance that misses by roughly the same amount in the same direction consistently, doing AFMA won't have much of an impact on your results. Note that AFMA is irrelevant for mirrorless cameras, as AF is done by the same sensor taking the photo in a process that is more of a closed loop.²
Color differences between different lenses are very subtle. Back in the film days, when it was much more difficult and time intensive to make small changes to the color of an image, this mattered a bit more than it does now. With digital photography light is the most important factor that affects color, followed by the skill used to post process the raw image data into a color image.¹ Differences between lenses with regard to color are relatively minor and have little effect on the final appearance of the image in a digital environment.
Your 18-250mm lens zooms to 250mm. That's longer than the 200mm 70-200mm lens at 200mm when focused on infinity. When used on an APS-C camera, the 35mm equivalent field of view is that of 400mm and 320mm, respectively, for the two lenses. Going from 250mm on a 1.6X crop body to 200mm on a FF body will give you exactly half the "reach" you have now.
That's not to say the EF 70-200mm f/2.5 L IS III isn't a better lens in many ways. It's certainly much sharper than the Sigma 18-250mm f/3.5-6.3 DC OS HSM when both are used on a Rebel T6i. But brighter colors and more "zoom" are not in the list of advantages.
With that much difference between the acutance of the two lenses, it can probably be argued that using the better lens and cropping to make up the difference in "reach" will produce a better image. With a maximum aperture of f/2.8, there's also the possibility of using a teleconverter or extender. The EF 70-200mm f/2.8 L IS III takes Canon extenders very well. The EF 1.4X III would give you 98-280mm f/4 and the EF 2X III would give you 140-400mm f/5.6.
Both Sigma and Tamron make 150-600mm f/5-6.3 stabilized lenses. Both the Tamron and Sigma Contemporary lenses have been well received by many hobbyists that shoot in daylight. Both are compatible with the maker's respective USB lens dock that allow the end user to perform firmware updates and detailed AF calibration without sending the lens to a service center. Tamron also makes the SP 70-200mm f/2.8 Di VC G2 that holds its own rather well against the Canon EF 70-200mm f/2.8 L IS II/III. That lens is also compatible with Tamron's Tap-In console that allows the end user to calibrate AF even if their camera does not have AFMA capability.
Sigma has also recently introduced a 100-400mm f/5-6.3 as part of their Contemporary group within the Global Vision lens series.
The older pre-Global Vision series' 50-500mm, 80-400mm, 120-400mm and 150-500mm offerings from Sigma weren't on the same level as their recent Art, Sports, and Contemporary lines. Particularly at the longest focal lengths, they tend to be fairly soft.
I know the photos could be even better with better equipment.
Please don't misunderstand the following as flippant or taking a cheap shot at a budding photographer. It isn't. It is an encouragement to decide to put in the learning and practice to develop the technique and compositional skills that better images truly require, rather than chasing better image quality through endless GAS (Gear Acquisition Syndrome). For another take on GAS, please see the well circulated Letter to George.
There's a saying that has been around photography for a long, long time: Gear doesn't matter.
This is true, but it is only a part a larger truth.
The fuller truth is: Gear doesn't matter... until it does.
So what does that mean? It means that until the gear in some way limits the photographer from doing something that the photographer already envisions and has the technical knowledge, skills, and ability to pull off then the gear is not the limiting factor - the photographer is.
The best way to improve image quality is, in fact, to improve one's skill as a photographer. Even when quality of gear becomes an issue, the ability of the photographer to diagnose the issue and know what is needed to correct it is still the paramount consideration. For the most part that ability comes with experience and practicing proper technique.
For more about how to know when one needs to upgrade, please see:
When should I upgrade my camera body?
Should I upgrade my Canon body or lens for upcoming travel?
After 2 years of amateur photo, buy a new body or a great lens?
Which lens for portraits using an APS-C camera?
Should I upgrade my body or lens first?
¹ All raw image data must be processed to produce an image. It may be processed based on the skill of the software engineer that wrote the camera's firmware that produces a JPEG from the raw data collected by the sensor, it may be the skill of the software designer that wrote the default processing routines of a raw conversion application, or it may be the skill of someone sitting at the computer and making their own decisions - but it is certainly someone's skill that results in an image appearing the way it does.
² At the risk of breaking SE's product recommendation prohibition, if you're serious about FF there are several mirrorless EOS R models on the horizon and one already in the wild. The issue with the EOS R already out is for some of your use cases, such as surfers, sports, and active wildlife, the frame rate when doing AF tracking between frames is woefully slow. The models slated for late 2019 might be better than the initial EOS R is.
I think that there is no reason at this time to upgrade the body. If you buy the lens and it works fine for you then you can use the mony saved later. If you ONCE you own the lens finds the body limits you, be it minimum focal length or focus speed etc, THEN consider buying a new body. You lose nothing by upgrading one at a time and potentially save a lot of money if the current body works fine for you.