I believe that you are correct about the 1D series bodies, but incorrect regarding the 5D4. Only 1D series have a menu option that overrides the spot metering only at the center AF point behavior. You can link spot metering to the selected AF point. My 1D4 allows linking of the AF point to spot metering.
The fact that Spot Metering only takes place with the center AF point on most of my camera bodies is why I do not bother with moving AF points around to focus on subject. You are not able to [spot] meter at the AF point, not unless you are using the center AF point.
To get around this I enable all AF points in AI Servo mode, using the center AF point as the starting AF point. I lock focus and exposure with the center AF point. I then recompose the shot. As I recompose, the AF tracking will keep the subject in focus. The exposure remains locked until I fire the shutter.
I believe what you are asking about is called "AF Linked Spot Metering".
Canon has had it on some cameras since the EOS-3 film camera in the 1990s. With the 45-point AF system of that camera, you could link spot metering to select points... 11 or 13, if I recall correctly. To use the spot metering feature, you first have to set a Custom Function that restricts the camera to use only those points.
The EOS-1V and most (all?) 1D-series cameras have had the same or similar AF Linked Spot Metering.
No other Canon DSLRs offer it. They have Spot Metering at the center only. That includes all the 5D-series, 6D-series, 7D-series, xxD, xxxD and xxxxD models.
I don't know about the EOS R or EOS RP. (Searching online, several sites suggest AF Linked Spot Metering is not offered on either of those mirrorless cameras... not surprising since they have such a huge number of AF points and that those points cover almost the entire image area... far more points and much greater coverage than typical with DSLRs.) I'm pretty sure the EOS M-series mirrorless don't have it either. (Note: I don't have EOS R or EOS M cameras, so am just going by what I've read online or in the user manuals for the cameras. Not from hands-on experiences, like I have with a number of the DSLRs and film cameras.)
Supposedly there is an upcoming EOS R-series aimed at very high performance sports shooting (I'd wager in time for the Summer Olympics, next year).... I wouldn't be surprised to see AF Linked Spot on that model. Another EOS R-series that's rumored is an ultra-high resolution model, but I wouldn't expect to see AF Linked Spon in it, just as it isn't on the 5DS/5DS-R.
FYI: "Evaluative Metering" is semi-AF linked. While it meters the entire image area, it puts extra emphasis on the area immediately around active AF point(s). This give results similar to AF Linked Spot Metering. I used the latter many years ago, when I shot with a pair of EOS-3. I've used Evalutive Metering frequently with more modern cameras.
I occasionally use Spot Metering with them, which is no problem since I use the center AF point only a lot of the time... but Spot Metering is more specialized and requires more careful attention to adjustments with Exposure Comensation or setting exposure manually. I shoot a lot of fast action/sports where it's simply not practical to use Spot and have to constantly be tweaking things with E.C., etc.
If you are doing professional portraits, then you should have a light meter. Are you using a Speedlite? A fill flash would help to correct the exposure for backlit portraits. So would strobe mono lights.
I have an M3, it seems able to do spot metering at any of its' AF points. I'm not really sure how many AF points it has, anymore. I can move the AF point all over the rear LCD, and the exposure changes when I move from light to dark areas.
Thanks everyone! My understanding is that Nikon's do meter on the active / selected AF point(s). It's one of the reasons I considered switching a while back. I do a lot of backlit portraits so using anything other than spot metering will not give me an accurate reading.
Right now with my Canon I basically have to look at my LCD to check if my subjects face is exposed properly which means that every time we move spots I have to take 'test shots'... It's a big waste of time.
I tend to shoot at f2.8 and not that far from the subject so sometimes focusing and recomposing will result in an image that is not sharp.
I was hoping that with the mirrorless line Canon took the opportunity to address metering.
If there is a solution other than my constant test shot approach please let me know!
No need to switch brands or even take a test shot. You're making Spot metering more difficult than it needs to be.
With your Spot meter centered on the subject, half press the shutter release to activate the meter and then press the AE Lock (*) button. This will retain whatever exposure the Spot meter determined for some 10 or 15 seconds, during which you can recompose as you see fit, focus and take the shot. Very simple and it works great for one or a few quick shots of a backlit subject.
Practice this a little and it soon becomes second nature.
If you are going to be shooting a series of shots of the subject, I'd recommend using the above to make a test shot, then reset the camera to manual mode (no Auto ISO) and set everything yourself. Once that's done, you can shoot away without concern, until the light changes or you and the subject move to different lighting.
If you need to shoot really fast, try Evaluative metering. It gives sort of a "semi" spot metering effect. While it measures the overall scene, it puts extra emphasis on the area right around the subject. Depending upon how much of the image is filled by your subject and how strong the backlighting, it can work pretty darned well.
I love backlighting, look for it and use it a lot. It's often helpful with portraits, so that subjects aren't "squinting" against the sun and look relaxed. Rim lighting can be nice, too.
I agree with Wadizzle, though, that sometimes fill light is needed. In the image below, it was simply the sun bouncing off a white painted wall to my left that served as fill (shot would probably have been impossible, without that fill light)....
Or, sometimes flash is needed, such as with this backlit subject...
All the above were done using Evaluative metering... not Spot.
Along with Spot metering and AE Lock, you can use any AF setup you like... Pre-selecting a single off-center point will usually give you the best focus accuracy. Keep in mind that your particular cameras have one dual axis/high performance AF point at the center. The 6D's other ten AF points, and the T1i's other eight AF points are all single axis and slightly lower performance. (Regarding the older film SLR... if I recall correctly it has three AF points, but whether all are singe axis or dual axis or a mix, I simply don't know.)
Some newer DSLRs including the 6DII, 80D, 90D, T7i, 77D use a 45-point AF system where all points are the higher performance dual axis type. 70D and 7D use 19-point, all dual axis type. 7DII use 65-point, all dual axis type.