Basic 5DM3 Light Meter Questions

I'm still pretty new to photography. I'm using a 5DM3 and trying to learn metering.

1. Can you enable the light meter to show how much light you are letting in while in any mode other then M?
2. If not, how do you properly meter a given photo using AV or TV?
3. In Spot Metering mode, if the meter is showing +3 based on what I'm metering on, does this mean it will absolutely be overexposed and clipped? Just want to make sure I understand the concept of this.
4. Is BBF the only way to lock into a given metered setting, and still be able to change the focus without it re-adjusting the metering?

Thank you.

Understand that there are three different concepts to camera use:

1. Setting controls:

   shutter, aperture, and ISO can be done all by you (Manual setting) with no automation,
   or with one or more controls automatically set by the camera (Tv, Av),
   or completely autmatically by the camera (Green Box)

2. Metering: measuring the light reflecting back from the subject
   ...this is a key principle to embed in your brain: the camera meter always tries to suggest settings which will result in the target being exposed so that it results in 'mid-tone gray'.

   If you aim the camera at a white wall, it suggests settings which record that wall as mid-tone gray!
   If you aim the camera at a black car so it fills the frame, it suggests settings which record that car as mid-tone gray!
   If you aim the camera at a mid-tone gray rock, it suggests settings which record that rock as mid-tone gray!

3. Metering area:

   a very small area (Spot),
   a slightly bigger area (Partial),
   the full viewfinder with a priority bias toward the middle (Center weighted),
   the full viewfinder with a priority bias toward the area where AF (Evaluative)
   

BBF is merely a means of taking the focus activation away from the shutter button. So you can pre-focus at a certain distance and not worry that camera refocuses simply because you pressed the shutter button...it makes the AF camera behave more like a traditional manual focus SLR, which also does not alter focus simply because you pressed the shutter button!

BBF does NOT 'lock a meter setting'...the '*' button does that! (Exposure Lock).
So you can use Exposure Lock to still be able to change the focus or reframe the shot without it re-adjusting the metering.
Or you can simply use Manual mode in which YOU do the all the setting of controls, and not the camera.

When in other modes than M mode, the camera's metering system shows you the level of incoming light by showing you the combination of exposure time, aperture and amplification (ISO) you need to render a standard exposure, as explained by Wilt above.

I full M mode, you set the combination of time, aperture and ISO. Since the camera can't change any of them, it will instead show how far away from standard exposure you are.

When you spot meter, and lock the exposure at the level metered from some part of the subject, then subsequent meterings will be in reference to that point. +3 implies that what you are aiming at now is three stops (eight times) as bright as the spot you used for reference. Whether +3 renders clipping or not depends on the reference. The camera can be set to underexpose the reference (exposure compensation) even in automatic modes. Thus +3 doesn't have to imply three stops above standard exposure, just three stops above the reference.
Cameras today typically have a dynamic range of above twelve stops, so +3 will not automatically imply clipping.

Canon cameras only lock exposure implicitly when focusing if you use One Shot AF together with evaluative metering. If you think about how evaluative metering works, you quickly realize that this is necessary. But in all other combinations of modes, there's no implicit exposure lock when focusing, regardless of how focusing is initiated.

southwestform wrote in post #18320623
2. If not, how do you properly meter a given photo using AV or TV?

What was said above, +...
Even in M, you have to learn to interpret the settings that your meter is giving you. Here's an example of what happens in one of the "Auto" modes, including Auto ISO: Post #47

This is what I use most of the time. Maybe it will help: Need an exposure crutch?