What happens when a camera in Av or Tv mode cant meet my exposure/meter requirements?

On my 5D3 + 50mm 1.4, I am shooting a person outdoors with clouds moving so the exposures changing constantly, I am asking the camera to meter at 0/center. Constantly changing ISO and shutter speed slows me down too much (even though the process is fast) so I switch to Av mode.

I set my minimum shutter speed to 1/125, and maximum ISO to 1600. Works fantastic as I can vary the aperture as needed and not have to worry about the rest. But when the sun sets and we lose light, the camera has to raise ISO or slow down the shutter which I am telling it not to do. If I recall correctly, I saw my shutter speed dropping way below what I told it not to, but did not raise the ISO past 1600. Is this normal? And if so, is this how it meets my meter requirement by breaking the rule?

In Av mode, you control the aperture, and the camera controls the shutter speed. It will go to as slow a shutter speed as it needs, given the aperture and ISO you have set.

I'll have to check when I get home, but I think there's a setting that lets you choose whether ISO or exposure time gets compromised under those conditions.

paddler4 wrote in post #16669160
In Av mode, you control the aperture, and the camera controls the shutter speed. It will go to as slow a shutter speed as it needs, given the aperture and ISO you have set.

With some cameras, you can specify a lower limit for shutter speed.

Well yes the 5D3 has a minimum shutter speed option but I dont understand how its minimum if the rule is not followed? I am pretty sure the rule was being followed for the most part, until it had absolutely no other choice and it capped out the ISO already. Granted, I totally understand if it needs to break the rule to maintain the exposure. I just want to understand whats happening

For me dropping below 1/125 means most of the shots will be blurry so my portraits will be useless

My 6D does this. It is called 'safety shift'. You can select whether the aperture/shutter (Tv/Av) is adjusted, or the ISO. If you are happy with a metered "0", try setting the camera to MANUAL mode, select the aperture and shutter speed you want, and let auto ISO do the rest. Works like a charm on the 6D.

If you have a minimum shutter speed and a minimum aperture size in mind, the best auto-metered mode is M with auto ISO. This will always give you the lowest possible ISO value for the available light given your other hard limits.

Now that auto ISO works properly (excepting no EC, and not with flash) there are fewer and fewer situations where Av and Tv mode make any sense to use.

JeffreyG wrote in post #16669215
Now that auto ISO works properly (excepting no EC, and not with flash) there are fewer and fewer situations where Av and Tv mode make any sense to use.

Having no EC with auto ISO is a big issue for me so I shoot Av. When using flag, I just pick a reasonably high ISO value.

I expect EC with auto ISO in manual to trickle down to the 5D3 eventually.

As for flash with auto ISO, is there a reason why Canon sets it to 400?

frugivore wrote in post #16669256
As for flash with auto ISO, is there a reason why Canon sets it to 400?

Because they are totally stupid?

Maybe it's because they still think it's 1994 and you need to change out the roll of film in order to change the ISO value?

I don't know, but I can say that the general approach to controls on most modern cameras still acts like the cameras are film based in some ways, and the automation around ISO is one of the biggest ones.

EOS5DC wrote in post #16669207
My 6D does this. It is called 'safety shift'. You can select whether the aperture/shutter (Tv/Av) is adjusted, or the ISO. If you are happy with a metered "0", try setting the camera to MANUAL mode, select the aperture and shutter speed you want, and let auto ISO do the rest. Works like a charm on the 6D.

JeffreyG wrote in post #16669215
If you have a minimum shutter speed and a minimum aperture size in mind, the best auto-metered mode is M with auto ISO. This will always give you the lowest possible ISO value for the available light given your other hard limits.

Now that auto ISO works properly (excepting no EC, and not with flash) there are fewer and fewer situations where Av and Tv mode make any sense to use.

Yeah I agree. However, sometimes I like to overexpose 1/3 or 2/3 of a stop because the meter gets fooled by the snow outside or maybe because I like to make my skintones brighter so I really need EC. I think ill stick to Av mode and start going into manual only when the shutter speed slows down beyond usable.

OP,
With my 600D it underexposes. it never breaks the rule. In Shutter priority mode Aperture value will keep blinking as a indication after maxing out on ISO and Aperture. With my F1.4 lens (Shutter priority , 1/125, ISO 3200max and F1.4) I see 1.4 value blinking on viewfinder instead of solid number in this situation. I can still take the picture but it is underexposed. Hope, I am not missing something.
Probably, same thing with Aperture priority mode with blinking shutter speed.

I am still confused about better way to manage EC in Manual mode with Auto ISO. There are no 1/3 iso values if I set it manually. So, it is full stop change to adjust everything manually. If i need EC, I am using Av mode during the bright light situations and Tv mode during low light situations.

khwaja wrote in post #16669805
OP,
With my 600D it underexposes. it never breaks the rule. In Shutter priority mode Aperture value will keep blinking as a indication after maxing out on ISO and Aperture. With my F1.4 lens (Shutter priority , 1/125, ISO 3200max and F1.4) I see 1.4 value blinking on viewfinder instead of solid number in this situation. I can still take the picture but it is underexposed. Hope, I am not missing something.
Probably, same thing with Aperture priority mode with blinking shutter speed.

I am still confused about better way to manage EC in Manual mode with Auto ISO. There are no 1/3 iso values if I set it manually. So, it is full stop change to adjust everything manually. If i need EC, I am using Av mode during the bright light situations and Tv mode during low light situations.

I find similar results as this. Both with the 600d and the 60d. So are you using ec because you don't have enough aperture or iso to get the needle to he middle of the reading? Or so you find that the camera underexposes in certain light when the needle is pinned in the middle. Because these are two different things. I actually started a thread about this a couple days back.

khwaja wrote in post #16669805
I am still confused about better way to manage EC in Manual mode with Auto ISO. There are no 1/3 iso values if I set it manually. So, it is full stop change to adjust everything manually. If i need EC, I am using Av mode during the bright light situations and Tv mode during low light situations.

When the dial is in the "M" position, Exposure Compensation (EC) is totally non-functional even with Auto ISO selected.

When using Av, auto ISO, a max selectable auto ISO level and a limit for longest exposure time, what you saw is what you get.
The camera will use the aperture you set and the lowest ISO you set for the auto ISO selection.
When it gets darker the camera will make the time longer.
When the time reaches the longest you allow, the camera will start raising the ISO instead.
When you reach the max allowed automatic ISO level, the camera will again make the time longer.
If you reach the longest allowed time, you get underexposure if you haven't set Safety shift Av/Tv. If you have, the camera will open the aperture until it's at the limit for largest opening.

Canon set auto ISO to 400 when using flash not becuse they are stupid, but because they know how flash photography works. Since E-TTL flash metering happens just before exposure, there's no way to know what the most appropriate ISO is beforehand. So they need a reasonable default, and set that each time.

I have never used Auto ISO so I was curious about this one. I found this on page 128.