Hoping someone can help me understand what is going on here... I'm still learning about using my flash so maybe expecting the wrong thing.... I have my camera set to Tv, and auto ISO, the lights are low in the living room and i take a shot of my sleeping black and white dog. The camera beeps to say it is in focus. Camera takes the shot at 1/125, f/4, ISO 8000. Picture somes out a bit dark. I then attach my speedlight... try to take the same shot and now the aperture is flashing??? ISO tries to use 400 so that makes sense - since it's underexposed ... so i then set the ISO myself to 8000 or higher and the Aperture is still flashing (which i know means that the exposure is not correct as far as the camera is concerned). Why is it happy with the scene without the flash but when i add the flash it's not? I tried a lot of shutter speeds (much slower) in combination with higher ISOs (still in Tv proority) and stilll i have a flashing aperture. I would have thought that since the flash is set to ETTL the camera recognizes that it is there and will provide the necessary light??? is this not the case? If i ingore the flashing aperture (which by the way is always reading f/4 for this shot) and bounce the flash the shot comes out well. if i aim directly at the dog it way over exposes (as you'd expect). Why is the aperture flashing when the flash is on/attached but not when it's off?
I didn't think it would do that - glad to hear it is normal behaviour 😉
i guess i was wrong in expecting something different 😉
Assuming you have the flash set to ETTL, the problem is that you have the camera in Tv mode.
The way Canon cameras and flashes work together, any time you are using an suto exposure mode (Tv, Av or P), the camera assumes that you are shooting by the ambient light, tries to set exposure by that alone, and just uses the flash as FILL, held back about 1.5 or 1.7 stops. Since you were in Tv, the camera was telling you that a larger aperture than was available was needed.
If you put the camera in Manual mode, it will treat the flash as FULL... as the main (even only) light source. You can use any shutter speed 1/200 or slower. It doesn't really matter much (unless you use a slow enough shutter speed that ambient light starts to record, too). The short duration of the flash (about 1/720) acts as your shutter, when using it as your primariy light source. Adjust the distance with your ISO and aperture settings, as needed. (Be sure to turn off Auto ISO, too... if using it.)
You probably could boost the FILL flash with Flash Exposure Compensation.... but if you change distances you will have to reset it, or risk the subject either being too dark or too light from the flash.
So it's better when you want the flash to be your primary light, to simply switch to M. You can still dial the flash up or down with FEC, but will need a lot less of it.
Tv, Av, or P plus flash = FILL
M plus flash = FULL
The only modes where the camera behaves differently because a flash is present is in full auto mode and also the Program mode.
In program mode, if you switch the flash off and meter the shot in low-ish light and note the exposure, you may be wide-open with a fairly low shutter speed (set a deliberately low ISO so you can see this... e.g. as I'm doing it while writing this reply, with no flash my camera wants to use f/4 and 1/15th sec.) If you switch the flash ON and re-meter, you get a lightning bolt icon on the back LCD screen (so the camera recognizes the flash) and while the aperture stays the same... the shutter speed bumps up to 1/60th.
In every OTHER mode... the camera behaves as if there is no flash and uses the flash as if it's a "fill" light. BTW... you WANT the flash to do this.
Flash (and frankly any light) follows a light fall-off rule called the "inverse square law".
The best tutorial I've ever come across on this is Adorama's Mark Wallace: https://www.youtube.com/watch?v=nk9cTa3UthM
Assuming you watched that... suppose you are taking a photo of a friend who is standing 7' in front of your camera and flash. Suppose we point the flash straight ahead (even though we could get better lighting using other techniques -- we'll keep this basic) The amount of flash power needed to properly expose a subject at 7' (whatever that is) will continue to spread apart as it gets farther from the camera ... that's what light does... it spreads out. Every time the distance increase based on the inverse of the square of 2 (in other words 1.41 ... which is roughly the square root of 2) the amount of light reaching that distance will be HALF as bright. 7 x 1.41 is about 10'. So your 7' friend is nicely illuminated... but anything just 3' farther away is getting HALF as much light. If you double the distance (14') it's half again... or ONE QUARTER of the total light. At 20' it's half as much again... or 1/8th as much light. So this is why you get a bright foreground subject and the brackground looks hideously underexposed. There are several options to fix this, but one of the easiest ways to is to "drag the shutter".
"Dragging the shutter" simply means that you deliberately leave the shutter open LONGER than needed considering you are using a flash. This allows the camera to expose for the AMBIENT light, but punch the subject with flash to "freeze" that subject. The result is a beautifully will illuminated subject... and yet a nice collection of the available light illuminating the background making for a nicely balanced shot from front to back.
This happens almost automatically because the camera naturally wants to meter for available light EVEN THOUGH it's going to use flash. It will back down the power of the flash to keep from over-exposing your subject.
If the shutter speed drops too much (very dark) then you can get blur caused by camera shake and that wont look good. For this reason, I usually switch to manual mode so I can keep the shutter from being too slow (yes... the computer in the camera may warn you that it's under-exposed, but it will automatically set the flash power based on the E-TTL metering calculation). You want to collect as much "available" light as you can without going so long that you get blur from camera shake.