5D4 & 430EX II, slow in Av mode

This may be a simple oversight, but I can't figure it out.

When using my flash in Av mode, the shutter speed is much slower than in P mode. All same settings otherwise.

For example, last night I was taking a lot of photos. At 2.8, 400 ISO, E-TTL, the shutter was around 1/20-1/40. When I kept the setting the same in Program mode, the shutter was back to what I expected, 1/125 or so. I even put the flash into manual and it was still slow.

Apparently the camera and/or flash is doing something else here behind the scenes since all other settings were the same.

I think I found the answer here in the flash 101 thread?

Flash fact #4: Your camera measures ambient light and flash illumination separately. In Av, Tv or P modes, it will attempt to expose properly for the ambient light by adjusting either the shutter speed, aperture, or both. The fact that you have your flash turned on has no effect on this** ( one exception is that in P mode it will not use a shutter speed slower than 1/60 with flash). The camera’s metering system cannot predict how much illumination will be gained by the flash, so it doesn’t try. In manual mode, the meter in the viewfinder measures only ambient light, because that’s all it has to measure.

Think of your system this way. The flash does not care about what your camera is doing and the camera does not care what your flash is doing. Each has it's own separate job.

During flash photography there are two exposures going on - an ambient exposure followed by the flash exposure.

AV mode does not really care if a flash is on or off your camera. For this example lets say the flash is on your camera but is turned off. AV mode's only job is to maintain the Aperture and ISO you selected and it will automatically adjust the shutter speed accordingly. If in a darker environment it will go as slow as it needs to. Lets say 1/15. Now turn the flash on and press the shutter ½ way. Your ambient reading will not change. It will still be at 1/15 because the flash exposure has not occurred yet so the camera has no reference to it.

Flash is a separate exposure. When you press the shutter ½ way the camera takes the ambient reading. When you press the shutter all the way the flash fires a pre-flash. The system uses the pre-flash light that is reflected from your subject/s to determine the correct subject exposure. The camera's ambient light meter has nothing to do with flash exposure. It uses a separate flash exposure system that works when the shutter is pressed all the way.

There have been several occasions where I have come across threads where people have struggled shooting low light events in AV mode. Nothing wrong with using AV as long as one is prepared for it. You either shoot with a fast lens wide open and/or crank the ISO up to get the shutter to an acceptable speed.

When indoors and low light environments people like to set the camera on manual. Here you can set your ISO, Aperture and Shutter speed to your liking based on the ambient light. Most likely the cameras ambient meter will show underexposure. Don't worry about it. The flash will take care of the subject exposure. Trust the process.

AV works well for outdoors shooting as there is typically enough light to keep the light meter happy and maintain an acceptable shutter speed. Of course you can always use manual as well. Set your ISO, Aperture and Shutter speed to get your desired ambient exposure and then let the flash do it's job for the subject exposure.

Back to AV mode. One member at POTN - Snorter shoots exclusively in AV mode when shooting indoor events. Nothing wrong with it as long as you know how to control it. I have not seen posts from this member for a while.

https://www.youtube.co​m …vEnAhkL0i38&fea​ture=feedf

Ambient light metering is totally INDEPENDENT of whether (or not) any flash is being added to the scene!

The aperture+shutter speed suggested in Av mode never knows that flash will be used, and exposes based only on the assumption of the presence of the ambient light.
P mode does not try to do the same thing as Av, it is programmed merely to 'use flash' when there is too little light, so it simply sets aperture+shutter to (usually) f/4 and 1/60 -- regardless of the actual light intensity which is present.

Wilt wrote in post #18412689
Ambient light metering is totally INDEPENDENT of whether (or not) any flash is being added to the scene!

The aperture+shutter speed suggested in Av mode never knows that flash will be used, and exposes based only on the assumption of the presence of the ambient light.
P mode does not try to do the same thing as Av, it is programmed merely to 'use flash' when there is too little light, so it simply sets aperture+shutter to (usually) f/4 and 1/60 -- regardless of the actual light intensity which is present.

That's exactly what I was seeing. I erroneously said 1/125 in my OP, but it was in fact f4 and 1/60 most of the time in P.

Thanks everyone!