The Canon flash system has some interesting quirks (or features) that may trip those coming from other systems, especially Nikon.
The biggest confusion is how the camera P and Av modes handle the flash exposure differently and this is the primary intention of this post.
Flash in Program Mode
The P (Program) mode flash behaviour is set up to be like a point and shoot mode; i.e. fire the flash to illuminate the scene properly no matter what happens and deliver decent, non-blurry results.
It will select a balanced fill-flash mode if the scene is fairly bright (13EV and higher) and it will go into normal flash mode if the scene is dim (10EV and lower), with a blend of both between 10 and 13EV. Naturally, the result of these will be very different – the first method will expose the subject with the background lit fairly well, while the latter will favour the subject exposed properly, but against a darker background — this is the standard direct flash look.
The camera will set a minimum shutter speed of 1/60 up the max sync speed in P mode, hence it should freeze most action fine. If that isn’t sufficient, then you need to use a different mode.
Trouble with Flash in Aperture Priority
That brings us to camera Av (aperture priority) mode shooting. Canon’s default behaviour in Av mode is to be in a fill-flash mode in all lighting situations, which can cause the shutter speed in low light situations to drag out to long durations (up to 30 seconds), rendering people photography in low light conditions useless. The camera is in effect exposing just for the available light with no consideration for the supplemental light from the flash.
This is somewhat troublesome default behaviour, at least for me, as I’m normally shooting people in Av mode and need at least 1/60 or faster to freeze movement. And it can certainly catch people that are not expecting this behaviour, including myself the first few times I used flash.
I can see the merits of both approaches, and illustrates the complexities involved in flash photography and making a design decision. Nikon, on the other hand, forces 1/60 for aperture priority mode, and bundles the options for enable longer shutter speeds to a flash setting: Slow sync or Rear Curtain mode.
Luckily, there are two ways on Canon to bring up the sync speed to that critical 1/60s level or faster and still be in control of depth of field via aperture:
Solution 1: Set Flash sync speed in Av Mode Option
The first solution is to force the camera to only select a shutter speed between 1/60 and the max sync speed (1/200 or 1/250 typically). This option on the 1DxII and 5DS R is under the ‘External Speedlite control’ menu option under the camera settings tab. There is a menu option called “Flash sync. speed in Av mode” where there are three possibilities: “Auto”, “1/250-1/60sec. auto”, and “1/250 sec.(fixed)” [the maximum sync will be 1/200 on the 5D series bodies].
The default setting of “Auto” would be the equivalent of enabling slow sync mode and the long shutter durations. Therefore select either of the two non-Auto options to lock the sync speed to a more hand-holdable or subject movement friendly range.
Solution 2: Manual Exposure
The other solution is to work in manual exposure mode coupled with the flash in an automatic exposure mode (Ext.A, Ext.M, or E-TTL on the 600EX). I personally prefer this way of working.
The camera manual settings will determine the amount of ambient light contribution to the image and action-stopping potential. The flash settings will determine the amount of power output by the flash. Both Ext modes on the 600EX use the sensor on the front of the flash (a.k.a. the external flash sensor in “Ext”) to measure the scene while the E-TTL mode uses the full features of Canon’s E-TTL / E-TTL II system.
For wedding or event work, I will normally work in manual camera + ETTL on the flash for changing situations. I’ll typically dial in a suitable shutter speed, usually 1/125 or faster, and adjust the aperture to set how much ambient light I want, and let the flash do the rest.
For controlled studio situations, then manual camera + manual flash works for full control and consistency from shot to shot.
Tip: Working with Auto ISO
Auto ISO can confound your best efforts to balance ambient and fill, so I find it best just to turn it off when using flash. In fact, for flash, you may as well go full manual mode for exposure and ISO.
This was the case when I was shooting Nikon – Auto ISO would float the ISO all over the place with flash and nothing really seemed to look right unless I turned Auto ISO off on the Nikon. The reason is the camera doesn’t know what it should do – should it meter and set ISO to get a proper exposure for the scene no matter how dim, or should the flash be the primary light source? Note that this behaviour is also different across Nikon cameras – some will drop to the lowest ISO with flash attached, some will go up to the highest. So again, it’s probably good advice to go manual ISO on Nikon unless you know what your camera’s going to do. Picking an intermediate ISO like 400 will give you good results – not noisy, and you get a little more ambient light than going with the camera base ISO (usually 100).
The good news with Canon is that the Canon bodies I use will fix ISO at 400 when a flash is installed and turned on with Auto ISO enabled. This is a handy feature that saves you remembering to disable Auto ISO when using flash, and you can still override ISO manually if you want a specific look. I am happy to push the ISO up to 1600 or more to gain more ambient light.
Of course, If you truly want pro results, I feel you’re better off shooting manual at least on the camera, and of course getting the flash off-camera!