Effect of Shutter Speed and Aperture on Ambient and Flash Exposure (Basic and Intermediate)

An oft-repeated mantra in flash photography is that shutter speed controls ambient exposure while aperture controls flash exposure.  Long time photographers don't usually get confused with this mantra but for amateurs and especially flash novices, it can be misunderstood.  In this post, I want to clarify the effect of changing aperture, shutter speed, and ISO, on both ambient exposure and flash exposure.  I'm also going to add exposure compensation and flash exposure compensation to that mix.  Hold on for this roller coaster ride...




BACKGROUND:
In flash photography, it is absolutely essential to distinguish between ambient exposure, flash exposure, and overall exposure.  Ambient exposure is the amount of existing light (light sources other than what you're adding from the flash, such as sunlight) recorded in the image.  Flash exposure is the amount of light from the flash that is recorded in the image.  Overall exposure in flash photography is the combined exposure from both ambient and flash exposure.  (Note: If this concept is new to you, I recommend the TTL Flash Tutorial and TTL Flash FAQ).
- ISO is a measure of the sensitivity of the film or sensor.  The higher the sensitivity, the more light is recorded from both ambient light and flash.
- Aperture is the size of the opening in the lens.  With a wider aperture, more light enters the sensor or film from BOTH ambient light AND flash.
- Shutter speed controls the duration that the sensor or film is exposed to light.  Shutter speed directly affects ambient light.  However, it *generally* has no effect on flash.  The reason: the duration of the flash burst is so brief that all of the flash reaches the sensor at almost any shutter speed. 
There are a couple of exceptions to this.  First is the sync speed -- above the sync speed the front and rear curtain form a slit that travels across the sensor.  The higher the shutter speed, the smaller the slit becomes.  Thus, above the sync speed, the sensor is never fully exposed and the flash won't reach the entire surface of the film (you'll see a black bar - the shutter curtain).  This poses no problem with some cameras that have unlimited sync speed such as those with electronic shutters, hybrid mechanical/electronic shutters, and cameras with leaf shutter lenses.
That brings us to the second exception to the shutter speed's effect on flash.  If you have a camera with unlimited sync speed, shutter speed might be set so high that it's faster than the duration of the flash burst.  For example, a Nikon SB-800 at full power has a flash duration of around 1/1050.  If you chose a shutter speed of 1/2000 for example, then not all of the flash will be recorded.
APPLICATION:
The effect of changing aperture, shutter speed or ISO varies depending on two factors:
1. which exposure mode you are using (Program, Aperture Priority, Shutter Priority, or Manual).
2. whether you are using Manual flash or TTL flash
Combo 1: Manual exposure + Manual flash
In this combination, changing the shutter speed will change ambient but has no effect on flash exposure (with the exceptions above).  Changing aperture or ISO will affect both ambient exposure and flash exposure.
Combo 2: PAS + Manual flash
In semi-auto modes (PAS), the camera will do its best to set ambient exposure to its best guess of the correct exposure.  A corollary of this is that the camera will do its best to MAINTAIN ambient exposure according to its best guess.  Therefore:
If we are in Aperture Priority and we widen the aperture, the shutter speed will increase proportionately, and vice-versa.  Meanwhile, in manual flash mode, the flash output doesn't change.  Therefore, changing aperture in A mode while using manual flash results in a change in flash exposure (because of the change in aperture), while there is no change to ambient (because the camera will maintain ambient exposure).  Changing ISO while in A mode results in a change in flash exposure (because of the change in ISO), while again there is no change to ambient. 
As for exposure compensation, if we change exposure comp while we are in A mode, then the camera will adjust the shutter speed while holding the aperture constant (to our chosen value).  The net effect of exposure compensation in A mode is to keep flash exposure constant (because the aperture didn't change).  Meanwhile due to the change in shutter speed, the ambient will change.  Therefore, changing exposure comp in A mode will change ambient exposure, but with manual flash does not change flash exposure.
In Shutter Priority, the effect is the converse of the above.  This time, as we change shutter speed, the camera adjusts aperture proportionately, in the process maintaining the camera's guess of ambient exposure.  Thus, if we change the shutter speed in S mode while using manual flash, then flash exposure is affected (due to the change in aperture), while ambient won't change.  Changing exposure compensation results in constant shutter speed but changing aperture, therefore both flash exposure and ambient are affected.
As for Program mode, I honestly don't know the different algorithms that cameras use to select exposure. I do know that with program shift, the aperture and shutter speed both change (while ambient exposure is constant).  Therefore, flash exposure will change while ambient won't.  When using exposure compensation the ambient will change (that's the point of exposure compensation).  As for flash exposure, it will change if the aperture value changes.
Combo 3: PAS + TTL Flash
Again, the camera will maintain the ambient exposure to its best guess of the correct exposure.  As for flash exposure, the camera will also attempt to choose and maintain the flash exposure.  Therefore, with this combo, changing aperture, shutter speed or ISO has no effect on either ambient exposure or flash exposure. 
To change the flash exposure, we instead use flash exposure compensation (FEC).  Changing FEC will only adjust the flash exposure and has no effect on ambient exposure. 
To change ambient exposure, we use exposure compensation.  Changing exposure compensation MAY affect flash exposure depending on whether you're using Canon, Nikon or some other brand.  With Canon, changing exposure compensation has no effect on flash exposure.  With Nikon, changing exposure compensation will affect flash exposure as well.  I'm not sure what happens with other brands - you'll have to try it out for yourself.

Combo 4: Manual exposure + TTL Flash
In this combo, the camera will maintain the flash exposure, while leaving you to choose ambient exposure.  Changing aperture, shutter speed or ISO will directly affect ambient exposure but because the camera is maintaining the flash exposure in TTL, then changing these variables has no effect on flash exposure.  As with combo 3, changing flash exposure is done by adjusting FEC which affects only flash exposure but not ambient exposure. 
As for exposure compensation, it's not usually used in manual exposure, but if you do change exposure compensation, then as in Combo 3, the effect on flash exposure depends on which camera you're using.

Summary:

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CONCLUSION:
Going back to the mantra, I would revise it this way:
Shutter speed *affects* ambient exposure and not flash exposure.  Flash exposure is affected by aperture (not shutter speed), however aperture will also affect ambient exposure.  How you *adjust* ambient and flash exposure depends on your exposure mode (PASM) and flash mode (manual or TTL).

COMING UP:
If with combo 4, ambient is controlled either by aperture, shutter speed or ISO, how would we choose which variable to change?

RELATED POSTS:

TTL Flash FAQ http://betterfamilyphotos.blogspot.com/2009/08/ttl-flash-faq.html

TTL Flash Tutorial http://betterfamilyphotos.blogspot.com/2010/06/ttl-flash-tutorial-in-10-lessons-part-1.html