True or False? "Aperture Controls Flash, Shutter Speed Controls Ambient"

One of the often-repeated rules of thumb for flash is that "aperture controls flash, while shutter speed controls ambient."  In this post, we'll see why this adage can be misleading.


IS THE ADAGE WRONG?
At first glance, the adage sounds correct.  When using manual flash, changing the aperture will affect the flash exposure directly.  Using a wider aperture will increase the flash exposure, while using a narrower aperture will decrease the flash exposure.

It is also true that generally, the flash duration is so brief that shutter speed usually has no effect on flash exposure (note: there are flashes with very slow flash durations which are affected by shutter speed).  At the same time, shutter speed does affect the ambient exposure.  So, indeed, shutter speed can be used to control ambient.

For these reasons, the adage is not exactly wrong.  However, as I said, it's misleading for a couple of reasons.

EFFECT OF APERTURE
While aperture affects the flash exposure directly, it also affects the ambient exposure.  Here are some test shots to illustrate (all of them use manual exposure with manual ISO).

In the first shot, the light is all ambient.  I used f/5.6, 1/60, ISO 800:

In the second shot, I added manual flash at 1/64 power (popup flash).  The ambient exposure is the same as the previous shot: 1/5.6, 1/60, ISO 800.  The light is therefore a mix of ambient and flash:

In the third test shot, I widened the aperture to f/2.8 in order to increase the flash exposure.  The manual flash was still 1/64 power.  However, because I widened the aperture to f/2.8, the flash appears brighter.  On the other hand, widening the aperture also had the incidental effect of making the ambient light brighter.

 

Therefore we can see that when using manual flash, aperture controls not just flash exposure but also ambient exposure.  If you really want to change just the flash exposure without affecting the ambient exposure, you can do it by adjusting the flash output instead of changing the aperture.

ADAGE DOESN'T APPLY TO TTL

More relevant to candid shooters, the adage doesn't apply at all to TTL flash. Just to recap, with manual flash, you set the flash power output.  With TTL flash, the flash emits a pulse and measures the reflection to guess the approximate power output to achieve a correct exposure.

With TTL, the adage doesn't work because when a TTL flash sets the power output, it already takes into account the aperture and ISO. When you adjust aperture or ISO, the TTL flash will adjust output automatically to give the same exposure.  Here are sample shots to illustrate.

#1: a baseline shot, all ambient: f/5.6, 1/60, ISO 800:

 

#2: Here is the shot with TTL flash added.  Still at f/5.6, 1/60, ISO 800:

 

#3: In this next shot, I widened the aperture to f/4.  Now at f/4, 1/60, ISO 800.  Note that the flash exposure (illuminating the surrounding carpet) did not change. What was affected was the ambient light (the flashlight), which now appears brighter.

 

#4: Here, I narrowed the aperture to f/8.  Now at f/8, 1/60, ISO 800.  Note that the flash exposure still did not change. What was affected was the ambient light (the flashlight), which now appears dimmer.

 

#5: What if instead of changing the aperture, we changed the shutter speed?  I increased the shutter speed by one stop from the 1/60 baseline to 1/125.  Now it is at f/5.6, 1/125, ISO 800.   Relative to the baseline, the flash exposure still did not change. However, the ambient light (the flashlight) appears dimmer.

 

#6: Similarly, decreasing the shutter speed 1 stop from 1/60 to 1/30 has no effect on the flash exposure.    Now it is at f/5.6, 1/30, ISO 800.   Relative to the baseline, the flash exposure still did not change. However, the ambient light (the flashlight) now appears brighter.

#7  Instead of adjusting aperture or shutter speed, we can also try to adjust ISO.  Suppose we decrease the ISO to ISO 400.  Here it is at f/5.6, 1/60, ISO 400.  The flash exposure hasn't changed, while the ambient light is dimmer.

#8 And let's try to increase ISO to 1600.  Here it is at f/5.6, 1/60, ISO 1600.  Once more, no change to flash exposure, while the ambient is brighter.

In all these examples, changing the aperture, shutter speed, and ISO had no effect on the TTL flash exposure.  While that may sound limiting, it's actually liberating.  As you saw, adjusting aperture, shutter speed or ISO did have an effect on ambient exposure (of course!).  Therefore when using TTL, you can adjust the ambient exposure using aperture, shutter speed or ISO, not just shutter speed.

But what about the flash exposure?   Are we stuck with the flash exposure chosen by the TTL flash?  No, we're not.  That's what Flash Exposure Compensation (FEC) is for.  You use FEC to adjust the flash exposure upward or downward.  Let's say you want to make the flash exposure brighter by 1 stop.  Just dial +1 FEC.   If you want to decrease flash exposure by 2 stops, just dial -2 FEC.  It's that simple.

RELATED POSTS

1. TTL Flash FAQ
2. TTL Flash Tutorial (Part 1 and Part 2)