Monday, February 14, 2011

Setting the Ambient Exposure with TTL: Aperture, Shutter Speed or ISO?

I got a great question from one of our readers, Nabeel.  The question was: in manual exposure mode combined with TTL flash, one can control the ambient part of the light in a frame by adjusting the aperture, shutter speed, or the ISO... so which one of these 3 should we choose to modify to control the ambient lighting?  Does it matter or can we just chose any one?  As I emailed Nabeel, it does matter.
 
Last week, we discussed how aperture, shutter speed and ISO affect both ambient and flash exposure.  That post lays the foundation for Nabeel's question.
REVIEW:
In manual exposure mode combined with TTL flash, you can indeed change ambient through aperture, shutter speed, or ISO.  Which one you change does matter, and should depend on your intent, similar to how you would select your exposure without flash, with one difference: shutter speed. 
 
If you change the aperture or ISO in manual mode while using TTL flash, then the ambient exposure will change but the flash exposure will remain the same.  Flash exposure remains the same with TTL flash because the camera will automatically adjust the flash output (based on the chosen aperture and ISO) to maintain the same flash exposure.

If you change the shutter speed in manual mode while using TTL flash, then as with changing the aperture or ISO, the ambient exposure will change while the flash exposure will remain the same.  However, this time, the reason flash exposure won't be affected is because the flash burst is so brief that at almost any shutter speed, all of the flash burst will reach the sensor.  With changing the aperture or ISO, the camera has to adjust the flash output in order to maintain flash exposure.  With changing shutter speed, there is no need for the camera to adjust the flash output.

APPLICATION:

With flash photography, just keep in mind that shutter speed does not affect flash output or flash exposure.  It therefore usually makes sense to adjust the ambient using shutter speed first, then setting aperture and ISO as you desire. 

If, for example, you're using TTL flash and you want to minimize ambient light, you can reduce ambient light with a narrower aperture, a lower ISO, or a higher shutter speed.  However, if you use a narrower aperture or lower ISO, you're forcing your flash to put out more light to maintain the same flash exposure.  On the other hand, if you minimize ambient by choosing a higher shutter speed, then your flash isn't affected at all.

Similarly, let's say it's somewhat dark, and you want to increase the ambient light.  You can do so with a wider aperture, a higher ISO, or a slower shutter speed.  If you adjust ambient using a wide aperture or higher ISO, you may be requiring your flash to put out less than the minimum flash output.  The result will be flash overexposure.  If instead you increase the ambient light with a slower shutter speed, you will have no issues.
Sample: I was able to balance the full power flash of the YN560 with its much dimmer LED display - by using a slow shutter speed:
http://betterfamilyphotos.blogspot.com/2010/08/balancing-flash-and-ambient-light-part.html

There are occasions when you may need a particular shutter speed, in which case you can change the aperture or ISO first.  In those situations, it's good to know that TTL has you covered and can adjust the flash output automatically to keep flash exposure constant (within limits).