5D MK III and AV and shutter speed relationship?

[Buzz57]

When shooting with AV in a dimly light environment like inside a mall, after the algorithym opens the aperature all the way, does the camera automatically adjust the shutter speed to get the correct exposure?  In other words, once it reachs the limits set by me in the ISO settings, will it attempt to adjust the shutter speed to get the correct exposure?  

[TCampbell]

It depends on which mode you use.

If you use "Manual" mode then the ISO, aperture, and shutter speed will be whatever you set.  If you half-press the shutter button, the light meter will indicate if it thinks your settings will result in either a correct exposure or in an over-exposed or under-exposed shot.  It will give you advice, but it will not actually change the settings.

If you use one of the automatic modes (such as Av mode) then the camera will automatically adjust the shutter speed to compensate for your change in aperture value.  

For any given amount of light, there are numberous different combinations of shutter speed and aperture value that would collect the same net amount of light to the sensor.

For example, outside on a sunny day (middle of the afternoon in full sun) you have a "Sunny 16" rule exposure which says that you can set the aperture to f/16 and set the shutter speed to the inverse of the ISO setting.  So at ISO 100, you can use a 1/100th sec shutter speed and get a correct exposure.

If you use Av mode, you can dial in f/16 on the aperture, and (assuming you set the camera to ISO 100) you'll probably get a 1/100th sec exposure (it can vary depending on the reflectivity of your subject (which can fool the light meter) and the rule only works in "full" sun (no shade, no partial shade, no partial overcast etc.))    But if you were to change the aperture by 1 full stop and open up to f/11 instead of f/16, then the camera will respond by using a faster shutter speed -- also by 1 full stop -- and would likely shoot at 1/200th sec instead of 1/100th sec.    This is automatic because you chose to use "Av" mode.

It will do the same thing in Tv mode -- except in Tv mode you dial in the Time value (shutter speed) and the camera will adjust the aperture. 

The caveat to all of this is there are limits to the aperture range available for a given lens and obviously the camera cannot exceed those limits.  So if you uses a shorter exposure time and the camera needs to compensate by using a larger aperture size -- but the lens was already set to it's widest aperture, it has nowhere to go and you'll just end up with an underexposed shot.

Program mode is slightly different because the camera sets both Aperture and Shutter values based on an algorithm called the "Program line" (which tries to find middle "safe" exposures).  But you can use a feature called "Program shift".  Meter the shot and through the viewfinder you'll see the Aperture and Shutter values that the camera plans to use... but roll the main dial (the wheel by the shutter button) and you'll notice that as you roll the wheel, the shutter speeds wil get faster and the aperture size will get large (smaller values) -or- in the other direction the shutter speeds will get slower and the aperture sizes will get smaller (larger values).  Program mode lets you "shift" through pairs of "equivalent exposures".

[Buzz57]

Thank you very much for your responses, they are appreciated.  

Regards,

Buzz57

[jrhoffman75]

Possibly more than you wanted to know , but a good article.

http://dougkerr.net/Pumpkin/articles/Program_AE.pdf

[ScottyP]

Yes.  In Tv mode you set the shutter speed and if it can, the camera sets an aperture that gives you proper exposure.  In Av mode, you select an aperture and if it can, the camera sets a shutter speed that would give you a correct exposure.
 
If you are in Tv (shutter priority) mode, the aperture will blink on and off if your camera will not be able to achieve a correct exposure for your selected shutter speed with your lens's widest aperture due to insufficient light.  You need to help it out by slowing the shutter speed or raising the ISO or by mounting a lens with a brighter max aperture. 
 
If you are in Av mode in crazy bright conditions the shutter speed will blink on and off if the camera can't achieve a good exposure for your selected aperture value, even using its highest shutter speed (1/8000th i think) because there is too much light.  You need to stop the camera down to a smaller aperture, or lower the ISO.