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Variable Shutter Speeds in AV

Posted: Mon Jan 09, 2017 10:53 pm
by WDCarrier
Can someone explain to me why, with the setting on AV, the focus on spot with the camera mounted on a tripod and the subject is stationary so that all frames look identical in composition, in a high speed burst of 6 shots #s 1,2,5 and 6 are at 1/200 and 3 and 4 are at 160 (and overexposed).  Is this a lens or camera issue? 

Posted: Tue Jan 10, 2017 1:36 am
by Kim
Could it be that to get the number of frames per second the camera needs to compensate, as in, reduce the shutter speed to achieve the frames required?

Re: Variable Shutter Speeds in AV

Posted: Tue Jan 10, 2017 7:51 am
by Vivek
WDCarrier wrote:Can someone explain to me why, with the setting on AV, the focus on spot with the camera mounted on a tripod and the subject is stationary so that all frames look identical in composition, in a high speed burst of 6 shots #s 1,2,5 and 6 are at 1/200 and 3 and 4 are at 160 (and overexposed).  Is this a lens or camera issue? 
This is interesting. The "spot" is really not a "spot" but a small area. If the area covers dark and light object or an edge, this could happen. Can you please share a photo of what you are shooting exactly with the focus point highlighted?

Re:

Posted: Tue Jan 10, 2017 7:54 am
by Vivek
Kim wrote:Could it be that to get the number of frames per second the camera needs to compensate, as in, reduce the shutter speed to achieve the frames required?
This is not the case. The shutter speed would have to *increase* if the frame rate is in question. Say 7D2 is capable of 10fps. It means that each frame has 100ms or 1/10th of a second. The current shutter speed is 1/240 or much higher. Even if we included the mirror travel time etc., your suggestion would not make sense.

Posted: Tue Jan 10, 2017 9:47 am
by rajandesai
You mentioned Av - so I assuming Canon. I always assumed that when shutter is half pressed, exposure is locked and wouldn't change during high speed burst.
I typically shoot in manual (or use the exposure hold when using Av) so I haven't noticed this behavior. May be it is worth experimenting with my cameras too.

Re:

Posted: Tue Jan 10, 2017 10:58 am
by WDCarrier
rajandesai wrote:You mentioned Av - so I assuming Canon. I always assumed that when shutter is half pressed, exposure is locked and wouldn't change during high speed burst.
I typically shoot in manual (or use the exposure hold when using Av) so I haven't noticed this behavior. May be it is worth experimenting with my cameras too.

The body is a7DII, the lens a 500mm f4, shooting in silent mode.  I use backbutton focusing so the shutter button doesn't affect the focusing.