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Exposure set with available light, so why does photo still come out good with flash?


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#1 Sea Lion

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Posted 11 January 2014 - 05:27 PM

Hi all - this is my first real question on Wetpixel, and it probably seems pretty dumb to experienced u/w photographers.  It's been knocking around in my head for a few years since I started thinking about getting a u/w still rig.  Let's say I'm shooting aperture priority at a given ISO.  What I'm wondering is, given that my camera determines a proper shutter speed based on that aperture and the available light, why does an u/w photo still come out when a bright strobe flashes - i.e., contributes a sudden burst of light far greater than the available light?  It seems like if a shutter speed was determined based on available light, wouldn't the photo be overexposed due to additional light contributed by the strobe flash?  Or, does something occur within the camera/strobe system to make an instantaneous adjustment to compensate for the flash?

 

A related question has to do with white balanance.  If I've determined a proper white balance based on available light, why doesn't a strobe flash totally mess that up and ruin the color in a photo?

 

Thanks in advance for any expertise you all have on this.  I'm hoping the answer will help me understand the internal workings of a camera/strobe system better.


Edited by Sea Lion, 11 January 2014 - 05:28 PM.


#2 CheungyDiver

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Posted 11 January 2014 - 07:28 PM

Hi Sea Lion

 

Good questions. We all have to start from the beginning somewhere. 

 

Check out this site. Everything you asked is probably answered in some ways:

http://www.seafriend...graph/water.htm

 

Just a quick answer to the obvious question : Water absorbs light and the warm colours get absorbed quicker than cooler ones. Downwards and distance from the subject to you. A strobe less than 3 feet from the subject will definitely provide better lighting.

 

 

phdwg10.gif

 

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#3 johnjvv

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Posted 11 January 2014 - 08:34 PM



What I'm wondering is, given that my camera determines a proper shutter speed based on that aperture and the available light, why does an u/w photo still come out when a bright strobe flashes - i.e., contributes a sudden burst of light far greater than the available light?  


No expert here but your camera would be making a calculation based on the fact that your camera flash will fire. However the light emitted by your flash would not be intense enough, which is where your strobe kicks in. It is still possible to over expose with your strobe set to higher than required.

#4 MikeVeitch

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Posted 11 January 2014 - 10:32 PM

Yes, good questions:  for the WB, if you set your WB manually based on natural light and then introduce strobes into the equation you will get a much redder photo than without, if you have it set on Auto WB then you will be fine.

 

For your first question, the strobe fires a very fast but limited burst, it goes off so quick that a longer shutter speed won't be affected by the strobe. Overexposure from strobe will not happen from a longer shutter speed, only a more open fstop.  For example:  think of a night dive (no natural light)  if you set your strobe to half power, and your fstop to f8 you will get the same exposure at 1/30 sec, 1/100 sec, and 1/250 sec as there is no natural light coming into the camera.  Easiest way to think about it:  shutter speed controls natural light, not strobe (except there is a max strobe synch speed on most cameras) 


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#5 Glasseye Snapper

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Posted 12 January 2014 - 09:08 AM

The best tutorial on this topic, and a must read for anyone who wants to understand this, is http://www.cambridge...era-flash-2.htm

 

If you just want to know the answer to the original question:

 

In Aperture priority mode with the flash forced to be on, cameras aim to achieve something close to balanced (1:1) exposure where half the light is from the flash and the other half is ambient. To do this the camera first measures the ambient light and calculates settings that give a 1 stop underexposed image. It then fires a very short pre-flash to determine the correct flash pulse duration needed to bring the total image up to the proper exposure level. [Note: In program mode a similar thing happens but most cameras treat it as a fill flash and produce a much weaker flash contribution than 1:1.]

 

To get a more dominant contribution of the flash to the overall exposure you can either go to manual mode, set ISO/aperture/shutter to values that give a more-than-one stop underexposure based on ambient light and then let the flash add whatever is needed to bring it up to the correct exposure level.

 

It is possible to define a different target exposure ratio than the default balanced exposure but you better read the link above for the details. Basically, to get a larger flash contribution you set the Flash Exposure Compensation (FEC) to a positive number. But at the same time you need to reduce the normal Exposure Compensation (EC) to prevent overexposure. I have never played with this but if the correct combination of FEC/EC to get 2:1, 4:1 etc ratios doesn't depend much on the ambient conditions then it could we worth investigating. If I get to it, I will report back.

 

Bart


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#6 Ted_Wideman

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Posted 21 January 2014 - 10:36 PM

I have the same question in mind, actually. As for the WB, I rely on the post processing, but thanks to David for the image. I'm just about to practice this one.