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What Is Exposure Compensation Affecting?


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#1 Madrugada

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Posted 08 April 2018 - 05:24 PM

Dumb question.

 

Aperture physically affects the size of the iris.  Shutter speed physically affects the length of time the shutter is open   ISO, as I understand it, physically affects the light sensitivity of the sensor.  What is exposure compensation physically changing that changing these other settings is not equivalent to?  Adding one stop of exposure comp isn't the same as, or does not itself affect, the ISO or shutter speed by one stop, right?



#2 geedee

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Posted 08 April 2018 - 11:44 PM

Welcome to the forum Madrugada. I do not know for sure though think it adjusts the  light meter centre point indication, the point at which the meter indicates correct exposure..  I guess the meter operates as normal but if the photographer assesses the scene knowing the subject or some aspect of it or its surroundings is/are brighter or darker then compensation for that difference can be dialled in to correct the exposure ?  Hope that gives you something to think about till someone comes along with the correct answer.. 



#3 Madrugada

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Posted 09 April 2018 - 01:26 AM

In watching some videos other than Tony's and playing with my camera, it seems like exposure compensation does indirectly affect ISO, shutter speed, or aperture. If the camera is in aperture priority mode then it seems that EC affects mainly the shutter speed. If I up the EC it effectively leaves the shutter open for longer. Similarly, in shutter priority mode, as I change EC, the camera essentially changes the aperture to get the corresponding exposure. I'm still not exactly clear what rules it uses in manual mode when an EC adjustment is made, and how auto ISO vs a specific ISO might factor into this. Anyway, although in A/S modes you have told the camera it can choose the other settings, it still seems important to me to understand that as you adjust EC you may get more motion or less bokeh besides just making the overall scene darker or lighter.

#4 geedee

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Posted 09 April 2018 - 08:13 AM

That which you type seems entirely logical.. I used the term "meter" in an attempt to simplify the explanation... as you typed dependant on which function you allow the camera to auto-select, that function will adjust to provide the correct exposure. I think in manual mode the only thing altered will be the meter read out as you will be required to adjust settings to centre the meter..?

 

Perhaps if you switch to manual, and focus on something to centre the meter.... then dial in a degree of EC and re-focus on the same subject, you will perhaps note that you now need to adjust one setting the re-centre the meter for a correct exposure read out...?

 

Of course the camera is only reading variations of light and has no idea as to what DOF might suit a particular element in a scene or whether the photographer wishes to freeze movement,...?

 

If you look in the ANIMALS section of this forum and beam up   John W,    he was one who regularly used EC when photographing birds.... given he may have been trying to capture in the darker plumage of Eagles with the bright blue sky as the backdrop... As with most postings John and others include their settings with their images

 

Hope this ramble is of some help and that I have not muddied the water.... I`m no expert.



#5 PeterPP

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Posted 09 April 2018 - 09:44 AM

Exposure compensation can  affect any of the other settings that are not locked in,

It is changing the way the metering system sees the scene, since all metering systems are trying to get what they see to be to 18% gray you are moving its 18% gray target up or down depending on which way you dialing in the compensation 

Basically it tells the camera that the scene is either darker or lighter than it appears to the metering system, and the camera tries to adjust the other settings accordingly.

 

If you are on aperture priority and have a set ISO the camera will adjust the shutter speed to the new setting

if you are on aperture priority and have auto iso set it might adjust both ISO and shutters peed to match the altered metering.

If you are in P mode with auto ISO it will try to use all three settings to match the new idea of what the exposure should be.

 

 

To your original q

Aperture physically affects the size of the iris: Yes

Shutter speed physically affects the length of time the shutter is open:  Yes

ISO, as I understand it, physically affects the light sensitivity of the sensor: No

 

The sensor only has one sensitivity, usually the base or lowest ISO setting on the camera.

What raising ISO does is increase the amplification of the base sensor output. On an audio amp it is like turning up the gain, the volume goes up but so does the noise until it starts to break up unacceptably.

It is the same with camera sensors, you turn up the gain(iso) with "appears" to boost light gathering but adds noise and other unpleasant artifacts until it breaks up and becomes unacceptably noisy and unusable. 

For the highest quality use the lowest ISO settings possible for what you are trying to shoot.

Modern cameras like the d850 5div or a7r3 control noise well to rather high iso settings and still produce good images!


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Peter
 





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