Exposure compensation, what changes?

[Martyn]

If I had a DSLR camera set to full manual and use the exposure compensation dial or the automated exposure bracketing setting, what changes? I have set the ISO, the aperture and the shutter speed; am I correct in assuming that the camera will only adjust the gain (volume or boost whatever it's called) on the image sensor to record images with an adjusted exposure?

 

 

[bugmeister]

Two slightly different subjects here.

 

AEB

With automatic exposure bracketing, the camera will take 3 shots in quick succession each with different exposure settings that you choose.

 

Use the menu to select the up-and-down exposures (e.g. +/- 1 stop or 2 stops)

 

In full manual, the centre point for the exposures will always be whatever your manual settings are (iso/shutter speed/aperture) and when you press the button the camera will take the images in quick succession centred around your nominal setting.

 

I believe the camera normally changes only the shutter speed, not the aperture or sensitivity to do this.

 

EC

Exposure compensation isn't something you would normally use in manual mode

I'm not even sure if it's possible, since in manual, you are already choosing the settings to give you the results you want.

 

in Aperture or Shutter priority, EC will allow you to tell the camera to always take an image with an adjusted shutter or aperture to consistently under or over expose a picture. (In aperture priority the camera will adjust the shutter speed and vice versa).

 

This can be very useful if for example you are taking a picture of a dark object with a light background (dark spider hanging in front of a lighter backgound anyone?). If you are using spot metering the camera may decide to use settings for the dark spider and totally overexpose the background and a negative EC setting could help reduce that while still keeping detail in the spider.

Conversely, if you were using a different metering mode, the camera may expose for the background leaving little or no detail visible because the spider is so underexposed. A positive setting will adjust what is recorded allowing detail to be seen in the spider.

 

*footnote* I have one camera that is always set to -1/3 EC because it consistently overexposes and I couldn't be othered sending it back for that, but in manual it isn't relevant, the EC indicator dsappears from the dial when I am in manual mode.

 

*footnote2* If you shoot in RAW, you have some latitude in correcting exposure settings during processing but if highlights are completely blown, you won't recover details there, and if the exposure is seriously underexposed, you will generate or increase noise in dark areas as you lighten them.

Edited July 13, 2013 by bugmeister

[Black Pearl]

If you are in Manual Mode and use the Exposure Comp dial or the AEB Mode the exposure/aperture/shutter speed/iso won't change - the meter reading will alter to say the 'correct' exposure is now different to what you original read and set but the actual shot will be as you have set it.

 

If you use an automated mode then the Exposure Comp dial will change:

 

In Aperture Priority Mode the shutter speed.

In Shutter Priority Mode the aperture.

In Program Mode the shutter speed will change first then the aperture depending on the light levels.

 

AEB works in conjunction with the auto modes very similar to above, again it won't do anything if the camera is set to manual.

[Martyn]

Thanks for the answers! As you've probably guessed by my questions on this forum I'm not happy to just know that things work; I need to know the how and why!

[Guest DaveW]

In film days exposure compensation was done by simply altering the film speed dial to a different film speed setting and I would guess that is all the exposure compensation does on a DSLR in order to fool the meter the sensor gain is slightly different to the ISO set? The meter reading of course controls all automatic exposure settings. Quotes from the Web:-

 

"Another way of saying the same thing is that Exposure Compensation changes the calibration of the built-in meter...it makes the meter either more or less sensitive."  

 

"Exposure compensation changes the reading from the camera's light meter. If the camera is in any of the automatic or semi-automatic modes, this will affect settings for one or more of the variables (aperture, shutter speed, flash lighting input, ISO) that control exposure. If the camera is in Manual mode, exposure compensation may either be disabled, or may just affect the light meter reading without causing any automatic change to any exposure control."

 

As I understand it Exposure Compensation simply modifies the meter reading so all the cameras automation then uses the new modified meter reading to adjust all it's functions.  In a DSLR the ISO Dial controls the sensitivity or gain of the sensor, unlike a film camera where the sensitivity of the film is set through it's manufactured film speed.  Therefore though you can alter the Film Speed Dial (ISO setting) on a film camera, but the film sensitivity remains the same and it just biases the meter into thinking a slightly different film speed is in.  With a DSLR the ISO Dial is effectively changing the gain or sensitivity of the sensor, which is like putting a different film speed in rather than fooling the meter as to the sensors sensitivity.  Therefore you need the Exposure Compensation control in a DSLR to fool the meter into thinking the sensors sensitivity or gain is not that set by that of the ISO Dial. Anyway If I have got it wrong correct me?

Edited July 13, 2013 by DaveW