Exposure Compensation - the 4th dimension of the exposure triangle?

[bbtom10]

I have a Nikon D40 which includes the exposure compensation function (+ / - 3).

My question, is Exposure Compensation the 4th dimension of the exposure triangle? Does it effectively boost or reduce ISO, or does it just digitally alter the image (much like digital zoom versus optical zoom in a point and shoot)?

I would like to understand how this relates to ISO if anyone has any clue?

Thanks
Tom

[wulf]

I think it depends what mode you are in. However, there is an easy way to test it - take some shots of the same set up (preferably indoors where lighting is not variable) and see what settings are used as you adjust the exposure compensation. AFAIK, it only has aperture, shutter speed and ISO to play with but it can access intermediate settings on at least ISO which aren't directly accessible via the menus.

Wulf

[RichardTaylor]

Under normal circumstances It you just change the the shutter speed (if in aperture priority) or the aperture when in (shutter priority).
In manual you decide which one to adjust or you could adjust the ISO (if possible) & do it that way if the aperture & shutter speed are critical
.
I normally only vary the ISO to give me the "correct" exposure (with or without exposure compensation dialed in).

I just tried it in P mode on a Canon G11 (P&S) and it changed the shutter speed (it was in low light and the lens was wide open anyway). The ISO did not change


Maybe your Nikon works differently, as Wulf says try it and see.

To me there is no 4th dimension in the "exposure triangle"

[dcclark]

Yeah, exposure compensation is not another variable in the exposure triangle. All it does is cause the camera to "skew" your shutter speed, aperture, or ISO towards under or overexposure (depending on your setting).

In other words, the camera chooses what it thinks are correct settings (shutter speed, aperture, and ISO), and then uses exposure compensation to either increase or decrease some of those to give you a slightly different exposure. It only affects existing settings, it's not an entirely new one.

[bbtom10]

Thanks for the feedback - I had assumed that this wasn't a 4th dimension, just thought it might attract some debate!

Appreciate the comments and will have an experiment later - I am wondering if this is why sometimes I see ISO levels which I can't select (200, 400, 800, 1600 - but I have seen ISO 510 on some of my EXIF I am sure!). Just trying to understand what my camera does when I ask it to do different things.

I should add I shoot in Aperture Priority when doing 'normal' photography (kids playing, landscapes etc.) and Manual mode when playing with my light-box.

I'll keep experimenting.

Thanks
Tom

[dcclark]

Originally Posted by bbtom10

I am wondering if this is why sometimes I see ISO levels which I can't select (200, 400, 800, 1600 - but I have seen ISO 510 on some of my EXIF I am sure!). Just trying to understand what my camera does when I ask it to do different things.

I have a D40 as well. You probably have "Auto ISO" on, which automatically chooses a higher ISO for you if necessary. The camera can choose "intermediate" ISOs (like 510) which aren't accessible normally. I'm not sure if these are "native" (as in, it's just varying the amount of charge on the sensor) or "fake" (as in, caused by the camera processing the image after it's already been taken).

In general, I haven't had trouble with those ISOs, however.

[OsmosisStudios]

Think of exposure compensation as a trick. It's tricking the camera into thinking there is more or less light in a scene than it thinks and therefore will lower or raise the EV for that photo.

For instance, if you take a photo in Av, and it comes out a bit bright for your liking, dial in a bit of negative exposure compensation. This tells the camera to under expose from what it would normally expose for that scene, effectively tricking it into thinking there is more light. The same works in Tv (S) and in Program as well.

In Manual it's a bit different, and different bodies interact differently. I'd never use it in manual: You are your own exposure compensation. If the image is too bright, just use a faster shutter speed. Too Dark: use a slower one.

[Sterling]

It sounds like the op knows what ec does. He's asking if anyone knows how it does it.

Does it effectively boost or reduce ISO, or does it just digitally alter the image (much like digital zoom versus optical zoom in a point and shoot)?

[dcclark]

Originally Posted by Sterling

It sounds like the op knows what ec does. He's asking if anyone knows how it does it.

Right, as explained, it adjusts the shutter, aperture, and/or ISO.

[candleman]

Its been explained well already... but here it is in my words.

exposure compensation can only change the exposure my making a decision to adjust one of the three dimensions of the triangle.
there is NO other method exposure can be changed. (forget flash for now.)

Assuming you have AutoISO OFF and have dialed in your own ISO

in Aperture Priority : only the Shutterspeed will change based on your metered zone
in Shuterspeed Priority: only the Aperture will change based on your metered zone.

EC is basically applying an offset to the meter, and forcing it to over or under expose by adjusting that one "dimension" of the triangle.


even in Manual mode, you can apply an offset to the meter.
in Manual:
say you dial in -1.0 EV to the meter (by holding the +/- button)
Center the meter so it looks like its giving a correct exposure.

take the shot.. and your image preview will show you an image thats exactly 1.0 stops under exposed....even though you had centered the meter.

SO, the cameras brain is the same... IT will always work to center the meter.. but by applying an Offset to the meter you are moving the mark it has to hit.

thats what we call Exposure compensation.