Exposure Compensation and Auto ISO

Exposure compensation in Av and Tv modes is fairly straightforward. Assuming that you have your ISO set to a specific values, using exposure compensation will adjust either your exposure time or f-stop value, depending which mode your camera is in.

So what happens you set your ISO to auto? With negative compensation, the ISO value will drop to it's minimum and then the exposure time or f-stop value will start to change. In Av mode, the exposure time seems to drop as low as the inverse of the focal length (e.g. with a 100mm lens, it will not go lower than 1/100s). In Tv mode, the f-stop will go down to the lowest possible.

So far, this makes sense. But what do you do when you want a specific exposure time and a specific f-stop, want your camera to meter the scene to pick the ISO value - and then want to apply exposure compensation? Well, I don't know which cameras, if any, have this feature but mine do not. Do Nikons have this feature? So what do you do in such a situation? Well, I recently started to set all three values manually, half-press the shutter release button to see how my camera has metered the scene in the viewfinder, and then change the ISO value as I'm looking through the viewfinder. Three clicks of the wheel and I'm at +1. I guess I'll have to live with this until EC in manual with auto ISO is available.

The next question is: When should one use exposure compensation? Here's what I do.

There are three situations with respect to a scene's brightness and your camera's sensor:

1. the dynamic range of the scene is greater than what your camera's sensor can capture
   
2. the dynamic range of the scene is less than what your camera's sensor can capture
   
3. the dynamic range of the scene is equal to what your camera's sensor can capture
   

Not a single Canon model offers EC in manual mode, even if you decide to use auto-ISO. The closest you can get to EC would be to add the "microadjust AE" option to your "My Menu" on the 1D4, and tweak it a multiple of 1/8 stops. Tedious but gives you about the same function.

Maybe Canon can introduce Iv where it locks in auto ISO, and you control just aperture and shutter, then EC could be used there.

I think you have covered the ground pretty well, as far as you have gone. There are, however, other scenarios when you will want to use EC. Conventional metering is based on the assumption that the average tone value of the metered scene is at least close to 13%. (In film photography the standard was 18%, digital cameras are calibrated to 13%.) A sunny day at the beach or on a snowy mountainside will provide a scene whose average tone is considerably above 13% and commensurate EC will be needed. A similar scenario is when spot metering is being used and the spot being metered has a tonality that is not 13%. Suppose you are photographing a landscape and you point the spot meter at the sky. A clear blue sky is about 1.5 stops brighter than 13%, so you would add 1.5 stops of exposure. If you are shooting RAW and want to gain the advantages of maximizing data capture and that sky is the brightest element in your photo, you can give +3 stops of EC, because RAW has a dynamic range whose top is at least 3 stops above medium grey.

Every camera has EC in manual mode, even my first camera fifty years ago, an Argus C3. You activate the EC by moving the speed control or the aperture control or both.

tzalman wrote in post #13513963
Every camera has EC in manual mode, even my first camera fifty years ago, an Argus C3. You activate the EC by moving the speed control or the aperture control or both.

It is pretty obvious when we talk about EC, we are talking about the camera's ability to automatically move one or two of the factors involved in exposure for you automatically based on where you set the EC marker. So no, there is no EC in manual mode. We are not talking about the person manually changing settings

Before there was auto-ISO, there was no need for EC in manual, everyone knew they had to control all 3 elements (aperture, shutter, and ISO). Once auto-ISO was introduced and usable in manual mode, it raises the need once again to be able to set EC, much like you would for Av or Tv.

tzalman wrote in post #13513940
I think you have covered the ground pretty well, as far as you have gone. There are, however, other scenarios when you will want to use EC. Conventional metering is based on the assumption that the average tone value of the metered scene is at least close to 13%. (In film photography the standard was 18%, digital cameras are calibrated to 13%.) A sunny day at the beach or on a snowy mountainside will provide a scene whose average tone is considerably above 13% and commensurate EC will be needed. A similar scenario is when spot metering is being used and the spot being metered has a tonality that is not 13%. Suppose you are photographing a landscape and you point the spot meter at the sky. A clear blue sky is about 1.5 stops brighter than 13%, so you would add 1.5 stops of exposure. If you are shooting RAW and want to gain the advantages of maximizing data capture and that sky is the brightest element in your photo, you can give +3 stops of EC, because RAW has a dynamic range whose top is at least 3 stops above medium grey.

It's not the average tone of a scene that I'm usually concerned about. It's the range of tones. My goal is to capture the brightest tones in the scene without blowing them out (ETTR).

However, I can see where spot metering comes in handy. You can meter off of the brightest part of the scene and compensate according to how much leeway RAW gives you, ensuring that you don't blow it out. Or in a scene with a high dynamic range, meter off of the subject (e.g. person's face) that you want to expose for, apply EC, and let some of the less important highlights get blown out - ensuring that the face is properly exposed.

You can work around it on the Mkiv and end up with an effective auto ISO mode with Exposure Compensation functionallity by setting a min and max shutter speed in the custom functions while in AV mode. Haven't mucked around with it much, but recall it working when I tried it.

TeamSpeed wrote in post #13514175
It is pretty obvious when we talk about EC, we are talking about the camera's ability to automatically move one or two of the factors involved in exposure for you automatically based on where you set the EC marker.

But EC isn't the camera automatically adjusting settings - in fact quite the opposite. EC is the user manually adjusting those settings that the camera automatically set, in order to compensate/override the decision the camera made.

If you set the camera to shutter priority (Tv) the camera will automatically set the aperture. I don't know how it works on other cameras but on the 7D it only adjusts the aperture. It does not adjust the ISO, even if it is unable to obtain a proper exposure level (due to limitations of the lens/aperture size). When the user applies EC the only setting that changes is the one that was automatically set by the camera. In Tv mode EC manually adjusts the cameras automatic aperture setting and in Av mode EC manually changes the camera's automatic shutter speed setting. EC doesn't alter the ISO in either of these modes - the user has to do that themselves.

Dan Marchant wrote in post #13516621
But EC isn't the camera automatically adjusting settings - in fact quite the opposite. EC is the user manually adjusting those settings that the camera automatically set, in order to compensate/override the decision the camera made.

Which is exactly what Teamspeed implied.

Dan Marchant wrote in post #13516621
If you set the camera to shutter priority (Tv) the camera will automatically set the aperture. I don't know how it works on other cameras but on the 7D it only adjusts the aperture. It does not adjust the ISO, even if it is unable to obtain a proper exposure level (due to limitations of the lens/aperture size). When the user applies EC the only setting that changes is the one that was automatically set by the camera. In Tv mode EC manually adjusts the cameras automatic aperture setting and in Av mode EC manually changes the camera's automatic shutter speed setting. EC doesn't alter the ISO in either of these modes - the user has to do that themselves.

No, if you set the 7D on Auto-ISO and dial in EC in Tv/Av mode, it tries change both ISO and Aperture in Tv or ISO and SS in Av. It "maxes or mins" out on one of them and then starts changing the other value, depending on whether a +ve/-ve comp is being applied and in which mode.

nikmar08 wrote in post #13517189
Which is exactly what Teamspeed implied.

No, if you set the 7D on Auto-ISO and dial in EC in Tv/Av mode, it tries change both ISO and Aperture in Tv or ISO and SS in Av. It "maxes or mins" out on one of them and then starts changing the other value, depending on whether a +ve/-ve comp is being applied and in which mode.

And this is what I found happened with my 40D and T2i. So as long as you are satisfied with the minimum shutter speed being 1/focal length when in Av mode, or smallest f-stop in Tv mode, auto ISO works in conjunction with EC. I would just like to be able to set a longer shutter speed in cases like this where my lens has IS, or set a bigger f-stop when I need to deeper depth of field.