On a digital camera, what is the difference between ISO and Exposure Compensation?

Neiby · Aug 12, 2011

I just want to make sure I have this right. ISO affects the sensitivity of the sensor to light. Higher ISOs are more sensitive to light, so in effect one can think of ISO as the speed at which some level of exposure could theoretically happen. The exposure compensation just affects the starting point for the exposure, but doesn't affect the time that a particular exposure would take like an ISO.

That's horribly explained, but am I close to right? It seems that they're definitely related, but do different things.

PJL · Aug 12, 2011

You're close.

ISO is the sensitivity to light of the sensor. The speed of exposure, however, is still controlled by your shutter speed (while aperture controls the amount of light let through). What higher ISO means is that you can use a faster shutter speed (or smaller aperture, or combination thereof) to obtain proper exposure compared to a lower ISO.

Exposure compensation means that the camera has selected an ISO, aperture, and shutter speed based on its light meter to be "correct" for that particular scene based on your preferred metering method. You can then tell the camera to let a little more light in to "overexpose" the picture, or let a little less light in to "underexpose" it. When you "bracket" a shot, that usually means you take one underexposed, one at the correct exposure, and one overexposed.

Neiby · Aug 12, 2011

That makes sense. But let's say you are on full manual and have chosen your ISO, aperture and shutter speed on your own. In that situation, what is the exposure compensation knob doing? Or in your example of bracketed photos, what is it doing? It seems as if it would cause the camera to leave the aperture and shutter speed as selected and then slightly alter the ISO to get the expected compensation. It's almost as if the exposure compensation is an ISO adjustment relative to the configured ISO.

Another question comes to mind. Let's say I have a shot I want to take at a certain shutter speed and aperture but it's still too dark. A higher ISO would lighten it up, but higher ISOs also mean noisier images. I wonder if there is any substantial difference in noise level between using the ISO setting or the exposure compensation to get more light.

I'm running on just a couple of hours sleep, so forgive me if I'm rambling. lol I desperately need more coffee.

Neiby · Aug 12, 2011

I guess to answer part of my question, I was just reading another thread about this and someone mentioned that when shooting in full manual, their exposure compensation setting is disabled, which I guess makes some sense.

PJL · Aug 12, 2011

Neiby said:
That makes sense. But let's say you are on full manual and have chosen your ISO, aperture and shutter speed on your own. In that situation, what is the exposure compensation knob doing? Or in your example of bracketed photos, what is it doing? It seems as if it would cause the camera to leave the aperture and shutter speed as selected and then slightly alter the ISO to get the expected compensation. It's almost as if the exposure compensation is an ISO adjustment relative to the configured ISO.

Depends on what mode your in, what the camera wants to do, and how much exposure compensation you want to do. If you're shooting with a fixed ISO, then the camera will change either the shutter speed or aperture in order to adjust exposure compensation. If you're shooting in "P" mode, where the camera selects everything, then it may simply adjust the ISO, or it will change the shutter speed or aperture. In manual mode, you perform the exposure compensation by using the light meter.

Another question comes to mind. Let's say I have a shot I want to take at a certain shutter speed and aperture but it's still too dark. A higher ISO would lighten it up, but higher ISOs also mean noisier images. I wonder if there is any substantial difference in noise level between using the ISO setting or the exposure compensation to get more light.

It really depends on the camera and what ISO's you're talking about. If you're going from 100 to 200 or 100 to 400, you're not going to see an appreciable noise difference. If you're going from 400 to 1600 or 3200, you'll see a lot more noise at 3200. Changing the ISO by less than a stop (ISO is considered one stop higher if you double the number, so 200 ISO is one stop faster than 100 ISO; 1600 is one stop faster than 800) should not add any significant amount of noise. Also, noise can be mitigated through on-camera or post-processing.

GooniesNeverSayDie11 · Aug 12, 2011

Typically, if in Manual mode, it is my understanding that the camera changes the Ambient light ( shutter speed ) and possibly the flash as well. ( I believe Nikon does this ) Some Cameras do not allow any exposure comp in full manual. I am pretty sure Canon doesn't allow this ( although it does in Av and Tv mode )

This may help, its one of the first things I found when googling ( which is also a tip for you )

Camera Exposure Compensation Vs Flash Exposure Compensation | Daily Tips and Tricks for Digital Photography

clanthar · Aug 12, 2011

Neiby said:
That makes sense. But let's say you are on full manual and have chosen your ISO, aperture and shutter speed on your own. In that situation, what is the exposure compensation knob doing? Or in your example of bracketed photos, what is it doing? It seems as if it would cause the camera to leave the aperture and shutter speed as selected and then slightly alter the ISO to get the expected compensation. It's almost as if the exposure compensation is an ISO adjustment relative to the configured ISO.

If you have the camera set on manual and you have selected the ISO, shutter speed and f/stop manually then the exp. comp. will do nothing. On Nikon cameras the exp. comp factor will be added into the meter, but if you're ignoring the meter then you'll get the ISO, shutter speed and f/stop you've selected.

Neiby said:
Another question comes to mind. Let's say I have a shot I want to take at a certain shutter speed and aperture but it's still too dark. A higher ISO would lighten it up, but higher ISOs also mean noisier images. I wonder if there is any substantial difference in noise level between using the ISO setting or the exposure compensation to get more light.

Exp. comp. can't help the situation you're describing. For a given ISO, shutter speed and f/stop if the photo is then underexposed the only way to correct that error is to alter one of those three settings -- ISO, shutter speed or f/stop. Exp. comp. is appropriate when you're using the camera in P, T or A modes and you know the meter in your camera is about to make a mistake. Exp. comp. will allow you to intervene at that point but it will alter one of those three settings. Let's say you have the ISO fixed at 200: Put the camera into T mode and set a shutter speed of 1/500th sec. The camera meter will then pick the f/stop to match given the light intensity. You look at the scene and know from experience that the camera is wrong and is about to overexpose. So you dial in a -.7 exp. comp and that forces the camera to change the f/stop to a smaller stop.

Joe