ISO vs exposure compensation in a digital

[RonFrank]

Using exposure compensation is telling the camera how to think. You're saying, "I want you to under-expose or over-expose. How the camera achieves that depends on which auto mode it's in.

Technically all it is doing is changing the meter sensitivity by the factor one selects. So in effect the camera does not change anything, it just reacts to an exposure that is biased and does the same things it always would do which is why changing the setting in aperture priority changes the shutter and not the aperture.

One can achive the same thing by using the meter in manual mode, and adjusting the settings to the point that the meter is showing over or under exposure. Exposure compensation is useful if one is shooting in Shutter priority as one can quickly adjust exposure without changing modes, and then setting the manual settings.

I use it all the time topside shooting, especially the exposure compensation on the flash which I often have set to -2 stops and is a bit different than what we are discussing.

 

[RonFrank]

- slow your shutter speed (1/200 to 1/160 etc). Underwater your shutter controls the background colour (light blue, dark blue black etc as you speed up your shutter)

This is true only if you are using a strobe as a primary light, and include the open water that the flash does not expose as the background.

The downside for using exposure compensation is that you haven't learned how to expose the image properly. EC is used as you shoot, not after.

I have to respectfully disagree with this statement. (How is that for PC )

There is no downside to exposure compensation, nor does it necessarily mean that the photographer has not learned to expose the image properly.

Rather than go into technical details, I'll give two examples, and how I would shoot them.

Example 1: Shooting larger fish using a WA lens against a coral backdrop, sunny sky's, no cloud cover, and the light is NOT changing, nor is the depth. This is a perfect setting to shoot in manual mode. Why? because once you get the exposure set, you can just shoot as the light is not changing. Bright and Dark subjects fool reflective meters, and manual settings will ensure that once you find a good exposure setting based on a nuteral scene, you can forget about it, and not worry about the meter getting fooled.

Example 2: Sunny day, a lot of clouds that result in changing light conditions -or- anything that results in changing lighting from shot to shot. This is where I would use a mode like shutter or aperture priority. Why? Well the light is changing, so my settings need to change often as well. The camera will do a good job of making those changes, but as I indicated in Example 1, the meter is fooled by some situations. Here is where exposure compensation comes into play. If I'm shooting a very reflective or bright subject, I will generally add +2/3 of a stop as the subject is going to make the meter think there is more light available then there really is. With digital the great thing about exposure compensation is that you don't even need to set it initially, just a quick review of the histogram, hmm some blocked blacks, solution, open up the aperture a bit by adjusting the exposure compensation. In the film days there was only ONE way to make sure you had things right... bracket.. bracket... bracket...

I think what alcina was getting at is that exposure compensation should be not used blindly without an understanding of what the camera is doing, and that I agree with 100%. But it's just a tool in the box, and like any tool, it can be very useful.

 

[alcina]

. But it's just a tool in the box, and like any tool, it can be very useful.

 

[bladephotog]

Technically all it is doing is changing the meter sensitivity by the factor one selects. So in effect the camera does not change anything, it just reacts to an exposure that is biased and does the same things it always would do which is why changing the setting in aperture priority changes the shutter and not the aperture.

One can achive the same thing by using the meter in manual mode, and adjusting the settings to the point that the meter is showing over or under exposure. Exposure compensation is useful if one is shooting in Shutter priority as one can quickly adjust exposure without changing modes, and then setting the manual settings.

I use it all the time topside shooting, especially the exposure compensation on the flash which I often have set to -2 stops and is a bit different than what we are discussing.

And it changes the aperature in shutter priority. Using ex. comp in aperature priority you're telling the camera to jack the shutter speed up or down respectively. Maybe it's more of a command, ie., "under expose by one stop from what you want to expose." The meter sensitivity though is still the same as you haven't changed the ISO but told the camera how to change the shutter speed or aperature.

In my daily shooting I use aperature priority a great deal of the time to deal with changing light conditions. And with my EOS 1D MKII's I can adjust the ex. comp with my thumb using the main control wheel on the back. This way my "auto" mode is going to get me close exposure-wise, yet I can still have control almost as if I was shooting in manual.

It is a great tool and if used enough it becomes very intuitive. You'll automatically know how much to over or under expose depending on the situation, as experience has taught you how your camera will try to expose the scene in front of you.

 

[alcina]

OK...so what, in quick terms, are the advantages & disadvantages to using exposure compensation? I guess I am not seeing any real advantages, at least for my type of shooting...if you KNOW you want your camera to under/over expose from your settings by a certain amount, why wouldn't you just set the aperture/shutter etc the way you want it in the first place?

This is not a question about the validity of using this method - we've established that everything on your camera is a tool. This is a question to make it very easy for everyone to see clearly what the advantages/disadvantages are with this method.

Example: benefit of using M over P mode is that you can control the background colour with your shutter speed.

Can we get a list of these for members to look at?

 

[bladephotog]

One advantage is if you know your camera has a tendency to over expose, which mine does in bright light. So if I'm using aperature priority I set exposure compensation to -1/3. The camera will still pick the correct shutter speed for me but it will be 1/3 faster than what it would without using any compensation.

Or if I'm shooting a scene that is backlit I can set the ex. comp. to + 1/3 or more and it will automatically select a slower shutter speed for me. Then if I want a smaller or larger aperature the camera will pick my shutter speed for me still using the ex. comp. I set earlier. The advantage over doing this in manual mode is that I only have to change one setting . The camera changes the other. But UW I use manual the majority of the time. I'm shooting wrecks which aren't moving very quickly.