ISO vs exposure compensation in a digital

[dlwalke]

I understand both the effect and the principle of shutter speed and aperture vis-a-vis correct exposure. However, the difference between ISO and exposure compensation eludes me [Don't know if this is a common, but on my Olympus, there is an exposure compensation function that can be adjusted in increments from -2 to +2]. In my Olympus C5060 manual, they both say, basically, that they increase sensitivity to light (my paraphrasing anyway). When would one use one and not the other, or vice versa? And what is the difference in terms of what they actually do at a concrete level. I'm talking digital (I know what ISO is for film) BTW.

Clueless in Atlanta

 

[Jamdiver]

Hi dlwalke,

I'll try and offer some advice here, disclaimer though I too don't fully understand exposure compensation but I can offer my practical experience with it and ISO.

First i'll start off with a definition of ISO from a great site http://www.scubahound.com/?page=basics

ISO is a rating of the light sensitivity of media, either a digital sensor or film. Higher ISO numbers require less light to expose the photo. Film photographers know that you select your film based on ISO numbers, but in the digital world, you set the ISO through the menu system on the camera. Using higher ISO settings comes with a price however. High ISO films exhibit more grain, while high ISO settings in digital cameras result in more 'noise' (stray artifacts) in the image. The rule:

Higer ISO = Less light required = Lower quality images

The lower quality images reference in the last sentence, refers to the fact that with most point and shoot cameras if you increase the ISO, noise is increased in the image.
This noise is usually seen as this weird orange/reddish dots all over the image, it's hard to explain you should probably try it on your camera to see for yourself.
On most cameras increasing ISO to 100 or 200 produces very useable results.

If I were at 80ft, and my shots were consistently dark/underexposed then I would probably increase the ISO to 100 or 200 for this, providing that I couldn't decrease shutter speed anymore or increase the aperture.

Exposure compensation is like greek to me, in my view it's just like darkening a whole image...
Negative EV values darken the overall image while positive EV values seem to brighten it.

Hope this helps with the caveat being that i'm a newbie at this and mostly self taught.

 

[MarkH]

On my video camera, exposure compensation is just adjusting the automatic exposure. For example, if you think the automatic exposure is always too light for your tastes, you can darken it up overall by up to 2 stops.

 

[devolution365]

Swankenstein is right. Exposure compensation is just a way of telling your camera to make the picture lighter or darker than it usually would have.

Auto exposure assumes a normal picture ie green grass, blue sky, etc. If you're taking a picture of a snowy scene, it'll try to make it darker -more like "average". Likewise, if you're shooting a darker scene, it'll try to lighten it. So, you use exposure compensation to fix that. If it's a light scene, use exposure compensation to lighten it by one or two "stops", or if it's dark, go one or two darker. (Lighter = positive, darker = negative.)

My advice would be to play with it -experiment with it a little. It's digital, right? So, no film or developing to waste...

 

[RonFrank]

but on my Olympus, there is an exposure compensation function that can be adjusted in increments from -2 to +2]. what is the difference in terms of what they actually do at a concrete level. I'm talking digital (I know what ISO is for film) BTW.

Clueless in Atlanta

Well you are NOT just talking digital, because they generally all work the same way.

Exposure compensation is based on the mode you are shooting in. In manual mode it does nothing, or messes with you meter. In aperture mode it adjusts the shutter -/+ the number of stops you select. In Shutter priority it adjusts the aperture +/- the number of stops one selects.

So exposure compensation is exactly that, and it does NOT impact the ISO but rather the exposure based on the setting. I'm not sure in program modes what it would change, but it's likely based on manufacture.

 

[bladephotog]

Ditto what Swankenstein said. By adjusting the exposure compensation you are telling the camera to under-expose or over-expose what the camera thinks the exposure should be.
When do you want to do this? Say for example you have a very bright subject and a very dark background and you're using either aperature or shutter priority. Your camera will weight the exposure heavily towards the background, which in turn over-exposes your subject. So you'd dial down the exposure to correctly expose for the subject, since that's what's important. This is all regardless of ISO. In aperature priority, your camera will either use a faster or slower shutter speed depending on whether you decrease the exposure compensation or increase it.

 

[dlwalke]

OK. I kind of get it, but not entirely (and maybe not at all). In terms of practical usage, let's say I take a pic and it comes out too dark. One option, I assume, would be to decrease shutter speed, another would be to increase aperture. Let's say I don't want to do either of these because I am worried about blur or depth of field or something. Wouldn't another option be to increase ISO or, alternatively, to change exposure compensation. Insofar as increasing ISO potentially increases noise, I would think that the prefered method would be to change exposure compensation value. Casting aside, for the moment, blur, depth-of-field, and noise issues...am I wrong in thinking that these would all have the same effect of lightening up the picture? I assume that I am either wrong about the similarity of the outcome of these different approaches or, alternatively, that there is a downside to exposure compensation that I am not aware of. Is there a downside?

Thanks again.

Swankenstein is right. Exposure compensation is just a way of telling your camera to make the picture lighter or darker than it usually would have.

Auto exposure assumes a normal picture ie green grass, blue sky, etc. If you're taking a picture of a snowy scene, it'll try to make it darker -more like "average". Likewise, if you're shooting a darker scene, it'll try to lighten it. So, you use exposure compensation to fix that. If it's a light scene, use exposure compensation to lighten it by one or two "stops", or if it's dark, go one or two darker. (Lighter = positive, darker = negative.)

My advice would be to play with it -experiment with it a little. It's digital, right? So, no film or developing to waste...

 

[alcina]

In terms of practical usage, let's say I take a pic and it comes out too dark. One option, I assume, would be to decrease shutter speed, another would be to increase aperture. Let's say I don't want to do either of these because I am worried about blur or depth of field or something. Wouldn't another option be to increase ISO or, alternatively, to change exposure compensation.
.

If your photo is too dark (which is better to post process than too light btw):
- 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)
- open your aperture. Go to f4.5 instead of f.6.3, for example. Aperture controls how much light you get in (it's a hole that gets smaller and larger). Aperture controls your depth of field. Which is why, IMHO, you shouldn't use your camera in P mode ... a lot of times the camera opens up to 2.8 (or whatever on your particular camera) and the photo looks soft. On a compact digital, f8 is often your smallest aperture (least light) and gives you the best DOF - I use this for great details on macros etc.

The downside for upping the ISO, which will lighten the photo, is potentially more noise.

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.

Unless you are shooting in dark waters (PNW etc) a shutter of 1/100 or 1/125 should by fast enough for most subjects.

In your scenario, yes, perhaps one of those would be a viable option. You'll have to do some testing!

 

[Monkey Knife-fight]

Well you are NOT just talking digital, because they generally all work the same way.

Exposure compensation is based on the mode you are shooting in. In manual mode it does nothing, or messes with you meter. In aperture mode it adjusts the shutter -/+ the number of stops you select. In Shutter priority it adjusts the aperture +/- the number of stops one selects.

So exposure compensation is exactly that, and it does NOT impact the ISO but rather the exposure based on the setting. I'm not sure in program modes what it would change, but it's likely based on manufacture.

Read this post again it explains what exposure compensation is. It won't miraculously make pictures brighter or darker. It just changes the settings (shutterspeed, aperture) that the camera chooses when in auto modes. It does nothing in manual mode.

 

[bladephotog]

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.