Exposure compensation - what is it?

captain_boycott

Here's something thats been bugging me about camera theory, so hopefully someone can explain...

I understand the so-called "exposure triangle" - ISO, Aperture and Speed. The first sets the sensitivity, the second controls the size of light allowed in, and the final is the length of time the shutter is left open. Changing any of these will modify the overall exposure. But here's the thing...In all my camera's theres also a dial for exposure compensation.

So, suppose I go into manual mode, set my ISO, Aperture and Speed and take a pic. Its a bit under exposured, but DOF, sharpness, etc are ok, so I dial in a little exposure compensation. My question is, what is the camera actually doing in this case? Is it modifying aperture, speed or ISO, or is there a 4th factor that effects how the camera controls the exposure?

Heebie

If you are in a semi-automatic mode like aperture or shutter priority (or any of the others like iso priority I suppose) If you find that it's consistently over or underexposing something you're trying to shoot, you can tell it to over or under expose to compensate. (so.. if it's always coming out a stop or two too "hot".. you can use exposure compensation to tell the auto-exposure system that you WANT it less exposed, or that you want it MORE exposed than it's "normal")

It's also useful if shooting slides. With slide films you generally want an overexposure of 1/2-1 stop

If you're using exposure compensation AND full manual exposure..that's just kinda weird. (although, I suppose if you set the compensation where you like things to come out.. then shoot where the meter says is "right".. you'll get things that consistently look the way you want.)

charybdis

Exposure compensation changes the way the auto-exposure modes of your camera expose relative to the readings from your camera's light meter. Your camera will suggest exposure parameters for a given scene, setting exposure compensation to -1 stop will set your auto-exposure mode to under-expose the scene by 1 stop.

Heebie wrote: »

It's also useful if shooting slides. With slide films you generally want an overexposure of 1/2-1 stop

Are you sure? I would've thought you'd typically want to underexpose slide film as it doesn't deal with blown highlights very well.

Heebie wrote: »

If you're using exposure compensation AND full manual exposure..that's just kinda weird. (although, I suppose if you set the compensation where you like things to come out.. then shoot where the meter says is "right".. you'll get things that consistently look the way you want.)

I don't think any camera will actually allow exposure compensation and manual exposure control to be used together, it wouldn't make any sense.

Heebie

Charybdis worded this a bit more succinctly than I did, I think.

charybdis wrote: »

Are you sure? I would've thought you'd typically want to underexpose slide film as it doesn't deal with blown highlights very well.

Yes, I'm sure. Slide film, for some reason, has an enormous lattitude for overexposure. People like to overexpose because it tends to give more saturation to the color. (only 1/2 to 1 stop.. so it's not drastic.) We used to have some people who'd shoot it over by 1/2 stop, then have us push process it 1/2 stop..and some who just had it pushed a full stop.

charybdis wrote: »

I don't think any camera will actually allow exposure compensation and manual exposure control to be used together, it wouldn't make any sense.

It seems pretty silly to me too.. and I know neither of my SLR's lets me use compensation when shooting in full manual.. but I haven't used every SLR in the world.. so I didn't assume.

AnCatDubh

To add to the above - it may depend on camera and manufacturer, but in terms of what exposure compensation is actually doing is, if you are in Aperture Priority, then adjusting the dial will tell the camera to compensate + or - across the aperture value. If shooting in shutter priority, then the camera will compensate across the shutter value/setting.

Check your camera manual for more information on the approach taken by your manufacturer.

In theory, if you are shooting manual then in effect you have the control over all the settings so exposure compensation shouldn't have any effect. You are in effect controlling both the aperture and the shutter speed settings so you control under and over exposed image (and of course properly exposed image).

I think that's the jist of it but i'm open to further correction / clarifications.

davmigil

Heebie wrote: »

It's also useful if shooting slides. With slide films you generally want an overexposure of 1/2-1 stop

It's the other way around with slide film: you generally want to underexpose as overexposure will result in burnt out highlights. Slide and digital sensors are similar in that respect. (slight) Underexposure will give more saturation.

Captain Flaps

I came in here expecting to hear about the OPs fines for public indecency.

charybdis

Heebie wrote: »

Yes, I'm sure. Slide film, for some reason, has an enormous lattitude for overexposure. People like to overexpose because it tends to give more saturation to the color. (only 1/2 to 1 stop.. so it's not drastic.) We used to have some people who'd shoot it over by 1/2 stop, then have us push process it 1/2 stop..and some who just had it pushed a full stop.

I'm fairly sure slide film has very little latitude, and surely underexposure would lead to better colour saturation?

Heebie

Underexposure leads to it looking washed out & spotty. Shooting at 1/2-1 stop over just gives it a tiny bit more "oomph" as it was explained to me by more than one customer back when I was doing photographic work for a living. Anything more than 1 stop and you just ruin the film. (I, personally, used to shoot slide film with 3x 1/2 stop bracketing or 5x 1/2 stop bracketing and then choose the best of those.. I found this "shutgun" approach pretty much always got the job done the first time.) =D

charybdis wrote: »

I'm fairly sure slide film has very little latitude, and surely underexposure would lead to better colour saturation?

davmigil

Heebie wrote: »

Underexposure leads to it looking washed out & spotty. Shooting at 1/2-1 stop over just gives it a tiny bit more "oomph" as it was explained to me by more than one customer back when I was doing photographic work for a living. Anything more than 1 stop and you just ruin the film. (I, personally, used to shoot slide film with 3x 1/2 stop bracketing or 5x 1/2 stop bracketing and then choose the best of those.. I found this "shutgun" approach pretty much always got the job done the first time.) =D

Please stop :eek:, everything you say is right............. only backwards!!!!!!
Just swap the word under for over and it all falls into place.

Ghost Train

davmigil wrote: »

Please stop :eek:, everything you say is right............. only backwards!!!!!!
Just swap the word under for over and it all falls into place.

are you sure?

reading about it maybe its to do with the processing being slightly reversed

davmigil

The more light slide film gets exposed to, the brighter the image. The more overexposed the more washed out the slide. Overexposing it too much will result in at best blown hightlights and ultimately to a clear slide.

Underexposing it a little will result in darker colours. So a little underexposure would boost the saturation. Too much you will be heading towards a black slide.

Slide film is just like digital in this regard. You watch the highlights. Negative film is the other way around, you generally expose for the shadows. Unlike negative film, slide film is pretty unforgiving when it comes to exposures.

CabanSail

The EV Compensation is really a correction you put on the Light Meter in the Camera. This will then calculate the expose values (Aperture, Speed & ISO)

When I used to shoot slides I would UNDERexpose by about half a stop to increase saturation & avoid burnouts.

Cameraman

captain_boycott wrote: »

So, suppose I go into manual mode, set my ISO, Aperture and Speed and take a pic. Its a bit under exposured, but DOF, sharpness, etc are ok, so I dial in a little exposure compensation. My question is, what is the camera actually doing in this case? Is it modifying aperture, speed or ISO, or is there a 4th factor that effects how the camera controls the exposure?

If you're in manual mode - simply don't use exposure compensation - change your exposure (ISO, Aperture, Speed).
The effect of any exposure compensation would depend on the specific camera - I have no idea what it might do (should be in the manual - or try it and see which of the 3 values are changed).

Regarding Slide Film (and digital) - avoid blown highlights. When I took slide pictures, I always underexposed by 1/4 to 1/2 stop.