Exposure Value and Exposure Compensation

I just saw a Facebook post about exposure compensation today, and thought I’d do a quick post on this very important little feature that is often overlooked by many new photographers

Exposure and Exposure Value (EV)

Ok, so to know what exposure compensation does, you should have an idea of what exposure and exposure value means. Alright. Exposure, as you probably know already, is the amount of light that falls on your camera’s sensor (or film). The whole idea of setting the proper aperture and shutter speed on your camera is to get the correct amount of light to fall on your sensor (or film!) to get your picture to look right – for your picture to be well exposed. This amount of light is usually determined by a light meter (built in to your camera) or by yourself, and it is generally determined as a value known as exposure value, or EV. This value is a numeric value, a whole number, and could be something EV1, EV6, or EV10…you get the idea. The darker the scene, the lower the EV number will be, and, like shutter speed or aperture, EV increases in stops…meaning the change of 1EV (one stop) means double or half the exposure. Pretty simple stuff. However, based on any one of these values, there are a whole range of shutter speed/aperture combinations that will result in this same exposure value i.e. the same amount of light entering the camera

For example, using my light meter right now in my living room, it tells me that to get a properly exposed picture, I need an exposure value of EV7 at ISO 100. To get this exposure value, it suggests a combination of f2.0 and 1/30 sec. However, the combination of f2.8 and 1/15 sec will also result in EV7, and so will f4.0 and 1/8 sec. F5.6 and 1/4 sec will give me EV7 too. And so on. Any one of these combinations can be used to get a perfectly exposed shot in my living room – because they all give an exposure value of EV7

There’s a whole series of aperture/shutter speed combinations that will result in an exposure value of EV7. And yes, these are fixed values: a given aperture/shutter speed combination will always result in the same EV. Always. As they are constant fixed values, you can find a chart that shows various shutter and aperture combinations and which exposure value they result in – check out the one below:

Source: Wikipedia

So, you can always know that f2.8 and 1/15 will always be EV7. Cameras don’t show you the EV, though – none that I have used ever showed the EV for a particular shot – they just show the aperture/shutter speed combinations. But know that this is how your camera’s light meter figures out the aperture/shutter: by determining the correct EV for the shot

NOTE: in the first example, I mentioned that in my living room, I needed an exposure value of EV7 at ISO 100, right? What happens if I double the ISO sensitivity to ISO 200? Then I need half the light that I previously needed, right? So now I need an exposure value of EV8 (at ISO 200) to expose my living room perfectly! That’s EV8, which is one stop more than EV7. Half the exposure! Ok, so the point of this note? EV is independent of ISO sensitivity

Exposure Compensation

The Compensation Icon

So this post started off as a quick write-up on exposure compensation, the little feature that almost all automatic cameras have…but I went on about exposure value for a long part of the post (for your own benefit, I assure you!) so much so that I had to edit the title of the post to read both exposure compensation AND exposure value! But alright, let’s move on to exposure compensation

Ok, so once your camera meters the scene and says, alright this scene is EV10, that’s the only amount of light that your camera will allow onto the sensor/film, no matter what you do, unless you shoot manual and set the aperture/shutter/ISO yourself. But if you’re shooting some form of automatic exposure on your camera, like the Program Auto mode on your camera, or Aperture/Shutter Priority, once the camera meters and says you need EV10 for this scene, it will not change that easily

This isn’t usually an issue because a good camera meter is spot-on most of the time. However, there are situations, situations that can fool the light meter of any camera, situations that require a thinking human brain to figure out what’s going on! In these cases, you will need to adjust the exposure value that the light meter has set for the particular scene. This is done by the exposure compensation setting on your camera! There, I finally got to the point!

So, the light meter says you need EV10 to expose the picture properly – but you disagree. You think it needs to be exposed to more light…so you dial in an exposure compensation of say +0.3EV, or +0.5EV, or even +1EV .0 or 2.0EV, depending on how wrong the camera’s got it. Let’s say you set an exposure compensation of +1.0EV. This means you want an additional stop of exposure for this image. What this does is, it doesn’t simply change the aperture/shutter speed combinations on your camera (which will end up with the same EV) – no, it actually tells the camera that you need an exposure value of EV9, not 10, to expose this picture properly (lower EV = camera allows more light inside, so +1EV compensation will result in the total EV reduced by 1). THEN, the camera will change the exposure settings to get a value of EV9

So, if previously the camera metered EV10, and set f2.8 and 1/125 sec, and if you tried to adjust the aperture to allow more light in but the camera remained set on EV10 (i.e. no exposure compensation), the resulting exposure would’ve been the same i.e. f2.0 at 1/250  – which is also EV10. But once you dial in +EV1.0, that makes the camera want to allow EV9 onto the sensor or film…meaning that instead of the original f2.8 and 1/125, the camera now shoots at f2.8 and 1/60! Or f2.0 and 1/125! These are settings that allow more light into the camera than the original settings, meaning you succeeded in telling the camera what you wanted

This gives you so much more flexibility, so much freedom, it allows you to do anything, really – you can express anything you want with a particular scene, and you’re not held back by what the camera wants. Without that freedom, you will find yourself very limited. Use this feature. I spent a long time explaining it – use it for me, at least!

When to use Exposure Compensation

When will you need to use this feature? Well, the common example is when you’re taking a picture of something or someone that is backlit i.e. a person standing in front of a light source. When metering such a scene, the camera detects a lot of bright detail, compared to the small (darker) area which is the subject – and without realizing that it is the background that is bright (the camera isn’t that smart!), it takes the entire scene and averages it to get an exposure value is that is right only to expose the majority of the scene, the background. The result = the subject is either in complete darkness, or is severely underexposed, while the background is nicely exposed

What do you do?! You could use a flash to brighten up the subject – this would probably be ideal, as then you will have both subject and background properly exposed. Or you could go for the silhouette effect, and leave the subject in darkness – or you could use exposure compensation! Compensate by however much depending on how underexposed the subject is – maybe +EV0.5 or 1.0, even 2.0 or more if necessary – which will tell the camera to brighten things up, and although it’ll complete blow out (overexpose) the background, the subject should be well exposed

Exposure Compensation being used

Note, in the example above – the subject was in front of a window, completely ruling out the use of a flash (flash would reflect off the glass) – and as you can see in the first picture, the ‘silhouette’ look really isn’t working, that could be a silhouette of a box of chocolates for all you know – no, the only option was to brighten things up by compensating with +EV2.0

Other instances where you’d need to compensate exposure is when shooting scenes that are mostly pure white, or complete black. When shooting white scenes (think snowy landscapes) the camera detects all that white as additional light, and tends to underexpose the scene. So if you’re shooting the Swiss Alps, this will result in all your lovely pure white snow looking a dull, boring grey. Compensate +EV1 or so, and boom: pure white snow again!

Using exposure compensation when shooting primarily white objects

In this example of my white wall, see how the first picture looks grey? Only after adding a compensation of +EV2.50 was I able to make it look white. Note that this is an extreme example…and not the best example at that, as you probably see the picture on the right as a blank image…but you get the idea, right?

With black, the camera assumes the scene is very dark, and often will overexpose the picture, making the black look like grey again (the meter works a lot with greys, read up on how light meters work!) – so, to get the blacks to look nice and…black…compensate by maybe –EV2. That should do the trick

Using exposure compensation to shoot a pure black subject

Another extreme example here, a black subject filling the frame completely – see how the camera tries to make the black look grey? A huge compensation was necessary again, -3.0EV…although maybe in my excitement I overdid the compensation – I think -2.5EV would’ve been better!

And that’s it for this post. I hope you get the idea of what I was trying to explain here. Your camera meter is usually very accurate (depending on your camera of course), and basically all new automatic cameras are very capable of getting correct exposure in most cases…but in the event it doesn’t, it’d be a shame to lose the shot just because you didn’t know about a feature that’s already built into your camera. I’m sure anyone who’s had a DSLR or similar for more than a month is very aware of this feature…but the thing is, its available even on the cheapest compacts, so some of you may not have figured it out. Go try it out and let me know if it worked for you by leaving a comment. Or just share your thoughts, ideas, or whatever…I’d love to hear back from you guys. Thanks

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By Heshan Jayakody
All content in this post is my own, except where noted

Technorati Tags: photography, landscape-photography, studio-photography, photography-tips, pixelogist

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