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View Full Version : Exposure compensation - why and when?


yelvertoft
24-12-05, 15:29
Having posted a thread on the basics of manual exposure mode, I thought I’d put together another beginner’s article on another exposure feature, “Exposure Compensation”.

To begin, it’s worth noting that a camera’s exposure meter monitors the amount of light entering the camera (the days of non-TTL metering are long gone in this digital era). Sounds obvious I know, but it should be remembered that it’s simply measuring the amount of light, not the colours. Despite all the technology that’s gone into cameras these days, cameras cannot interpret what they see, that is down to the user. The camera’s light meter detects a certain amount of light and it doesn’t know if this is a dark surface with a lot of light falling on it, or a light surface with a small amount of light falling on it. To cope with this uncertainty, the meter in the camera will be calibrated to give the correct exposure when it is measuring the light reflecting off a mid-grey surface.

So, if you take a picture of a completely black surface and nothing else, left to its own devices, the camera will tend to give settings that would result in more of a dark grey than black. Likewise, if you take a picture of a completely white surface and nothing else, the camera will give settings that will tend towards light grey rather than white. In each case, the camera does not know that the surfaces are plain black or white and the meter is calibrated so that it hedges its bets on grey.

Ok, the photographer isn’t so clueless about what he or she is looking at. The photographer knows they are looking at a black or white surface. Knowing this, and knowing that the meter is calibrated to assume everything is mid-grey, when faced with a scene with lots of bright/dark areas, the exposure compensation feature can be used to, errrmmm, compensate the exposure.

If you want the brighter areas of a scene to be remain bright in the picture, the compensation control should be used to give a positive compensation. If you want the darker areas of a scene to remain dark in the picture the compensation control should be set to a negative value. You know the camera will try and turn everything to grey and are telling it to keep the whites white (positive compensation) or the blacks black (negative compensation). If the scene is an equal mix of light and dark, assuming you are using an evaluative exposure metering mode, the camera will get things pretty much right on its own as it averages things out.

Scenes that will get it wrong would typically be snow scenes or where the subject is in shade against a bright background (positive compensation needed), or alternatively a light coloured subject against a very dark background (negative compensation needed).

How much compensation to apply will vary depending on the lighting, subject and background but +/- 1.0 is not at all unusual. Remember, with digital, it costs nothing to try a range of different exposures so fire away with a range of different settings and see what the results are like. Examine the EXIF data of each shot and learn the effect that each level of compensation gives. Many cameras have an “auto-bracketing” feature where a range of (typically) 3 shots can be taken, the camera automatically applying “correct” and a degree of under/over exposure compensation to the next three shots taken. This is a useful feature to help you learn the effects of under/over exposure and to help understand the meter readings your camera will have taken.

Hope this helps someone out there who is struggling with their new Christmas toys.

Regards and happy Christmas to everybody,

Duncan.

Joe Bellantoni
25-12-05, 22:33
Duncan: When I am shooting white birds I make the exposure with a -1 or even a -2 on the exposure compensation...If I don't it will usuall blow out the whites. This is the opposite of what you are saying...When I need detail in the blacks I add + 1 or +2 compensation....

yelvertoft
26-12-05, 18:30
Duncan: When I am shooting white birds I make the exposure with a -1 or even a -2 on the exposure compensation...If I don't it will usuall blow out the whites. This is the opposite of what you are saying...When I need detail in the blacks I add + 1 or +2 compensation....

Hello Joe,
It all depends on the light/dark balance of the whole picture. If you are taking a shot of something small and white against a predominantly darker background then I can fully understand that the whites will get blown out as the camera will try and get the majority of the picture to the "right" exposure. A smaller area of white in the shot will then be overexposed. Likewise for the black objects against a lighter background, the camera will try and set the exposure to get the majority of the shot "right" and the smaller dark area will be underexposed.

The camera has no inherent idea what the "subject" is. Without you telling it, it doesn't know that the small patch of white or black is the bit you are interested in. This can be achieved by spot metering off the subject rather than using evaluative exposure. If you take a spot meter reading directly off the white object it will tend to shift it towards grey.

If you read my paragraph "Scenes that will get it wrong would typically be ......[snip]........ against a bright background (positive compensation needed), or alternatively a light coloured subject against a very dark background (negative compensation needed)", you can see that we are not saying opposites, we are entirely in agreement.

Regards,

Duncan.

Joe Bellantoni
26-12-05, 18:34
I just wanted to clarify it for any newbee photographers because if the use it without truly understanding it they will drastically over or underexpose...

John
02-01-06, 11:29
I have just read Yelvertoft's informative thread on E.C. I fully understand the techniques he describes and how they apply to spot or partial metering. However, I have a problem with compensation when using 35 segment metering. I don't really understand how this works and so I am never sure how to compensate. Can anyone enlighten me? Thank you.

yelvertoft
02-01-06, 20:28
Hello John,
In the original article I tried explaining the way that the camera's metering worked when faced with a completely black or completely white surface initially to try and get the concept across. As you quite correctly point out, using spot metering off a black or white point in the image will give the same tendency towards dark grey or off-white as a completely black/white surface.

If you think about multi-segment metering, it is like a whole bunch of individual spot meters (35 in your case). Each segment will take a meter reading and the same rules will apply for each segment, the exposure will only be exactly correct if the surface is a mid-grey. However, the camera will take all 35 readings, off all the different segments and try and compromise to give a balanced exposure over the whole picture.

The same rules apply as I’ve explained in the original article, it’s just that there’s a debate going on inside the camera between 35 delegates before the camera comes up with an agreed solution! If we take the classic case of air-show photos (Stephen Fox/Snappy will jump on me if I get this wrong!), we have a relatively dark plane flying in a bright sunlit sky. Let us say that 25 segments are metering the sky and 10 are metering the plane. The camera does not know that the subject is the plane in a bright sunny sky, it just knows it’s a given amount of light that’s coming in to it. The metering is calibrated to give a correct exposure off a mid-grey surface, the sky is a lot brighter than that, so there is a tendency for the camera to underexpose the sky. “What about the plane?” you ask. In our example, 10 of the segments will be saying “Errrm, we’ve got a much darker bit in the middle here, can we have it a bit lighter on the exposure please?” The camera will bias the exposure a little towards the brighter side having had the input from the 10 segments covering the plane, but the 25 segments will win the vote against the 10 and the bias will not be enough to ensure the plane is exposed correctly.

Knowing that this compromise is taking place between your 35 segments, you should evaluate which part of the picture you wish to be most accurately exposed. Look at the light/dark tone of the part you wish to be exposed correctly. Look at the remainder of the picture and decide if it is a lot brighter/darker than the part you want to get right. Look at the relative areas of the part you want to get right versus the remainder of the picture. Knowing that your camera will try and balance the whole image towards a compromise in the middle, you should be able to adjust the compensation accordingly.

Some examples:
Small mid-tone subject against bright larger background.
Spot metering of the mid-tone subject will give the correct exposure, leaving it on multi-segment will result in the subject being slightly under exposed as the majority bright background wins the vote. Exposure compensation to apply: A small amount of positive compensation.

Small mid-tone subject against a dark larger background.
Spot metering of the mid-tone subject will give the correct exposure, leaving it on multi-segment will result in the subject being slightly over exposed as the majority dark background wins the vote. Exposure compensation to apply: A small amount of negative compensation.

Small bright subject against a dark larger background (see post #2 in the original article).
Multi-segment metering will result in the subject being over exposed as the majority dark background wins the vote. Exposure compensation to apply: A moderate amount of negative compensation.

Small dark subject against a bright larger background (see post #2 in the original article).
Multi-segment metering will result in the subject being under exposed as the majority bright background wins the vote. Exposure compensation to apply: A moderate amount of positive compensation.

Exactly how much to apply will vary depending on the exact circumstances, amount of light/dark areas and just how far off mid-grey the subject and surroundings are.

Hope this helps.

Duncan.

John
02-01-06, 21:09
Thank you Duncan for yoour exhaustive reply. Ive got it now!

robski
03-01-06, 14:11
Multi-segment metering mode. This mode also associates itself to a focus point and assumes that the area in focus is the subject. It then applies a set of rules to try and correctly expose the subject. However, if your subject lighting does not match the design criteria of the metering system you are still in trouble.

So early meter reading systems assumed that the reflected light from the subject is 18% of the incident light. This is because scientist measured that value from a typical landscape scene. A Gray Card is designed to reflect this value so they are useful to take reading from in tricky lighting conditions.

However, whenever you step outside of the typical normal scene you have to make a judgement on how best to get a good exposure. Then with experience it becomes intuitive as outlined in Duncan's examples.

Remember also many of the DSLR cameras have a histogram feature with the image preview. Ideal if you are able to take a number of shots to get the exposure correct.


Rob

John
03-01-06, 19:17
Thank you Rob for the further information. Yes, my 20D has the histogram and I use it, but I wanted to learn more for when I am using my film camera the EOS30.

John

robski
03-01-06, 23:46
Arrrrrrrrh it's that film word again. I remember those days when you had to wait to get the film processed to see the results. This is where digital is a god send. Maybe you can use the 20D like a polariod camera in the old days to test the exposure setting and then transfer them to the EOS30.

Rob

John
04-01-06, 15:00
Arrrrrrrrh it's that film word again. I remember those days when you had to wait to get the film processed to see the results. This is where digital is a god send. Maybe you can use the 20D like a polariod camera in the old days to test the exposure setting and then transfer them to the EOS30.

Rob

What a good idea. Why did it escape me. Thanks.

John