image sensor / metering question

[theregoesjb]

I have a point and shoot, canon SX 130 is. Its pretty good with manual options and i am running the CHDK firmware which gives a little more control.

I see the term 'metering' a lot with explainations of image sensors (i have been trying to learn about exposure compensation), what I am wondering is if there are any atributes in metering that cannot be manually controlled?


For example, say I am taking a picture of something outdoors, at early evening such as my dog, and there is an interesting sky. If i just take the picture of my dog then the sky will usually wash out. OR- I can half-press the shutter button while pointing it at the sky and then point it at the dog and fully press it to take the picture and get the sky looking good and with the dog looking like its in more dim light.

By 'metering' on the sky and then pointing it at the dog, has the image sensor done anything that I would not have been able to manually do with aperture, shutter speed, ISO ?

hope this makes sense

 

[Big Mike]

Welcome to the forum.

'Metering' doesn't necessarily have anything to do with the sensor or what it's doing.

When we 'take a photo', we are creating an exposure. To control the exposure, there is the shutter speed, the lens aperture and the sensitivity of the medium (ISO setting).
To get a brighter image, you adjust those settings to get more exposure. To get a darker image, you adjust for less exposure.

Metering is the process of determining what combination of settings to use....or at least, what EV (exposure value) to use (a specific EV value can be reached by several different combinations of those settings).

Cameras have built-in light meters. So when you point the camera at something (unless you are in fully manual mode), the camera changes the settings to give you what it thinks is a good Exposure Value. Exposure compensation just tells the camera to change one of the settings to give you more or less exposure.

On a more advanced level...the act of metering is choosing settings to get the exposure that you want...not just what the camera is telling you. As a matter of fact, camera meters are designed to give you the wrong exposure value in some situations...but because it's predictable, we can compensate for that by changing the settings to get more or less exposure.

When you are pointing the camera at the sky, you are then using AEL (auto exposure lock). You lock in the settings that the camera suggested while pointed at the sky...you then use those setting to take a photo of something else. If you don't have a good understanding of how camera meters work...this can be very hit-and-miss, but if you have a good understanding, then it can be a good way to get 'proper exposure' and not just the average exposure that the camera might give you.

But either way, the sensor isn't doing anything different. You use the shutter speed, aperture and ISO to control the brightness/exposure of your photos.

 

[dots]

By 'metering' on the sky and then pointing it at the dog, has the image sensor done anything that I would not have been able to manually do with aperture, shutter speed, ISO ?

hope this makes sense

As i understand it ..no. If the viewfinder contains only sky (and you are seeing ~100% of the meterable scene through it) you've metered for the sky. You've told the camera(sensor) that the portion of the sky (or an average of it) is middle grey, in terms of luminance. The sensor can only see in b&w ..luminance...the Bayer color filter array handles color input. ..although Nikon cameras may meter "3D color metering" which is more sophisticated. Whether you now select ISO100 or ISO10,000 the sky represents middle grey luminance.

Use fillflash to expose properly the dog and retain sky detail.

 

[dots]

But if you spot or partial meter AND set an exposure compensation, you are telling the camera that the metered value is no longer "zone 5" it's biased to represent +/-1 Or +/-2 ..+/-5 higher or lower. So Middle grey ("Zone 5") with +2 compensation now means the metered value represents "Zone 7"
Exposure compensation - Wikipedia, the free encyclopedia

​

 

[DavidSutphin]

For example, say I am taking a picture of something outdoors, at early evening such as my dog, and there is an interesting sky. If i just take the picture of my dog then the sky will usually wash out. OR- I can half-press the shutter button while pointing it at the sky and then point it at the dog and fully press it to take the picture and get the sky looking good and with the dog looking like its in more dim light.

By 'metering' on the sky and then pointing it at the dog, has the image sensor done anything that I would not have been able to manually do with aperture, shutter speed, ISO ?

This actually sounds more like the difference in the light between the sky and your dog may be too great a range for your camera sensor to record all of it optimally. It's possible that manually setting an exposure between the 2 might produce something you're happy with (or something where nothing looks all that good). Our eyes adapt to a larger range of differences in brightness than digital camera sensors can. What would happen if you waited until the sky was just a little darker, or you shot at an angle where the sky was not quite as bright, or you persuade your dog to change angles. Or, you add a little flash on the dog?