Please explain Metering to me.

[MOmilkman]

This is what I know about metering. It evaluates the amount of light that is used in a particular frame.
Thats about it.

I know there is much more to it, I just need someone to explain it to me in laymens terms.

Theres something about this I just cant seem to wrap my head around.

[zona5101]

ok I’ll bite. So based on the sensitivity of the sensor (ISO) the meter will see the amount of light and determine the optimum amount of light to properly expose the sensor. The amount of light is controlled through adjustment of the aperture of the lens and shutter speed. The sensor is designed to render the proper exposure based on all objects in the universe being 18% reflective gray. We know that’s not the case so we make override adjustments to the aperture and shutter speed. We also will use the meter reading to determine our desired combination of shutter speed and aperture to gives us a faster or slower shutter speed; or more or less depth of field while still providing the sensor the proper amount of light to make the exposure. As cameras and their meters have gotten more sophisticated, the electronics attempt to make some of those adjustments decisions for us. As long as we understand what the camera is trying to do and how it “sees” we can step in and make the changes to capture the image the way we want.

[inkista]

Originally Posted by MOmilkman

This is what I know about metering. It evaluates the amount of light that is used in a particular frame.
Thats about it.

Yup, that's about it. Metering measures the amount of light coming into the camera. The camera does calculations based on the iso/aperture/shutter speed settings to try and tell you if the exposure you're going to get is a good one or not, but it's kind of dumb in how it thinks and evaluates the scene.

Basically, all the values of light and dark in the scene are averaged (or weighted, or ignored/counted, depending on your metering mode). The problem is that the camera doesn't know what the scene is or what you want it to look like. It's just mechanically computing an average(ish) value and mindlessly setting that value to the middle of the exposure scale (middle gray).

If you have a good distribution of dark-to-light, the average really will be what you want to have set to middle gray, and the AE system will be very close to what you want.

But if you're shooting a night scene, the majority of your frame is going to be very dark, and you may want to keep it at black or close to black. But the algorithm is dumb, and the auto-exposure system will still be trying to set the average, a very dark/near-black value, to middle gray: upshot? you're likely to overexpose. Similarly, if you're taking a shot of a snow scene, the AE system will try to see a very light/near white value to middle gray and you're likely to underexpose.

This is why we like having exposure compensation and/or Manual mode to nudge the exposure one direction or another, based on the scene we're trying to capture.

One tool your camera offers you, in addition to metering, to let you judge exposure, is the histogram. Using that together with metering can often help you get exactly where you want to be, exposure-wise, as the histogram lets you see where the light/dark values sit, related to your camera's dynamic range.

[MOmilkman]

(head spinning) lol

Ok I'll take a shot here and see if I'm following.

I normally keep my camera on Evaluative, well manily because the person I bought the camera from told me that's what they used 98% of the time. Evaluative mode basically evaluates the light coming in the lens from all angles and exposes for that.

If you were on center weighted metering, it would base your exposure on the center portion of the frame and expose the whole picture based on that.
So if I were to take a picture of a light bulb in a dark room in center weighted and I locked my exposure on the bright bulb the whole scene is going to be way overexposed? Am I on track here?

The reason I ask is because I tried a few pics (same scene) on different metering settings but they all exposed the same. I considered just putting it on evaluative and leaving well enough alone but I just can't. I have to know.

[teaking]

Well sounds to me like you know what the metering modes do its just a case of using that knowledge to either set the exposure compensation to allow for the over exposure or to go into manual mode and set your own exposure. In the light bulb example i you set your exposure off the bulb everything as you say would be over exposed so you would set your exposure compensation to -2 to adjust for that.

Heres a link that explains perfect exposure but im affraid it goes through about 7 video clips between 10-30 mins long lol but if your up to it, it may give you an understanding:

Photography Tutorials on Vimeo

[Doug Pardee]

Originally Posted by MOmilkman

This is what I know about metering. It evaluates the amount of light that is used in a particular frame.
Thats about it.

To be picky, that's it. Metering is the measurement of the amount of light. There are a number of variations on the specific technique, but that's what it is. There are hand-held meters and in-camera meters, there are various formulas for producing a single "number" from an entire screen full of bright and dark areas, but in the end metering is just the measurement of the amount of light.

Just about all modern cameras have in-camera metering. Furthermore, just about all modern cameras couple that in-camera metering to automatic exposure control.

Many people say "metering" when they mean "auto-exposure". Auto-exposure relies on in-camera metering, but metering doesn't need to be connected to auto-exposure. If you put your camera on M mode the exposure is disconnected from the metering, while the metering continues to operate.

[inkista]

Originally Posted by MOmilkman

So if I were to take a picture of a light bulb in a dark room in center weighted and I locked my exposure on the bright bulb the whole scene is going to be way overexposed? Am I on track here?

With spot metering set on the lightbulb, you'd likely have an underexposed (assuming you're letting the AE system set the exposure without compensation) scene, because the camera assumes you want to expose for just the lightbulb.

With center-weighted metering, you'd likely be a bit overexposed, since the camera would still be considering the whole scene, but simply giving more "weight" to the pixels in the center of the frame (not just the lightbulb).

With evaluated, even (slightly) more overexposed.

In practice, though, I've found what you've found: center-weighted doesn't make much difference from evaluative for most scenes. Try testing with spot metering vs.evaluative.

[BBenson]

Originally Posted by zona5101

The sensor is designed to render the proper exposure based on all objects in the universe being 18% reflective gray.

Yep. One of the things in my camera bag that has survived from my early film days is an 18% card. Gets pulled out a few times per year even now

Bob

[MOmilkman]

Originally Posted by inkista

In practice, though, I've found what you've found: center-weighted doesn't make much difference from evaluative for most scenes. Try testing with spot metering vs.evaluative.

I will surely do that. Thanks to all for the replies.