Please Explain Metering

[jamesev]

Ok so i've got to grips with everything else on my camera but never really paid much attention to metering....

I've read a number of books and websites that cover metering and theres a few things I don't get.

I read that an 18% grey card is needed and metering using "Sky Brothers" and "Mr. Green Jeans" but what I don't get is that you can meter on these points, but do these points then need to be in the shot? Does the camera remeter on every shot? Or do you meter once for the conditions / envirnment iei a nice green forest then you can not worry about it until you go down to the blue seaside.

I think I don't get the colour vibrance that I could get for example when I shoot greenery because i'm not metering properly

please can someone xplain metering and what the camera does needs to have a well composed metered shot?

[RecurrentNerve]

I think you are a little confused. The camera meters every time you point it at something; it meters constantly. Metering has nothing to do with colour per se, metering is how the camera reads the light in your scene.

What I think you are referring to is manual White Balance. For general shooting there is no need to take a grey card with you; if you're shooting JPEG just make sure your WB setting is appropriate for the situation, if you're shooting RAW (and you really should be) forget about it because you can adjust the WB easily when you import.

If you do want to go down the card route, yes, you would have to meter for different scenes. Technically you would have to meter for every different shot as the white balance changes with each one!

[RichardTaylor]

Are you asking about the colour of objects as recorded or are you asking about image "brightness"?

Do you know about white balance?
Do you know how to read a histogram?
Do you know about exposure compensation?.

[jamesev]

yes I understand white balance, reading histograms and exposure compensation, just not very much about how to effectively use metering

[inkista]

Think of it this way.

Every sensel on the sensor registers a specific value, right? Metering simply concerns itself with how bright that value is.

When you set a metering mode, you're basically telling the camera's AE system how to process all those values. You only want to consider a specific area. You want one area to be weighed more heavily than another. You want everything to be straight averaged...whathaveyou. But usually (aside from spot metering), all those different sensel values are going to be mashed together to give you an overall reading of how bright the scene that's hitting the sensor is.

So, let's just simplify this picture and say that all the values are going to be averaged.

When you point the camera at, say, the shaft wall of a coal mine at midnight all those values are going to be dark. When you point the camera at, say, a white rabbit in a snowbank at mid-day, most of those values are going to be light. And so will your average.

This is why metering isn't consistent from scene to scene. The values aren't the same from scene to scene. And the values themselves can bias the exposure because of how most camera autoexposure systems use that conglomerated "average."

The basic thinking is this. Most scenes will have a good light-to-dark distribution of values. You'll probably have an equal number of highlights and shadows and most of the stuff will be in midtones, so the average value is likely to be right in the middle of your dynamic range. So the camera exposure is then set so that that average is going to be in the middle of the dynamic range (that mythic 18% grey card).

So, if you're in the coal mine (etc.), that really dark near-black average that you got? it's going to be assumed you want that to be "middle grey" in your exposure. Result: overexposure. This is why so many first timers at nighttime photography have a brown sky instead of a black one to show for their efforts.

If you're in that snow scene, then a very very light near-white value is likely to be what the camera thinks you want as "middle grey". Result: under exposure.

This is why metering off the 18% grey card is one oft-advised method. Spot meter off that card, and you know that middle grey will be set to middle grey.

This whole "metering off the sky" thing, is basically about a particular scene ending up biasing the exposure in a way you don't want. Bright skies and a dark subject can lead to overexposure. Your skies go to white. To avoid this, you meter fully on just the sky, so it's properly exposed (and will be blue), then you lock the exposure settings (it's what the * button is for) reframe, and then take your picture. Voila. Blue skies.

You could just as easily twiddle about with exposure compensation if you wanted to (hence all the "you should always shoot in manual" advice you'll hear from the Manual Modians).

Does that help?

[RichardTaylor]

@ jamesev

You may like to look at this U Tube video (and the next couple in the series)

YouTube - Digital Photography 1 on 1: Episode 25: Metering Part I