question about metering

lilmomma · May 15, 2009

I'm having a hard time understanding the 3 metering types. I mean I get the concept of center, weighted, and whole and metering the light but what is it changing when going from one to another, the exposure? Does any of it matter at all if you shoot raw and manual?

Hopefully this makes sense....

Qarik · May 15, 2009

lilmomma wrote:

I'm having a hard time understanding the 3 metering types. I mean I get the concept of center, weighted, and whole and metering the light but what is it changing when going from one to another, the exposure? Does any of it matter at all if you shoot raw and manual?

Hopefully this makes sense....

The metering types are typically point, center weighted, and matrix metering. If you look at your manual you will see that poitn meter is meterin goff a dot in the center of your viewfinder, center weighted is metering off a medium circle at the center, and matrix is metering off a wide swath of your viewfinder. These are the different areas in which your cameras light meter measure for exposure.

If you are shooting an even kind of scene matric will work fine. If you are shooting something white stark contrast then you may want to go spot metering and meter off something neutral in the shot. Cneter weighted is in between.

It doesn't matter what mode you are shooting in..A,S, auto, manual etc.

lilmomma · May 15, 2009

Those are the terms...sorry I couldn't think of them on the spot. Ok I get how it works that I think but I'm still a little confused. So it's measuring the light for exposure? What does that mean exactly? (Forgive me for my terms, i'm trying to learn this all) but I thought exposure was the combination from the shutter speed and aperture? How does it measure exposure three different ways when your shutter speed and aperture are set? Say if you're shooting something outside on a cloudless day, aside from the right shutter speed and aperture settings, could, say, the sky be more or less blown out based on the metering? Maybe I'm confused on the term "exposure"?

Qarik · May 15, 2009

lilmomma wrote:

Those are the terms...sorry I couldn't think of them on the spot. Ok I get how it works that I think but I'm still a little confused. So it's measuring the light for exposure? What does that mean exactly? (Forgive me for my terms, i'm trying to learn this all) but I thought exposure was the combination from the shutter speed and aperture? How does it measure exposure three different ways when your shutter speed and aperture are set? Say if you're shooting something outside on a cloudless day, aside from the right shutter speed and aperture settings, could, say, the sky be more or less blown out based on the metering? Maybe I'm confused on the term "exposure"?

yes..it takes your shutter speed, aperture, and ISO and meters the light in that particular metering mode. Say spot metering. It ignores the entire frame except for the tiny spot at the center. In that tiny spot..it reads the light and gives you a corresponding value on your on camera light meter. Why would you use this? Take for example..you are taking a shot of small rock in a plane of white bright snow. If you use matrix metering your meter is going to go crazy with all the white biright snow and give you a n exposure that is way too dark. In this case you just move your center spot on the one rock and meter off that. Then you set your shutter, ap, iso based on that reading.

lilmomma · May 15, 2009

I think i might sort of understand, and if I understand correctly then my thinking has been off this whole time...Old way of thinking was that the metering was used for P,A, or S modes to determine the S and/or A settings for the shot. So for even when having full control of both, it is still beneficial?

In your case of the rock and the white snow, for this example lets say I'm shooting M and have set the shutter speed to 1/100 and the aperture to 5.6, ISO 200 (have no idea if those would be the actual settings i'm just using an example)
if you use the matrix, the image will be blown out.
If you use spot and meter the rock, the image could be rightly exposed, without changing any other settings.

So the aperture determines how much light is let in, the shutter speed determines how quickly it hits the image (again excuse my terminology!) and the ISO is the sensitivity, but when it all boils down, the metering determines how the light measured and burned into the image???

I've never really messed with the metering, but I suppose I should. My old thinking is if the image or parts of my image was blown out (like outdoors) i'd just increase shutter speed or aperture, or vice versa, depending on what i'm going for.

Qarik · May 15, 2009

The metering mode determines "where" in the frame the light is measured from. A small spot, a medieum circles, or nearly the full frame.

pathfinder · May 15, 2009

lilmomma,

Do you understand why we need to meter to get a correct exposure? What we are actually doing when we set the aperture and shutter speed?

To capture a correct exposure requires a specific amount of light - think of it as one full bucket full. You control the amount of light getting to your sensor via the aperture and the shutter speed. One full bucketful will correctly expose the sensor for a middle grey tone. More light causes the tone to get whiter and whiter until it is totally blown out white. Less light than one full bucketful causes the sensor to capture darker and darker tones until a total black is captured.

Aperture and shutter speeds are set up in full stops f2,f2.8, f4, f5.6, f8, f11, f16 each allow one half the amount of light of the previous one reading from f2->f16. Likewise, shutter speeds are set up in the same way 1 sec, 1/2 sec, 1/4 sec. 1/8th sec, 1/16th sec, 1/30th sec, 1/60th sec, 1/125 sec etc. Each allows 1/2 the amount of the previous longer shutter speed. Using aperture and shutter speed allows us to vary them so that the total amount of light is always the same - 1 bucketful - but the aperture and shutter speed can be altered reciprocally as needed.

When you alter the aperture or shutter speed one full stop ( f2.8 -> f4 or f4-> f5.6 ) you either increase or decrease the amount of light passing through the lens to the sensor by 1/2 a bucketful or 2 bucketsful. That is why metering is necessary - so that the amount of light getting to the sensor is always the correct amount. Raising or lowering the ISO alters the sensor sensitivity so that it does not require one bucketful, but 1/2 a bucketful, or 1/4 bucketful of light, or more or less.

ISO changes are just alterations in the sensitivity of the sensor from 1 bucketful, to 1/2 bucketful, to 1/4 bucketful etc.

To have the correct exposure means that you need to know exactly how much light is going to be captured coming from your subject. A light meter in your camera can't tell you that exactly, because it does not know what you are pointing it at. A reflected light meter ( like the one in your camera ) always assumes you are metering off an 18% grey card.( Do you own one? You should!) If you are metering off black coal or white snow you will need to interpret the reading and alter it to be correct. An incident meter just reads the light falling on your subject, and needs no interpretation from you. It is handheld and separate from your camera.

All of these comments are accurate whether you shoot in Av, Tv, Auto, Manual or Program. When you shoot the camera in one of its automatic modes, it is making those decisions for you. When you shoot in manual mode you need to read the light meter and set the aperture and shutter speed appropriately to the task at hand.

Matrix, center weighted and spot determine how much of the frame to actually measure, once you understand what setting the aperture, shutter speed, and ISO are doing. If you meter the face of someone with their back to the sun do you meter the sunlit background or their shaded face? How do you do this - spot meter, center weighted, or matrix. All can work, some better than others perhaps, but understanding how the meter measures light helps you intepret what it is telling you.

One good way to learn exposure is to put a 16 step gray scale image in you frame and photograph it with various exposures. You will quickly learn that only a very correct exposure will capture the entire grey scale without over exposure of the whites or underexposure of the blacks. Here is an image of a stepped grey scale - notice that you can see each of the steps - this was shot as an out of the camera jpg - no post processing whatsoever.

MooreDriven · May 16, 2009

lilmomma,

This is a topic that takes time to learn and understand. It also takes a lot of practice. I would recommend picking up Bryan Petersons Understanding Exposure. It's not manufacturer specific, but rather topic specific. I found it a well written book with examples on how to get the correct exposure.

Something else to consider, and learn, is how filters can effect how the meter reads the scene. I have a D300. When using a polarizing filter, it's recommend to not use matrix metering.

Pathfinder and Qarik, thanks for the excellent post.

Dale

pathfinder · May 16, 2009

I agree, Bryan's book is excellent.

lilmomma · May 16, 2009

Thank you pathfinder for the detailed post! And everyone else for taking the time to explain. I looked at my camera some more to see if I could figure it out and it clicked that the metering reader is the line on the screen that I've never paid attention to before! Duh! I usually just take some pictures and judge the exposure by looking at the photo, checking the highlights and also checking the histogram. Qarik, thank you for responding and I think I had a blonde moment yesterday! I get it all now!

question about metering

Comments