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« Academy Director - Behind the Scenes, Part Two |Main| Exposure - little helpers »

August 29, 2012

Exposure - metering modes

exposure metering.jpg
It was easy stuff for cameras before we, photographers, get lazy. They were simple mechanical things which opened shutter curtain for time which photographer said will be good and let the light reach the film with dedicated ISO through the aperture, again, set by the photographer. But then we got lazy and we makes the camera thinks about exposure and pretty much anything. 

You can find out about how good or bad it is and what are the possibilities how camera can makes your life a misery with automatic modes here. Today we'll find out, how camera does the metering for "correct exposure" if you really want to leave it to poor camera.

So, in auto, or semi auto modes camera has to do the job for you. I auto modes completely, in semi auto you tell it what aperture, eventually shutter speed you'd like and camera calculates the rest. Let's not forget, camera calculates "correct exposure" as 18% grey tone average and metering modes are based on this fact and work with mid-tones. To give you a bit more control, how the exposure will be measured, camera offers you a few choices of metering modes. What's the difference then, if all modes work with mid-tones? Why do we need a variety of them? Well, every mode works with mid-tones, but every mode measures it in different pattern.

Spot and partial metering
Most spot meters have a precise metering circle that reads off 3% of the image frame, partial metering is little bit less precise and covers 9% of image frame. What that means? That means on whatever you point your camera, camera will think that is 18% grey mid-tone and will calculate the exposure based on that assumption. That also means you must be careful what you pointing your camera at and you should really point it somewhere where the mid-tones are. If you take the reading from highlight area, camera still assumes that is a 18% grey so the rest of the picture will be underexposed. On the other side, if you take reading from dark area, you'll end up with overexposed picture.

Spot and partial metering are ideal for portraits with back light, where multi-zone metering can be fooled by highlights in background and will underexpose your subject. Spot metering is also ideal for snowy conditions, where you should take reading form shadows to avoid underexposed image if you would do the reading form white snow. Over all in tricky light conditions with plenty of highlights, spot metering is the best choice.

Multi-zone metering
Multi-zone metering is the most sophisticated metering system available. It is reliable and will do correct exposure for vast majority of the scenes. How it works? It takes reading from whole frame, but to make it more precise, the whole frame is divided into zones. Yes, that's why it's multi-zone. It depends on model of your camera how many zones there are. Each zone takes a separate reading of whatever is in front of it, sends it to the processor where, using clever algorithms, camera decides what the correct exposure will be. 

Many times this works perfectly, that's why this metering mode is ideal for most situations without extreme light conditions. You can set it without any worries for most of your shooting and only change it if light conditions will change to something unusual.

Centre-weighted average.
This mode is a predecessor of multi-zone metering. It works on similar principle, it also takes in account the whole frame, but it hasn't got zones and it's most favorite area is center of the frame. Therefore is not as accurate as multi-zone, I'd say it is something between spot and multi-zone metering. 

But it has got it's usage. It's ideal for general portraits, where you can take reading from the face of the subject, what will be the metering's key area, it also consider light conditions in the background, but it won't give it that much attention to ruin your picture.

Next time we'll find out what aids and cheats there are available and you can use to bluff the exposure and create something different.

Posted by pkuzmin at August 29, 2012 9:21 AM