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General photography questions and answers: Discuss Metering Modes...I'm after a bit of help and clarification required on metering modes (in particular Canon 20D) please If I have ...
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Old 11-01-2008, 22:40   #1 (permalink)
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Metering Modes

I'm after a bit of help and clarification required on metering modes (in particular Canon 20D) please

If I have understood correctly (in simple terms and perhaps not quite the correct terminology!):

Evaluative - takes into account the whole of the frame
Partial - meters on 9% of the viewfinder at the centre point

Centre-Weighted I am finding harder to grasp - it's not really centre weighted is it because it's "centre weighted average". My understanding (probably wrong!) is that centre-weighted should meter/expose based on the central area of the viewfinder/frame, whereas centre-weighted average takes this central area and then averages for the rest of the frame?

So does this mean that "Partial" is really "Centre-Weighted", or am I being really dim?
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Old 11-01-2008, 22:50   #2 (permalink)
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Re: Metering Modes

Ali, centre-weighted means: The whole frame is taken into account, but more emphasis is placed on the centre area. This mode is good for portraits, whereas you'd use Evaluative for landscapes.

When using centre-weighted for a portrait in which you don't want the subject to be placed centrally in your image, you'd have to remember to meter off the subject, then recompose your shot.

Don't know about 'Partial'; like you, I would imagine it's the same as centre-weighted I'm sure a Canon user will come along to clarify that one
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Old 11-01-2008, 23:56   #3 (permalink)
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Re: Metering Modes

Ali

The partial metering is like spot metering but with a large area rather than a very small area typical of one degree spot meters. This is shown as a circle in the centre of the viewfinder of your 20D

Evaluative uses the auto-focus points of your camera as areas to meter from hence 'evaluating' the scene.

Centre-weighted is usually a roughly shaped triangular area with its peak around the centre of your viewfinder expanding downwards and then outwards. This mode was traditionally used for landscape pictures since it is biased towards metering the lower half of the scene, i.e. the land rather than being influenced too much by the sky (which would usually cause the land to be too dark).

Hope this helps.
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Old 12-01-2008, 00:12   #4 (permalink)
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Re: Metering Modes

Thanks Charlotte and Les
Your posts do help, but I am still a little confused about "centre weighted average" fit? I assume this is not the same as true 'centre weighted'?

Or have I missed the point (highly likely!)
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Old 12-01-2008, 00:31   #5 (permalink)
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Re: Metering Modes

Any metering mode that uses an area of the scene to meter from, including one degree spot meters, has to average the information within the area being read to provide you with one set of exposure info i.e. shutter speed and aperture. So they are all averaging methods.

The result from a light meter assumes the meter has actually read a single brightness value i.e. a flat grey card. It is not possible for a meter to do anything else but average the reading when the area being metered contains more than one brightness value.

Forget about the word average! The important thing to know is which part of the viewfinder is being used when you meter a scene (as described above)
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Old 12-01-2008, 00:34   #6 (permalink)
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Re: Metering Modes

Ali,

'Centre weighted average' metering is the same method which I described as 'Centre-weighted' metering.

Here's Wikipaedia's page on metering modes, and if you Google "centre weighted average" you'll find other web sites, but I suspect they will all say the same thing

Crossed with Les' post
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Old 12-01-2008, 08:12   #7 (permalink)
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Re: Metering Modes

Ali, I am an EOS 20D owner. The information already supplied is largely correct but you also need to know which mode to use when.

A description of each mode is on page 71 of the 20D Handbook.

Evaluative metering uses 35 zones. Consider the frame to be divided into 35 equal areas. Each of these is metered (an average measurement being used). This gives 35 measurements which are compared to many (thousands?) of stored patterns. The nearest stored pattern is used for the exposure. So if your shot is vertical and against the light, there is a good chance that the camera will recognise this and set a suitable exposure. From experience I have found that this mode will be fairly good for 90% of the shots of an average photographer.

Partial is really like a spot meter but instead of a single spot the "spot" is average over the 9% central area. This allows you to point the "spot" at a face or other subject and obtain a good exposure.

Centre weighted is the least useful as this is easily fooled by bright skies etc and can easily introduce large exposure errors.

I pesonally use the Partial all the time. This allows me to know exactly what the camera is metering. However, a much lower proportion of my pictures would fail with evaluative just because of the subjects I take.
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Old 12-01-2008, 13:36   #8 (permalink)
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Re: Metering Modes

My confusion arose from the word "average" tagged onto the end of centre-weighted.
I shall worry no more. Thank you
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