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Saturday, January 16, 2010

Metering modes .

My test subject : I've chosen extremes , black and white , to exaggerate the differences .

Spot metering :

Exposure is calculated entirely from the centre 2.5 % of the frame . With some cameras it can be set to meter off a chosen focus point.
It will try to make that small area an average gray regardless of whether it is white or black or anything in-between so you will have to be ready to dial in exposure compensation depending on the subject .

spot metering on white tries to make the white surface an average gray ......

Moving that spot onto a black surface results in a huge difference in exposure as it tries to make the black surface an average gray ....

Centre weighted metering :

The centre of the frame gets 75% priority , the rest of the frame contributes 25% toward the final exposure .
This means that the centre of the frame has 3 times the effect on the final exposure as the rest of the frame . If the centre of the frame changes drastically the exposure changes drastically but if the outside of the frame changes drastically it only has 1/3 of the effect as when the centre changed .

For the maths geeks we could say :
" Exposure = [outside of frame + 3Xcentre of frame ]/4 " = "average gray"

Here we can see that centre weighted metering finds a reasonable average in a high contrast scene and changes only slightly as we move from side to side .....

Once I move right across to the black surface it responds like spot metering where it tries to make the black an average gray because now the entire centre of the frame is black - it has blown the whites on the side . Spot metering only required a slight movement either side for such a drastic change while centre weighted metering gradually changes .

And the same can be said for moving till the centre is only on the white surface , it tries to make the whites an average gray and all detail is lost in the blacks and as mentioned before , this happened gradually [ with regard to moving to the side ] until eventually the whole centre was on one colour ........ it required a larger area than spot metering uses to have this effect .

Matrix metering:

Matrix or ''pattern'' metering is a bit more complicated - it's a computer program that tries to work out what the subject is for you and will try to make that an average gray . It changes slightly [ sometimes drastically ! ] with each new camera model - and between brands of course .
It varies like the difference between xp and vista so we can't make any definite statements about matrix/pattern metering for all cameras .
With many of the newer models from Nikon I can state that they tend to meter strongly toward the focus points so simply selecting a different focus point for exactly the same scene can cause up to two stops difference in exposure !

Now look at these images carefully before deciding which metering mode you ' trust ' .
These were taken with a Nikon D90 , my D40 behaves very much the same but other models and brands might behave very differently . This is what Matrix metering did with the scene .

Then I moved the active focus point onto the black surface , I use "dynamic auto-focus" which allows me to select the main focus point while it uses the others to keep an eye on the subject and help with maintaining the auto-focus .... I shifted the active focus point across to the left .

Then I shifted the active focus point to the right which caused a big change in the metering .
You could say that I have moved the camera a bit but it's almost like it spot metered for the active focus point which is not necessarily what it will always do - you can't guarantee it will be consistent with a computer program .

Here's a real world example . The same scene , taken using a tripod , and all I've done is move the active focus point ! ....

Focus point on the left ....

focus point on the right ....

So which metering mode should you use ? You can use any one of them and never get perfect results in every situation .
Just remember that if matrix/pattern metering does a perfect job for you now - it doesn't mean the next model released will do the same so it would make sense to first get accustomed to using centre weighted and spot metering before seeing how well matrix/pattern metering works on your particular camera .


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