11 Aug 2014

Mode, Meter and Autofocus

This question was asked on a certain Facebook page and I thought I should share my answer with everyone not just the poster.

"... if I am shooting manual and manual focusing, does the metering mode matter? Evaluative, partial, spot etc..."

In this question you see a confusion of what the modes are and what interrelations they have to one another, so I felt it necessary to define what each part of the settings does. I hope you find the following information useful.

The meter's JOB is to measure incoming light and give the nearest shutter/aperture value that produces %18 gray for the area you are metering for given the current ISO setting and taking into consideration the compensation value you are asking for.

Meter mode is the "area" you are metering for. It is the area of light that is taken into the consideration of the meter.

If you are metering for a spot, the meter will tell you what settings give ONLY that spot mid-gray tones. It ignores the rest.

If you're metering centre-weighted, the meter will incorporate the rest of the frame, giving priority to about %20 in the centre of the frame, attempting to make it appear mid-gray in the end result.

If you are metering with Evaluative, the meter is measure different light values across the frame and taking all those into consideration to sum up the best time/aperture values that give an overall nearest thing to mid-gray. Then of course compensation value if you specify one.

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The photography mode you set on your camera (P, T, A, M) is what tells the meter's algorithm what setting it is responsible for calculating automatically:

If you are in Aperture priority mode, the meter will give the nearest Time (shutter) value to achieve mid-tones and your compensation value for the area you are metering for.

If you are in shutter priority mode, the meter will give the nearest Aperture value to achieve mid-gray and your compensation value for the area you are metering for.

If you are in Manual mode, the meter will is asked to sit aside and just watch.

If you are in P mode, the algorithm is measure the combination of shutter/aperture settings that achieve mid-gray taking into consideration the compensation value you have specified.

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Metering has nothing to do with autofocus. The autofocus lock button and metering lock button can simply be the same button (usually half-pressing the shutter locks them both).

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To sum up:
Internal light meter tries to calculate values that achieve mid-gray.

Metering mode: what area of the frame are you metering?
Camera mode: what setting is the meter responsible for?
Metering and autofocus: different things. But are locked together in the default settings.


I really hope this is of help to anyone. Feel free to leave any comments or send any questions.

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