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12-07-2011, 05:55 AM   #1
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DSLR metering function

Having never had a DSLR until the other day (now have a K5) it occurs to me that I need to understand among other things how metering works.

My initial impressions are that:
  • centre weighted metering averages the readings with a higher weighting of the readings near the centre, then sets the exposure for the entire frame according to that single averaged result. (spot metering is a special case of this)
  • multi segmented metering is fundamentally different, in that it takes a reading on that 1/77 segment, then sets the exposure electronically for that segment accordingly, and so on for the other 76 segments. There presumably is also some clever merging across the segment boundaries to avoid the segment boundaries showing.
What this means is that if there are widely varying light levels across the frame, centre weighted will ultimately be a compromise with some areas over and some under exposed (and the centre about right), while multi segmented will get it right all over the frame.

I can understand that there are doubtless artistic pros and cons to each method, but I just want to understand the fundamental approach. Do I have it right?

12-07-2011, 06:20 AM   #2
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I think Multi segment just sets the exposure at a level that will ensure the best exposure for all 77 segments, though obviously some will be over and some will be under. Centre weighted is the same, but only for the central segments. So in multi-segment, it doesn't expose each segment individually, it just tries to expose the whole frame as well as possible.

So multi segment will generally give you a better exposure, though centre weighted works better for portraits and stuff where the background might be very dark or light compared to the face of the person.

That's how it works as far as I know, there's more electronic jiggery pokery to it than that, but multi segment essentially spot meters each individual segment and finds the exposure value that'd give the best exposure, as opposed to spot metering each [part of the image and doing it all yourself like they used to way back when.
12-07-2011, 06:21 AM   #3
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You've got center weighted correct, but not for multi segment. What you are describing is basically HDR (hi-dynamic range) and the sensor isn't capable of multiple exposures at once. Multi Segment uses all the points to get a general idea of the light across a given scene and then tries to average it all out to get you the exposure it thinks you want.

For a better read, Metering mode - Wikipedia, the free encyclopedia
12-07-2011, 07:00 AM   #4
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the idea behind multi segment metering is to be able to better descriminate the correct overall scene exposure, based upon not only the average value of light falling on the frame,. but also to, in some manner, determine the histogram of the scene, and the distribution of the lighting in the scene

Matrix metering can ignore for example a single bright spot, such as the sun in one spot on the frame, and expose correctly for the rest (which is too bad if you want a siloutte) where as average metering will be too influenced by the sun, and will cause a general under exposure.

at one time, back in the old days BD (Before Digital) I seem to remember seeing somewhere an article that nikon had reportedly used something like 5000 different scenes to program how to interpret the results of the matrix metering for auto modes, based upon things like subject modes (i.e., portrait, landscape, sports etc..... )

12-07-2011, 09:22 AM   #5
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Just because the camera can meter 77 different segments, doesn't mean it can magically expose correctly of all of them simultaneously. You still get one exposure, it's just harder to fool a 77 segment metering system with backlighting, or say a small light object against a large dark background.

If you think in terms of the Zone System, the camera can look at a scene and determine that some objects are in zone II and some are in zone IX and your subject (the focus point) might be zone IV, so rather than trying to expose your subject as zone V which would blow out parts of the picture, it renders it correctly in zone IV.
12-07-2011, 10:51 AM   #6
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But the real issue is whether the camera understands your composition. It could by mistake the subject as something to ignore in exposure, just the opposite of what you want, but in general yrs it is an attempt to evaluate more accurely based upon the perceived histogram
12-07-2011, 11:03 AM   #7
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As mentioned, matrix metering algorithmically computes an exposure (a very specific shutter-aperture-ISO setting) from readings from each meter segment across the frame -- hopefully an optimal exposure. Center-weighted tries to set an 18% gray value biased by readings from the central segments, and is the default when using non-AF and non-A-type lenses. It's like using an old film cam. Spot metering just reads the center segments. I use it for a simplified Zone system: spot-read the brightest and darkest points in a scene and set the exposure midway between, adjusted for highlight vs dark details.

But whatever the metering mode used, the result is a single shutter-aperture-ISO setting. To play safe, bracket bracket bracket. You can maybe use the bracketed frames to build an HDR image too.

Both center-weighted and matrix metering can lead to exposure shifts when reflected light values change in the scene -- shifting clouds and shadows and contrasts, etc. That's why we may use gray cards or incident light meters, to read the light falling on the scene rather than what's reflected. It's instructive and entertaining to compare multiple readings via varied techniques. Also, a trick from my film days: Look at a subject, then take a center-weighted reading from something nearby of similar brightness. I'll read off my hand or sleeve, or the ground at my feet. Extreme background values are thus excluded from the reading.
12-09-2011, 04:43 AM   #8
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Thanks for all of your replies - that clears it up!

Bit of a shame though that all of that information that is gathered to determine the amount of light present in each of the 77 segments is simply averaged and discarded instead of being stored and used to post process the brightness of the same segments of the captured image.

Oh well - maybe in the 2020 model...

12-09-2011, 05:52 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
the idea behind multi segment metering is to be able to better descriminate the correct overall scene exposure, based upon not only the average value of light falling on the frame,. but also to, in some manner, determine the histogram of the scene, and the distribution of the lighting in the scene
This is correct I believe but doesn't the camera not have multiple "scene presets" so to say.
Doesn't it know from the metering that it's dealing with a Landscape photo or a portrait and have different lighting algorithms for them and it also looks where the focus is?
12-09-2011, 06:05 AM   #10
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QuoteOriginally posted by Andrew Crouch Quote
Bit of a shame though that all of that information that is gathered to determine the amount of light present in each of the 77 segments is simply averaged and discarded instead of being stored and used to post process the brightness of the same segments of the captured image.
You're now talking about applying different ISO settings to each pixel, I believe i've read something about that actually.

I've also heard of theories about pixels that reset themselves when full which could lead to unlimited dynamic range and with that you might be able to change the lighting a lot in post processing.
The technique was based on the memory chips they used for SD cards and the likes.
12-13-2011, 08:33 AM   #11
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QuoteOriginally posted by Anvh Quote
This is correct I believe but doesn't the camera not have multiple "scene presets" so to say.
Doesn't it know from the metering that it's dealing with a Landscape photo or a portrait and have different lighting algorithms for them and it also looks where the focus is?
The K5 doesn't have scene modes.
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