evaluative metering

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i was wondering what the theory is behind the evaluative metering on canon eos cameras? i practically use it all the time (ellan iies partial metering is a joke) and was wondering what really makes it better than the normal center-weighted metering? thanks!

-- Jeff Nakayama (moonduck22@hotmail.com), December 19, 2001

Answers

Jeff, I'll try to answer your question. Using the EOS30/Elan 7 as an example ('cause that's what I've got), the image area is divided into 35 equal sized squares. the light is measured in each of these squares and then averaged out (or worked out using some other kind of calculation) to get the exposure setting.

Centre weighted metering is great if your subject is in the centre of the frame. In practice this is not always the case, of course. Evaluative metering should give you more predictable results more often than centre weighted metering.

Hopefully I've explained it correctly. Perhaps Puppy Face or Julian Loke could fill us in a bit more - I think they're more knowledgable about this...

Nik

-- NikB (ndb_letters@yahoo.com), December 20, 2001.


Ah, good one Jeff.

I'm not an expert on Evaluative metering, but here's a how I understand it. The brightness info from the segments is interpreted by algorithms--algorithms supposedly based on countless picture taking situations. The CPU distinguishes frontal and back lighting, foreground and background lighting, subject and background, flash and ambient light, etc., and applies appropriate compensation for the best exposure. It's like taking a bunch of spotmeterings and averaging them, but biasing exposure towards the main subject.

The trouble is, the CPU sometimes doesn't know what the main subject is or how I want it exposed. It took me years of wasting chromes and losing images before I figured out the situations Evaluative metering nails and fails. For example, very light or dark situations fool the meter. Snow and white sand beaches come out gray on chromes, so I add .5 to 2 stops. Large silhouettes against a setting sun are sometimes exposed as medium and, subsequently, wash out the sunset. I usually meter off a medium toned section of sky above the sun to get around this.

-- Puppy Face (doggieface@aol.com), December 20, 2001.


The other good thing about partial metering--and maybe you can set this on your IIe--is that it can be pegged to one of the focusing points. On the Elan 7 and other cameras, a command function moves it from the center point (default) to the active point.

-- Preston Merchant (merchant@speakeasy.org), December 20, 2001.

Puppy Face's explaination is probably as good as anyones on how evaluative meter works. I treat it as a slightly smarter version of the center weighted meter, which means I don't trust it in high contrast situations (or on snow, sand, with very light colored or dark colored subjects). The partial meter is not a joke, (just not as good as a tight spot meter) and is very useful in metering high contrast lighting.

-- Kenneth Katz (socks@bestweb.net), December 20, 2001.

No matter what camera or meter you use, please remember that they are set up for midtone (18%) gray. Snow takes +2 stops and black (like a blank computer screen) takes -2 stops from the meter; you have to "fool" the meter. The Olympus OM4 had buttons for this and that is where I learned about it when I had one. Take a roll of slide film, a gray card and experiment. It is much harder to explain than to put into practice but you will get great results.

-- Victor (Catmanman@aol.com), December 22, 2001.


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