Evaluative metering question

[Jeff Sagar]

When using the evaluative metering mode the camera's microprocessoris comparing thevarious segments of the scene itsees to a database and determines the exposure valuesbased on the comparison. For example, for a mostly white (or dark)scene I'm metering (using the evaluative metering mode) does thelight meter stilltry to makethe white (or dark) areaa middle gray or isthis taken into account during the database comparison?In other words will white be white based on the cameras exposure values ordo I still have to openup 1 to 2 stops like I would normally do in the other metering modes to makewhite look white in the final image?

[Don Burkett]

I don't know if Canon's evaluative mode is matrix based like Nikon's but yes, the meter will bias towards grey if the majority tone in the scene is light or dark.

[Ehcalum]

Every meter I've worked with (Nikon, Canon, Sony, Minolta, Petax, kodak) has metered to 18% grey.


Take a shot of a snowy field and it will be more grey. Take a shot of a black bear or a black lab and it will be more grey. This is even more visable on cloudy days.


Over expose a little to make the whites white and underexpose a little to make the blacks black.


A fun way to "cheat" the meter is to use a tone similar to 15% grey. Clear blue skies, the palm of your hand (my fav), the handy rock, roads or sidewalks. Meter them, remember the expouser, and dial it in for your scene.

[Colin]

I'm lazy.


I look at the histogram, and if I can, try to get the information placed in the middle.


I can make it brighter or darker after the fact []

[nerdmonkey]

I look at the histogram, and if I can, try to get the information placed in the middle.

I usually try to "expose to the right" when it makes sense to reduce noise. So when taking a picture of my black dog, for example, I often overexpose and then adjust the brightness later. There is an interesting article on Luminous Landscape about this.

[Jeff Sagar]

Hi Don,


Thanks for your reply. Yes, it is my understanding that Canon's meter is calibrated to 18% grey. I knew that it was, but I wasn't sure if some "adjustment" was made in the reading you got since the microprocessor is comparing what the meter is reading to an onboard database of exposure values. I'vedone some testing and it'sobvious that regardless of what metering mode you are using, you have to account for the fact that the meter is calibrated to 18% grey. So to make snow in your scene white you typically have to open upapproximately one stop or moreto render the snow as white inthe final image.

[Jeff Sagar]

Thanks for the tip on the Luminous Landscape artice, I've readit and others like it. The simple fact is that due to the way our digital sensors work, thegreatest number of tonal values, 2,048 for camera's with 12-bit sensors,are recorded within the first stop which ishalf of the 4,096 tonal values that can be recorded with a 12-bit sensor. Also, these 2,048 tonal values are brightest onesandthis is why imaging software can more easily correct overexposure since there is more information to work with.Comparatively, by the time you get to the "left" side of the histogram where the darker tonal values are, youhave maybe 512 tonal values to work with which makes it hard to correct underexposure without making "noise" visible in the dark areas of the image. To maximize the capabilities of your sensor, as you pointed out it does make sense to bias your exposure somewhat to the right especially if you are shooting RAW, but not too far to the right that you end up with blown highlights. Itry hard to get the exposure right in the camera to minimize as much as possible the amount of time I have to sit in front of the computer post processing my images. The imaging software available today can do amazing things, but I'd rather be out shooting instead of sitting in front of a computer.

[RDKirk]

For example, for a mostly white (or dark) scene I'm metering (using the evaluative metering mode) does the light meter still try to make the white (or dark) area a middle gray or is this taken into account during the database comparison? In other words will white be white based on the cameras exposure values or do I still have to open up 1 to 2 stops like I would normally do in the other metering modes to make white look white in the final image?

Yes, you are on the right track. As you say, the camera evaluates the scene and compares it to scenes in a database, looking for a scene that matches the lighting and the subject placement (it presumes the location of the active focus point identifies the subject). But like all meters, it's calibrated to presume that the subject is middle gray.


But...it might not actually expose it that way. The scenes in the datbase are examples selected by people and inserted into the database as "correct exposures." Sometimes in a "correct exposure" the subject is deliberately lighter or darker, and if Evaluative can select the correctly matching database scene, it will take this into account.


Sorta.


Evaluative can take into account the effect of the surrounding light patterns and attempts--by the database scene matching--to identify what kind of picture it is. It can determine, for instance, that "this is a backlighted picture" and will thus intentionally underexpose the subject slightly to retain the backlighting ambience...


...but that underexposure will be based on the presumption that the object under the focus point is middle gray. So if the subject is actually a bride in a white dress with the sun as a backlight, Evaluative will make that white dress a bit darker than middle gray.


That's what the compensation control does in Evaluative mode. If the subject (that is, the object you focused on) is not middle gray, use the compensation control to correct for it.

[Alan]

I'm lazy.


I look at the histogram, and if I can, try to get the information placed in the middle.


I can make it brighter or darker after the fact [img]/emoticons/emotion-1.gif[/img]
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I'm with Colin, except I'll simply bracket +/- 2/3 eV for those pesky scenes, and do an exposure blending in PP, if necessary.