White balance for metering

<p>I am trying to understand white balance and metering and how they intersect. I understand that the purpose of white balance is to prevent colour casts in a photo.<br>

Let's say I am taking a photo of a scene with a lot of snow. I am shooting in manual mode. I set custom white balance using an 18% gray card. When I am metering, do I have to overexpose a stop or two above zero to get accurate exposure for the snow?<br>

After all, I have set my white balance. If this is so, shouldn't my snow be 'white' in my photo? Why do I have to increase exposure a couple of stops above zero to get white snow in the exposure?</p>

<p>If you overexpose so much that the RGB channels are all blown out, yes the snow will be white no matter what the white balance. But that's not what you want. Overexposing a stop or two from the meter will give you bright snow, like it should be, but it can still have a cast. If you don't overexpose from what the meter is telling you, then the snow will be gray with or without a cast depending upon your WB setting. If you shoot RAW, worry about the WB later, for the most part.</p>

Back in the old days of film, people would correct for white balance by using a color filter over the lens. Indoors, under incandescent light, everything would have a yellow cast. White paper correctly exposed for the amount of light falling of it would still appear slightly yellow. A light blue 80A filter would correct for that. Today you have a whole range of color correcting "filters" known as white balance built into the camera. If the white balance had been set for indoors to correct for yellow light, the outdoors scenes would appear slightly blue as if there were still a light blue filter over the lens. It may be properly exposed but the light blue "filter" would make things look slightly blue. Setting the white balance for outdoors would remove the light blue "filter".

 

Metering sets the correct exposure. White balance removes the color cast.

<p>Thanks for your help so far, but I feel my question has not been answered:<br /> if I have already defined what 'white' is by setting custom white balance in the same light I am shooting in, why do I still need to overexpose a stop or two above zero to get the snow to look 'white' in the photo?<br /> Related: how do we define a colour cast?</p>

Don't think white balance and exposure has much of an intersection. If you set your WB using an 18% gray card you will likely end up with a good WB setting. If you also used the 18% gray card to set your exposure, you should also end up a reasonably good exposure for that image. If you set your exposure using the snow, I would add about +2 stops of additional exposure, which I would expect would be real close to the value you would get if you used the gray card to set exposure. This all assumes that the snow, the gray card, and your main subject are all in the same light and you know how to use a gray card properly. The only time I consider WB when setting exposure is when I shoot under incandescent indoor light, and the last thing I want is to underexpose knowing how much I will need to boost the blue channel in order to get the WB of the image to look OK.

<p>The color balance refers to the color temperature of the ambient light falling on the subject, not necessarily the the proper exposure to achieve what you see. Your meter will take whatever it sees and attempt to render it as 18% grey....so if you metered a white egg, no matter what color temperature of light you had shining on it, the reading of your meter will indicate the proper exposure to render it 18% grey; likewise if you metered a black cat, your meter would indicate the proper exposure to render it 18% grey. Obviously the egg and the cat aren't 18% grey in real life, and that is where we apply a correction factor to the meter reading to give us a rendering which matches what our eyes perceived....in the case of the egg - you would want to overexpose by about 1-2 stops, in the case of the cat you would want to underexpose by 1-2 stops.</p>

<p>White balance deals with color, exposure deals with brightness. Go on your computer monitor or an older TV set and you will find separate controls for brightness and for color. You can adjust the color back and forth to where a white object might have either a blue color cast or a red color cast. Separately from that, you can control its brightness. Even with the color neutral to where it has no color cast, the brightness can make the "white" object range anywhere from a shade of gray up to full white. That might help undertand the difference between the two.<br /><br />Generally, there is no need to do a custom white balance. In film days we had two choices -- daylight and tungsten. If you set your camera to daylight, you will get the correct white balance for most scenes that are lit by the sun or flash (which is daylight balanced). If you're in open shade things will look a little cooler, if you're in the golden glow of sunset they will look redder, etc. But those are what things look like naturally under those conditions. You can set the camera to tungsten if you are shooting under photographic or theatrical "hot lights." Household incandescent lamps will look a little on the warm side.<br /><br />The third choice, which we didn't have in film days, is fluorescent. Use that under fluorescent lights obviously.<br /><br />With those three, you will usually be within the range of what can then be corrected after the fact in Photoshop, but close enough that you won't have to be tweaking every shot unless you want to.<br /><br />The one place you do need to a custom white balance is when shooting under mixed light. And even then sometimes you have to choose which light source is dominant and correct for that.<br /><br />Whatever setting you use, always shoot in raw. It keeps your options open for changing things later. Only reason not to is if you need the images immediately, within a matter of minutes. But unless you're shooting on deadline for news, those situations are few and far between. Even then, shoot jpg plus raw. Shooting raw doesn't mean you have to sit and play with every image -- if you got the settings right in-camera, you just do a batch conversion to jpg.<br /><br /></p>

<p>There are 3 colors in printing that make slight color casts jump into your eye: Something known to be neutral grey, human skin tones and food. The latter are just sensitive but don't tell as clearly how you should adjust your image.<br>

"white"-ballance is a missleading word. It does not define "white" it defines "neutral" as neither cool or warm grey. <br>

keep the "white" of white ballance out of your metering decissions. your snow will have hints of shadow darker than 255, 255, 255 white on your screen. To keep these neutral i.e white not rhinestone or blue tinted you set your white ballance.</p>

So just so it is clear: if you have taken an exposure reading off the gray card, you can use that reading and do not need to adjust exposure for the snow.

To get a better understanding of how a camera light meter may be" fooled" by too much white snow (or too much of any lights or darks in a scene) click on this:

 

http://jdainis.com/exposure1.html

 

 

A color cast just means that there is a wash of color over the image like looking at a scene through a yellow piece of cellophane gives a yellow cast to what you see.

"White Balance" only controls color bias , exposure controls volume of light reaching the sensor.

 

What might be confusing you is that the metering systems in some cameras incorporate Red, Green, and Blue pixels that

are used to determine which of a matrix of algorithmsis used to bias the metering results for "typical" common scenes.

 

Further: in some cameras this RGB metering data and to some extent contrast data between the tens of thousands of

metering pixels is also used in the autofocus system to aid the AF system's predictive tracking when you are are in

continuous AF mode. The best example I've seen of this is the system in the Canon EOS-7D mark II and presumably in

the new 5D S and 5D SR.

I seriously doubt that the OP who has a hard time understanding white balance and metering and how they intersect is confused by the metering systems in some cameras that incorporate Red, Green, and Blue pixels used to determine which of a matrix of algorithms is used to bias the metering results for "typical" common scenes. Not confusing at all to a beginner who never even thought about that or knows what it means.