exposing for white

[danir]

According to Bryan Peterson's Understanding Exposure, grey objects reflect 18% of the light and white objects reflect 36%.
Does that mean that overexposing by 1 stop can replace a grey card when metering for snow?

Thanks,
Dani.

 

[JerryPH]

That is not always the case, because you do not know when the camera's meter will be "confused". Our cameras are cailbrated for that 18% reflectance (I really did find that the hand trick comes darn close if no grey card is available!) and you have to know how your particular camera understands the scene and how it meters the light to apply exposure techniques. It could be a full stop, it could be a third of a stop or it could be 1.5 stops... hard to put a single value on something that is so dynamic.

Also, evaluative metering will need a little more help. Spot metering (of a darker subject), will give you correct exposure... for the subject, NOT the scene.

Many factors to consider!

 

[Helen B]

36% is not a very bright white at all - in fact I'd call it a light grey. Bear in mind that 18% is roughly two and a half stops less reflective than a perfectly diffuse-reflective surface. In practice the brightest whites in a scene are around 96% or thereabouts. If you want white snow, you can meter off the snow and open up a couple of stops. If you meter off the brightest area, you can open up a little more - two and a third or so. The old palm-of-your-hand trick works as well.

The technique of metering off white objects is useful when your meter is running out of sensitivity in very dim light - it gets you an extra couple of stops of sensitivity in effect.

Best,
Helen

Edit:

It's a good exercise to meter off an evenly-toned surface that fills the frame, then over- and underexposing from the reading. That will give you an indication of how your camera's metering system behaves, and how many stops you have to open up or close down to get different tones in the final image.

36% is actually darker than a 50-50 mix of black and white. A checkerboard with black and white squares would have a reflectance of around 50%

 

[danir]

Thanks very much Jerry and Helen.
We have our anual snow today so hopefully I'll be able to use these tips.

Dani.

 

[That One Guy]

in the book, didn't bryan meter off of the sky right above an old schoolhouse to get the correct exposure.

it's been a while since i read that book so i may be mistaken