18 Percent Grey Clarification

[Biomech]

Good morning all

I'm hoping that someone could just clarify the whole 18% grey idea. Reading various material I'm unclear as to whether I'm supposed to be looking for 18% between black and white - hence 18% grey, or half way (50%) between the two as many books and articles refer to the 18% grey as "midtone".

TIA.

[Tiberius]

18% grey simply means it reflects 18% of the light that falls on it. It also doesn't create a colour tint. Light meters are designed to reproduce scenes so that they have an overall tonality of 18%, which is why they are good to take meter readings off. Meters can be fooled by light or dark scenes, such as snow or coal. an 18% grey card is giving the camera exactly what it wants.

[Biomech]

Thank you for your reply. So I should be looking for a tone in the image that is 18% grey from black (so a dark grey) I'm assuming. All the talk of midtones made the whole thing a little confusing :P

..... it's all a bit of a grey area at the moment

[inkista]

Originally Posted by Tiberius

...Light meters are designed to reproduce scenes so that they have an overall tonality of 18%....

Actually, 12%, according to Ctein, who says he got it from the Kodak engineers who design light meters and their specs.

Don't get too caught up in the numbers. Just know the concept of "middle gray" and the effect of the metering on autoexposure modes when it tries to set a certain value to it.

For the sake of simplification, let's say that your metering mode averages out all the values in the frame, and then sets the average value to middle gray. As long as the frame contains a good wide tonal range between black and white with relatively even distribution, this is a reasonable way of setting the exposure. But if your scene is predominantly black (like shooting at night)? Then this same mode is going to be setting a very dark value to middle gray, and the scene will be overexposed. Similarly, if you're shooting a predominantly white scene, a very light value will be set to middle gray, and you'll be underexposed.

This is why we have Manual mode and exposure compensation, and evaluative, center-weighted, and spot-metering metering modes.

[Tiberius]

Yeah, I gotta agree with Inkista. Don't get caught up in the numbers. If you are using it as a way to get your exposure right, you don't really need a grey card. A concrete footpath (sidewalk), caucasian skin, there are lots of things that are about the right brightness, even if they aren't the right colour.

[denja]

Using an internal meter off of a grey card is the same as using a handheld incident light meter.
An incident reading is much more effective than a reflected light reading. I have attached a link that shows the concept well.
Using Gray Cards

[kencaleno]

The problem is that meters are calibrated to 12%-13% and Kodak gray cards are calibrated at 18%. So your camera,by default will overexpose by between 1/3 and 1/2 EV stop-Which is why it pays to underexpose by 1/3 stop for correct exposure,and to avoid blown highlights.Ken

[kencaleno]

Originally Posted by Biomech

Thank you for your reply. So I should be looking for a tone in the image that is 18% grey from black (so a dark grey) I'm assuming. All the talk of midtones made the whole thing a little confusing :P

..... it's all a bit of a grey area at the moment

It's not a dark grey it is a reflected tone-18% grey is mid-grey-not half way between white and black which would be 50% grey


Here is an 18% grey:

[denja]

Originally Posted by kencaleno

The problem is that meters are calibrated to 12%-13% and Kodak gray cards are calibrated at 18%. So your camera,by default will overexpose by between 1/3 and 1/2 EV stop-Which is why it pays to underexpose by 1/3 stop for correct exposure,and to avoid blown highlights.Ken

If you meter off of a grey card, the resultant exposure will print in the middle of your dynamic range, or as Ansel would say, zone V.