Beginner 18% GREY TEST CARD??

Lauren Neal

Lauren Neal

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Lauren
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Hi,

So in my photography course, i'm currently learning about metering systems etc. My tutor keeps saying things like "this is like 18% grey blah blah" and i have literally no clue what it means. Can someone explain what an 18% grey test card is and anything else around it, please?

Thank, Lauren x
 
A grey piece of card that reflects 18% of the light that falls on it.
 
The assumption being that light meters are calibrated to assume that the average reflectance of a standard subject is 18%. Actually, it's a little more complex than that and the figure should be lower (which is taken into account by Kodak who recommend that you angle the card at 45 degrees to the light to reduce the value....).

If you photograph a grey cat against a black background, the meter will average the light out and reproduce the black background as grey; and if the cat is against a white background, in trying to turn the background grey the meter will underexpose. A grey card is one way to get round this. It's effectively measuring the light that falls on the subject and works the same was as a hand held incident light meter.

Lots more info on request...
 
You need to read up on the difference between 'incident' light metering and 'reflected' light metering. Incident light metering, measures the intensity of light falling ON your subject; reflected light metering measures the intensity of light reflected OFF your subject.

If you use a camera's in-built meter, that will almost always be a 'reflected light' meter reading; and most SLR's & DSLR's meters are actually metering 'TTL' or Through The Lens. (Some older SLR's, and many range-finders had more simple external meters above the lens, but with a fixed angle of view, independent of that of the lens, they might be measuring light from a much wider or smaller portion of the scene than the lens, hence not be so accurate.. not that that's particularly relevant here). If you take an 'incident light reading', measure how bright the light falling on your subject is, and make your exposure settings based on that... theory is, that you'll get an exposure that's more like what you see; dark things will come out dark, bright things come out bright, where-as, if you took a reflected light reading, bright stuff will chuck more light at the meter, dark stuff less, so if your scene has more light or dark stuff in it than 'average' the meter is likely to suggest an exposure value that will make for over or under exposure.

OK; f16-Sunny... old rule of thumb for 'metering by eye'.. and it works! No meter needed! But guide says, on good sunny day, with clear sky, your exposure value will be a shutter speed 1/ISO value @ f16... if its a bit cloudy, open up a stop, if the sky's actually a bit dark, open up two stops; if its starting to get dark, ad its a bit cloudy, open p three stops... you get the idea!

So, we have an almost f16-Sunny day,crack out the old hand-held meter, and take an incident meter reading of the light falling from the sky, and yup, meter suggests EV10, or ISO100, F16& 1/100th.. great.... now, point it at the scene, take the inver-cone off and take a 'reflected light' reading, now the needle suddenly falls two stops to EV8... suggesting f11 & 1/50th at ISO100, because the subject is a dark stand of trees on a mountain side, and they reflect less light than 'average'. Point the meter down a little or turn around, and ow the meter needle jumps two stops to EV12, begging f22 & 1/200th @ ISO100, because the subject is ow a white sow covered bit of hill, reflecting more light than 'average'... or it jumps to EV14, because you are pointing it at the sun, and your subject is strongly back-lit! So you try it again, with the inver-cone back on, to get a 'better' icident reading, remembering you are supposed to put your meter i the same light as the subject.. so you walk to the woods and try it there, and meter reading comes up to EV9.. walk back over to where. you pointed it at the hill-side that gave a reflected reading of EV 12, and it comes up to EV11..

Many meter readings, all perfectly 'accurate'.. but you have a range of maybe four or five stops of readings.. you can only use one... which one do you 'trust'? Which one is 'right'?

Answer is none of them... you trust your eyes! You evaluate the scene ad how bright or dark the ambient light is, and how bright or dark your subject, and you make your OWN call as to what is most likely to give a 'good' exposure... exposure meters are very good at measuring light levels, but they are bludy thick when it comes to knowing what they are looking at! That's where you have to do your bit!

But you probably need a little bit of help & you probably don't have the luxury of a hand-held meter to make independent incident or reflected light meter readings; you only have the cameras in-built TTL reflected light meter; so you use a Grey-Card to make a 'pretend' incident meter reading with the cameras TTL meter.... you put the grey-card in the same light as your subject, and take a reflected light meter reading off it with the camera. The Grey-Card is a known 'average' brightness, neither light or dark, so if your meter is being 'fooled' by an over or under average brightness scene to under or over exposure, grey-card meter reading should help you decide if you need to add any compensation.
 
I might be wrong here.
I was told/read somewhere that if you take a litre of black paint and mix it with a litre of white paint, the resulting colour should be 18% grey and this is the average that a lightmeter looks for in an image - ie a mixture of the darkest through to the lightest tones.

Why its not 50% grey is what I don't understand
 
If we assume that 50% grey means "reflects 50% of the light falling on it", then, since no white paint will reflect 100% of the light and no black paint absorb 100% of the light, the difference should be clear.

I don't know if this mix is 50%, I'm just assuming for the purpose of illustration.

From my book:

The most efficient artificial reflector can reflect as much as 98% of the incident light; natural objects can only manage about 90%. The best naturally light absorbing materials reflect only 1.5% of the incident light.
 
JulesP said:
I might be wrong here.
I was told/read somewhere that if you take a litre of black paint and mix it with a litre of white paint, the resulting colour should be 18% grey and this is the average that a lightmeter looks for in an image - ie a mixture of the darkest through to the lightest tones.

Why its not 50% grey is what I don't understand
Click to expand...

2 litres of grey paint would make a hell of a big grey card!:LOL:
 
Simplified & non-technical explanation.

The first thing to realise is that the light falling on a scene determines what the exposure should be, the light reflected from a scene varies according to what is doing the reflecting. As camera meters measure reflected light this can cause problems. If you were to measure the light falling on the scene, using an incident light reading, you would not have to worry about adjusting exposure for different scenes which reflect different amounts.

Your camera is taught that all scenes reflect 18% of the light falling on them (Digital cameras may be taught that it is 14% or 12% depending on the camera but I will continue with 18%)

First consider taking a picture of a landscape which, on average, is reflecting 18% of the light falling on it.

When your camera is pointed at the landscape it thinks ' That scene must be reflecting 18% of the light falling on it so I need to set an exposure to suit that light. '

If the light gets brighter the camera thinks ' That scene is still reflecting 18% but it is brighter now so the sun must have come out so I need to reduce the exposure to suit the new brighter light '

If the light gets dimmer the camera thinks ' That scene is still reflecting 18% but it is darker now so the sun must be behind a cloud so I need to increase the exposure to suit the new dimmer light'

So far so good, but what about scenes which reflect more than 18% such as snow.

When your camera is pointed at the snow scene it thinks ' That scene must be reflecting 18% of the light falling on it but it is very bright so it must be very sunny so I need to reduce the exposure to suit the very bright light '

This means that, although the camera meter shows the exposure as correct, it has actually reduced the exposure and will cause the white snow to be grey.

A similar thing occurs when you photograph a very dark scene, a black horse for example.

Your camera thinks ' That scene must be reflecting 18% of the light falling on it but it is very dark so it must be very overcast so I need to increase the exposure to suit the very dim light'

This means that, although the camera meter shows the exposure as correct, it has actually increased the exposure and will cause the black horse to be grey.

If you meter from a grey card the camera thinks 'that must be reflecting 18% of the light falling on it" and it is, so the exposure will be technically correct!
(Although it may not be artistically correct - that depends on you and what effect you want)
 
18% grey is mid-grey - think elephant grey. It reflects 18% of the light falling on it, and equates to the middle of the histogram on the camera's LCD. All light meters and light metering systems are calibrated to this, or a value very close to it (camera manufacturers never say exactly) and it's a universally used reference. The idea is if you take an average scene (say a few people, bit of green grass, blue sky etc) and scramble up all the bright and dark tones, you will end up with 18% grey. That's the theory on which it's all based and while it can be surprisingly accurate for a lot of subjects, it obviously has it's weaknesses - which is why we have exposure compensation controls, to adjust what the camera thinks it's looking at, to what it's actually looking at.

That's really all you need to know, though bear in mind whole books have been written about this.

You also need to get up to speed on some of the jargon used, that we all take for granted but it's not always obvious or logical. In this context, "a stop" refers to either the halving or doubling of exposure and it dovetails neatly with the camera's three exposure controls - shutter speed, lens aperture, and ISO (the sensor's sensitivity setting). Shutter speed is logical, in that 1/125sec is clearly twice as much time at 1/250sec, which is double 1/500sec and so on - a difference of one stop between each. ISO is also easy, with ISO800 being twice as sensitive to light as ISO400, that is double ISO200 etc.

The tricky one is the f/number settings for lens apertures that are not only illogical but also run counter-intuitively (there is sound maths behind it, but it's not obvious). F/2 lets in twice as much light as f/2.8, which is double f/4, which is double f/5.6, then f/8, f/11, f/16 and f/22. There are more f/numbers on some lenses, less on others, and intermediate settings on all of them. While each setting allows either double or half the amount of light to pass - one stop difference - the numbers are linked by the square-root of two, ie 1.4. Unfortunately, you just have to learn this stuff but you'll be surprised at how quickly it becomes second nature. Do you ever get the chance of a one-to-one with your tutor to discuss? I would have thought a few keen students like you would be a dream come true. Have a chat, the penny will drop, and you'll be away :)

Good explanation in that video link from Russ (y)

Edit: with most cameras, the exposure controls for shutter speed, lens aperture and ISO are all set in 1/3rd stop increments, ie three clicks on any of these controls will shift the exposure by one stop. Ditto exposure compensation settings.
 
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HoppyUK said:
Do you ever get the chance of a one-to-one with your tutor to discuss? I would have thought a few keen students like you would be a dream come true. Have a chat, the penny will drop, and you'll be away :)
Click to expand...


This. Very much this!!! A decent teacher will be happy to chat for a few minutes to clear up any doubts a keen student has, although the teacher may have further commitments so might not have the time for an extended chat.
 
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JulesP said:
I might be wrong here.
I was told/read somewhere that if you take a litre of black paint and mix it with a litre of white paint, the resulting colour should be 18% grey and this is the average that a lightmeter looks for in an image - ie a mixture of the darkest through to the lightest tones.

Why its not 50% grey is what I don't understand
Click to expand...
The human visual system is non-linear, it follows a gamma curve.
 
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