If 18% Grey Is A Midtone ....

[Ken Lee]

There are 3 issues here.

1) Even though it is supposed to reflect only 18% of the light that strikes it, its surface not perfectly matte: it is a little shiney, and so it has hot-spots and dark spots when the light is specular. Under soft light, the surface of the card reflects 18% of what strikes it, all across the card, no matter how it is oriented to the source.

2) No matter how much light there is, it merely reflects a percentage of it. If we double the light, the card will still reflect only 18%.

3) The eye and film do not repond in a linear manner to light. That is why we consider 36% as "middle" grey, even though common sense might suggest that it be 50%. Perhaps someone with a better grasp of the physics and densitometry can explain it. I presume it is a logorithmic function of some kind.

[Ken Lee]

Correction:



That is why we consider 18% as "middle" grey, even though common sense might suggest that it be 50%.

[robc]

John, you have a fixation on grey cards.

Lets use a brick instead. Say we have a brick which reflects 18% of the incident light. Anything which reflects 18% is said to be zone V. Right next to the brick in the exact same light and with its surface in the same plane as the brick face, we place a white card which has been coated with a highly reflective paint such that it is capable of reflecting 100% of light that falls on it. It is a 100% white card. Now meter the brick and then meter the white card. Now you work out how many stops brighter the white card is than the brick and give me your answer. Also tell me if its possible to have anything in the same light which is any brighter than the white card.

[tim atherton]

in addition, isn't the standard Kodak grey card something like 13.5%...?

[John Brownlow]

I've no idea what the point you are trying to make is, Rob.

If you put the gray card in full sunlight, it may be the case that a white card in the same light is only three stops brighter. However you are talking about diffuse reflection as opposed to specular reflection, which may be considerably brighter if the the surface has a sheen or gloss, or is a mirror. If you don't understand why that is, think about it.

Moreover, unless you are shooting in totally diffused light without any shadows, or chromes, you would not place Zone V on a gray card in the brightest light. You would find a bit of open shadow, where the incident light is less. Therefore your bright white would be more than three stops brighter... typically about five, which puts it on Zone X where we would expect it.

If you can't get it into your head that a gray card and reflective meter is *exactly* functionally equivalent to an incident meter (except that incident meters are typically calibrated to the equivalent of 13-14% gray) then there's no hope.

Anytime I take a reading with an incident meter, I can pull out a gray card, point it in the same direction as the incident meter, take a reflective meeting, and get *exactly* the same reading (taking into account the 14%/18% thing).

Sorry if you're having trouble with all of this.

[robc]

"Moreover, unless you are shooting in totally diffused light without any shadows, or chromes, you would not place Zone V on a gray card in the brightest light. You would find a bit of open shadow, where the incident light is less. Therefore your bright white would be more than three stops brighter... typically about five, which puts it on Zone X where we would expect it. "

Ah, now we're getting to the flaw in your thinking. It is not physically possible for the white card to be 5 stops brighter than a grey card in the same light. The maths is so blindingly simple, but your refusal to do it is misleading you.

Lets make two examples:

First if there are 1000 units of light falling on the two cards. The grey card reflects 180 units and the white card reflects a 1000 units of light. The difference between the reflecatnce values is 820 units. In terms of stops difference that is approx 2.5 stops.

Second if there 1024000 units of light falling on both cards (ten stops brighter) then the grey card reflects 184320 units of light and the white card reflects 1024000 units of light. Thats a difference of 839680 units of light which happens to be the exact same difference in stops as in the first example. i.e approx 2.5 stops. It is physically impossible that when both cards are in the same light that the white card is more than 2.5 stops brighter than the grey card.

How you come think that the white card could be 5 stops brighter than the grey card I simply don't know and you haven't attempted to explain.

My point is and has been that a grey card has no place in the zone system since it tells you nothing useful in zone system terms. If you use an incident meter then you should be pointing it at the light source and if you are using a spot meter as I do then you should be metering from the subject itself .

Let me quote AA from the "Negative" Chapter 3, sub heading "Reading middle grey" the very last sentence.

"If, however, the subject contains some areas in direct light and others in shadow, the incident light or grey card reading will provide no clue to the actual total range of luminances."

I don't think I've ever photographed a scene where some areas weren't in shadow and according to AA a grey card or incident meter doesn't cut it for determining the range of luminances.

Thats why I use a spot meter. Firstly because I can meter everything from the camera position which makes life much easier. Ssecondly because it is more accurate since the light at the camera is always the correct value(especially when the subject is at a great distance) whereas having to move about a scene to take different readings with an incident meter is very often impratical and frought with error. And thirdly an incident mter may tell me how much light there is but it doesn't tell me what zone any part of the subject will fall on.

Incident meters have their uses and I'm sure some use them to good effect in the field, but my guess is that more exposure inaccuracies are caused by using an incident meter in the field than by using a spot meter.

[John Brownlow]

Rob, I give up. You are tilting at windmills. Bye.

[robc]

there's none so blind as them that won't see...

[robc]

"in addition, isn't the standard Kodak grey card something like 13.5%...?"

Tim,

maybe and no. confused?

Depends what you measure it with it. If you measure it with a desitometer(I have) it measures 18%

However, a desitometer shines light a specific angle and measures reflected light from a specific angle. Your light meter collects light from a grey card which is reflected from many different angles. i.e. diffused reflection.

Same principle applies in prints. Measure the dynamic range of a B+W FB print with a densitometer and you may get upto 8 stops of range. Measure the same print with a spot meter and you'll be lucky to get even 5 stops.

So what you get from an 18% grey card with a light meter is debateable/uncertain.

[tim atherton]

this is the bit I was thinking of from the kodak instructions - you need to adjust the measurement 1/2 stop to adjust the 18% grey of the card to the 13% standard of light meters...:

"Meter readings of the gray card cshould be adjusted as follows- 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop. 2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop 3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"