General exposure question - probably zone system related

[alanrockwood]

OK, so the let's assume you take a reading from a 18% gray card, and lets assume the subject is being illuminated by an external source and not generating light itself and furthermore that the reflectance of the subject is not specular. So, 18% reflectance is zone V, 36% reflectance is zone VI, 76% reflectance is zone VII, and 152% reflectance is zone VIII. However, 152% reflectance is a physical impossibility, so you should never need anything much above zone VII.

However, the analysis above kind of goes out the window if there is more than one light source, e.g. if the gray card is placed at the main subject and the gray card is in the same location and it is illuminated by a relatively weak light source (e.g. in shadow) and part of the scene (e.g. background) is illuminated with a strong light source (e.g. full sun.) Then you might need to be able to capture parts of the scene at zone VIII or higher.

Do I have this right?

 

[RobC]

Yes BUT...

Your spot meter has a K Factor which is usually between 12.5% and 18%. That K factor is used to reduce the amount of light read from whatever it is to 12.5% (or whatever your K Factor is ) less so that the reading is approximately in middle of the curve.

So here's the thing, 18% is 2.5 stops of 100% and 12.5% is 3 stops. So if your meter K Factor is 12.5% then by using a grey card you are reading something which is 2.5 stops less than 100% and then reducing it another 3 stops by applying your meter K Factor. So with a meter with a K Factor of 12.5% reading a grey card you will get a reading which is 5.5 stops less than 100%.

So you should be asking yourself why the hell am I using a grey card which is going to give me the wrong reading. Answer is, DON'T, throw your grey card away and forget about it. Then it won't cause you confusion.

Fact is that mid point reflectance value percentage is variable depending on the Subject Brightness Range so a grey card will only work accurately in certain lighting conditions and depending on your meter K factor and only if you have calibrated development so it works with grey card and meter.

What I do is calibrate development to give me a curve which prints exactly a 10 stop SBR (that is 11 zones 0 thru 10) onto paper unfiltered.
I then meter and expose for the highlights, usually a zone 7 (which happens to be 12.5% less than 100%) and use standard development for a 10 stop brightness range.

Then if the actual subject was less than 10 stops I can increase printing contrast to push the shadows back down to visually correct level without fear of blocking them. If had exposed for the shadows then It would be more difficult to print because there is no room for increasing print contrast without blocking shadows. Always exposing for a highlight gives much more consistent print times and easier printing.

Only if SBR is greater than 10 Stops do I expose for the shadows but in the knowledge that I will have a more difficult job when printing. However, since most subjects aren't over 10 stops that is a rarity.

Using this metodology makes life far easier.

Yes I know everyone says expose for the shadows and you can be sure of getting everything on the negative but then that is true of my(and other peoples) method too. I would suggest that a lot of books and so called experts just haven't really thought it through fully and repeat parrot fashion what other people have written in books.

Your meter is calibrated for 2.5 to 3 stops less than 100% and not 5 stops less contrary to what most people using the zone system seem to believe.

The mid point reflectance of a 10 Stop range is 3.125% and NOT 18%. Anyone who understands the most elementary mathematics should be able to work that out for themselves. Photographers don't do mathematics, they just believe what other photographers write in books. Result = Confusion.

p.s. A kodak 18% grey card is based on the munsel colour system which is a perceptual colour system. Light meters and film don't work using perceptual colour systems (except a few colour meters which have options for perceptual metering).

 

[Doremus Scudder]

Well now...

Let's see; an 18% gray card is called "middle grey" because of how we perceive it, yes. It's in the middle of the logarithmic progression between black and white; black being 0% and white 100%.

But, the 18% value is determined by measurement. An 18% gray card reflects 18% of the light falling on it. Black velvet is about 2%, titanium white paint is about 97%. This is a measure of reflectance and has very little to do with subject brightness range, which is the range of luminances in a particular scene (which can be illuminated by light sources of varying strength.)

However, if you had a scene that was evenly illuminated by one light source, and you got the blackest black thing your could find and the whitest white thing, you will never get a reflectance of more than 100%. Even the brightest white absorbs or scatters away a small percentage of light. So, "152% reflectance" just doesn't work.

So, in the classic Zone System, the progression of reflectances would be approximately: Zone I = 1.125%, II = 2.25% III = 4.5%, IV = 9% V = 18% (our grey card and perceived middle value), VI = 36%, VII 72% and... (wait for it...) 97% would fall about in Zone VII 1/2. This might be a case for expanded development if you wanted that 97% reflectance to be rendered whiter than Zone VII 1/2. Simply put, an evenly lit scene doesn't contain all the Zones.

However, put your black in a weakly illuminated shadow, and your white in direct sunlight, and yes indeed, you can get a reading that is "152%" more than another; even more, in fact. We measure amounts of light reflected from the scene with our spot meters, not "reflectance." Given that lighting ratios can be very great, it is possible to have SBRs of 12 or more stops. And, yes, in these cases, the high values are way more than 100% brighter than the lower ones.

Fortunately, we have development controls to allow us to tailor our negatives to print with the desired range of tones on our photo paper (which, when evenly illuminated, is less than 100% from black to white).

As for meters and K factors and the like. Yes meters are often made to give an exposure reading for a value different than 18% grey. This is why we Zone System users do personal film-speed tests and development tests. That said, the only important thing is getting enough information on the negative in the desired relationship (contrast). For LF users, minimum exposure is not a goal, and a bit of a safety factor is usually built into the system.

Rob's method of placing the highlights is a more extreme example of this: He isn't worried about overexposure and takes care of the contrast at the printing stage, so it makes sense to measure SBR and just place a highlight, even if the shadows get "overexposed." I've advocated something similar for roll-film users for years: use the in-camera meter for average and low-contrast scenes, but overexpose a stop or two for high-contrast scenes. This latter gives a bit of overexposure, which needs to be taken care of when printing, but ensures that the shadows don't go black.

The whole "expose for the shadows" method was to ensure adequate information in the low-density areas of the negative. "Develop for the highlights" was to control overall negative contrast range so that printing was not impossible. These are still good advice, no matter how we achieve it.

Best,

Doremus

 

[RobC]

So, in the classic Zone System, the progression of reflectances would be approximately: Zone I = 1.125%, II = 2.25% III = 4.5%, IV = 9% V = 18% (our grey card and perceived middle value), VI = 36%, VII 72% and... (wait for it...) 97% would fall about in Zone VII 1/2.
Doremus

Incorrect. 18% is the middle of a 5 stop range and NOT a 10 stop range. The grey card only works approximately for a 10 to 11 stop range because the meter takes another 2.5 to 3 stops off what the grey card has already taken off 100%.

look at following

0=1/2
1=1
2=2
3=4
4=8
5=16
6=32
7=64
8=128
9=256
10=512

If
we put 11 pieces card in a blacked out room and shine 512 units of light at
them. Card number zero (which is black) reflects 1/2 unit of light. Card number
10 (white and is very reflective white) refelcts all 512 units of light. Each
card reflects twice/half as much as previous/next card.
If we metered each
card from 0 to 10 we would find that each one was 1 stop brighter than the
previous. So what we have is a perfect 10 stop range which matches exactly the
concept of the zone system where zone 0 is black and zone 10 is pure white. The
middle zone (5) would be a middle grey.

Card number 5 reflects 16 units
of light. 16 units is 3.125% of 512 units and NOT 18%.


Next....

0=0
1=0
2=0
3=0
4=0

5=1/2
6=1
7=2
8=4
9=8
10=16

This time we only shine 16 units of light at them which only gives us a 5 stop SBR.

The mid point on this 5 stop SBR is somewhere between card 7 and card 8 which is around 3 units of light
reflectance. 3 units is approx 18.75% of 16 units which is close enough to 18% for our purposes.

So 18% reflectance is the mid point of a 5 stop subject brightness range. And ONLY A 5 STOP SUBJECT BRIGHTNESS RANGE. It is NOT the mid point of a 10 stop subject brighness range.

And the zone system teaches a 10 stop subject brightness range and misleads people into believing that 18% grey card reflectance is the mid point of a 10 stop range and equates to zone 5. It doesn't. It equates to zone 7.5 on a 10 stop range.

However, because your meter subtracts 2.5 to 3 stops off the grey card which has already subtracted 2.5 stops from the 100% of light falling on it, it works. But only if your SBR is 10 to 11 stops. If it were only 7 stops (the average) then its way off the mark.

So bin your grey card....

And yes if different parts of your subject are in different lighting then you can mistakenly believe you have more than 100% but only because you haven't understood what is going on with regards to lighting, metering and the useless grey card. But hopefully now you do understand.

 

[frank]

This is why I don't bother with the zone system. Not my cup of tea. If it works for you, then by all means, have at it.

 

[RobC]

well it does work but it took me a long time to resolve for myself exactly what was going on, primarily because what is taught and written about it in books is plain misleading. Its really very simple once you have grasped a few fundamentals and works extremely well. AND works well for roll film too if you modify its usage as I and many others have done.

 

[markbarendt]

If I understand the theory involved correctly; the percentage scale relates to the reflective range from paper white to silver based black; for example a photographic print.

If a photographic print is used as the 0-100 reference for a scene, then it is normally pretty obvious that, in the real world, there are many subjects outside that 0-100 range; both darker and lighter.

A negative's range, while not necessarily covering the entire range of the scene, is still normally longer than the paper's. So if the photo print is still the reference, then a negative may cover from a significantly negative number (-100 or ...) to a significant number well past 100% (200, 400, ... %).

 

[Jim Noel]

Incorrect. 18% is the middle of a 5 stop range and NOT a 10 stop range. The grey card only works approximately for a 10 to 11 stop range because the meter takes another 2.5 to 3 stops off what the grey card has already taken off 100%.

look at following

0=1/2
1=1
2=2
3=4
4=8
5=16
6=32
7=64
8=128
9=256
10=512

If
we put 11 pieces card in a blacked out room and shine 512 units of light at
them. Card number zero (which is black) reflects 1/2 unit of light. Card number
10 (white and is very reflective white) refelcts all 512 units of light. Each
card reflects twice/half as much as previous/next card.
If we metered each
card from 0 to 10 we would find that each one was 1 stop brighter than the
previous. So what we have is a perfect 10 stop range which matches exactly the
concept of the zone system where zone 0 is black and zone 10 is pure white. The
middle zone (5) would be a middle grey.

Card number 5 reflects 16 units
of light. 16 units is 3.125% of 512 units and NOT 18%.


Next....

0=0
1=0
2=0
3=0
4=0

5=1/2
6=1
7=2
8=4
9=8
10=16

This time we only shine 16 units of light at them which only gives us a 5 stop SBR.

The mid point on this 5 stop SBR is somewhere between card 7 and card 8 which is around 3 units of light
reflectance. 3 units is approx 18.75% of 16 units which is close enough to 18% for our purposes.

So 18% reflectance is the mid point of a 5 stop subject brightness range. And ONLY A 5 STOP SUBJECT BRIGHTNESS RANGE. It is NOT the mid point of a 10 stop subject brighness range.

And the zone system teaches a 10 stop subject brightness range and misleads people into believing that 18% grey card reflectance is the mid point of a 10 stop range and equates to zone 5. It doesn't. It equates to zone 7.5 on a 10 stop range.

However, because your meter subtracts 2.5 to 3 stops off the grey card which has already subtracted 2.5 stops from the 100% of light falling on it, it works. But only if your SBR is 10 to 11 stops. If it were only 7 stops (the average) then its way off the mark.

So bin your grey card....

And yes if different parts of your subject are in different lighting then you can mistakenly believe you have more than 100% but only because you haven't understood what is going on with regards to lighting, metering and the useless grey card. But hopefully now you do understand.

You discuss 10 stops, but your chart contains 11 stops. If one researches the Zone System as formulated by Fred Archer ad Ansel Adams they will discover that in the early discussions and publications it contained 10, not 11 zones. The Zones were noted as 0-IX.

 

[RobC]

You discuss 10 stops, but your chart contains 11 stops. If one researches the Zone System as formulated by Fred Archer ad Ansel Adams they will discover that in the early discussions and publications it contained 10, not 11 zones. The Zones were noted as 0-IX.

It contains 11 cards which is 11 readings but that is a 10 stop brightness range.

 

[Bill Burk]

As you already figured out: This thought is not the right one...

...152% reflectance is zone VIII. However, 152% reflectance is a physical impossibility, so you should never need anything much above zone VII.

This thought is why you sometimes encounter Zone VIII through Zone XII...

...illuminated by a relatively weak light source (e.g. in shadow) and part of the scene (e.g. background) is illuminated with a strong light source (e.g. full sun.) Then you might need to be able to capture parts of the scene at zone VIII or higher.

I enjoy reading about the 18% gray card and K factor, no disputing there are misconceptions that deserve to be cleared up.

But if you are following Ansel Adams' Zone System in its entirety, his system ignores both 18% and K. Essentially you are calibrating the meter to Zone V

 

[RobC]

But if you are following Ansel Adams' Zone System in its entirety, his system ignores both 18% and K. Essentially you are calibrating the meter to Zone V

He's a bit vague and not explicit on the topic of an 18% grey card but I think most people come away from reading the Negative thinking a grey card is a zone V and all they've got to do is meter a greycard and their exposure will be bang on. It won't be most of the time.

 

[David Allen]

Sorry but that ‘truism’ (meters are calibrated for 18% grey) that has been repeated down the ages in wrong. Light meters are calibrated to an ANSI standard. This standard is related to a specific luminance value that can be roughly described as being the equivalent to the reflectance of 12% gray. The reason that it is linked to luminance is because it can be correlated to a measurable amount of light energy whereas reflectance will vary according to metering position, position of light source and angel - all of which vary the amount of light energy bouncing off any particular object. Because the meters are calibrated to 12% ANSI standard some people have taken to calling this the ‘K’ factor. The reality is that meters are calibrated to luminance that is equivalent to 12% grey and Grey Cards are calibrated to a reflectance of 18% grey.

If you meter a grey card with an ANSI calibrated meter you will be off by 1/2 stop plus any other errors that you might have such as incorrect EI, metering technique, etc.

Far better is to do your own tests and just keep the grey card for doing white balance on any d*g*t*l cameras you might have.

Bests,

David.
www.dsallen.de

 

[Bill Burk]

Sorry but that ‘truism’ (meters are calibrated for 18% grey)

Right, Gray card is a "known" reflectance of 18% - and you are expected to make the exposure correction to account for difference between the roughly 12% and the known 18%.

What's not well understood I think, is that the meter calibration ties together statistics of scenery and the results of excellent print viewing judgments... So that the meter isn't calibrated to a "middle gray", it's calibrated to statistics of the average scenery - so you can point your meter at the scene, and take the result as-is to shoot with statistically successful exposures that will give you excellent prints.

As soon as you begin to be precise about what you are aiming the meter at, and where you intend that to fall... Then Zone System calibration is more meaningful, because it IS tied to specific grays.

 

[RobC]

grey cards are a nonsense item. It has to be at precisely the correct angle to get 18% reflectance which is the midle of anything anyway, except munsell perceptual black to white. As I said earlier, meters and film don't work with perceptual colour system. It appears to work with zone system for those using one but it only gives you a ball park exposure indication and then only if you SBR is around 10 stops.

Bin your grey card and meter the subject shadows or better still the highlights.

 

[markbarendt]

grey cards are a nonsense item. It has to be at precisely the correct angle to get 18% reflectance which is the midle of anything anyway, except munsell perceptual black to white. As I said earlier, meters and film don't work with perceptual colour system. It appears to work with zone system for those using one but it only gives you a ball park exposure indication and then only if you SBR is around 10 stops.

Bin your grey card and meter the subject shadows or better still the highlights.

Gray cards are great tools, as are any other known references one might choose, if used consistently, the same way each time the % reflectance and angle doesn't matter. The tool just provides a reference that an individual's system can be tied to.

The advantage reference cards have is that they eliminate the guess work inherent in judging subjects with unknown %'s, like dirt and bark and one of 400 shades of white paint and ...

 

[Bill Burk]

...then only if you SBR is around 10 stops.

If you can think of counting just the textural zones... Then Subject Brightness (luminance) Range or SBR (because three-letter-acronym SLR was taken) in Zone System is around 8 stops... And the statistical standard considers average SBR to be about 7 2/3 stops.

At this point, if you accept the discussion so far, then the differences might be what you call insignificant. We are talking about two systems that only differ in their handling of SBR by about 1/3 stop.

Now how far you move the meter reading to the shadow placement, I figure there's 2/3 stop difference. Significant enough to justify using different Exposure Index (EI) when using different systems - This difference is where I believe Zone System testing anecdotally arrives at 1 stop lower EI than box speed (rated ISO/ASA speed).

Bin your grey card and meter the subject shadows or better still the highlights.

Agree with the whole angle of lighting issue throwing off readings significantly. It's like I found earlier in the week... Submerging a thermometer too deep in the beaker leads to readings that differ by 2-degrees F.

Even though I am a shadow-placement kind of guy, I like the way you present the highlight metering idea. I use high readings from time to time. In my photo of StoneNYC, I metered his face and placed it on Zone VI. Of course I had no faith in the actual shutter speed that I got.

 

[RobC]

Gray cards are great tools, as are any other known references one might choose, if used consistently, the same way each time the % reflectance and angle doesn't matter. The tool just provides a reference that an individual's system can be tied to.

The advantage reference cards have is that they eliminate the guess work inherent in judging subjects with unknown %'s, like dirt and bark and one of 400 shades of white paint and ...

Well the current light meter topic suggests that with a bit of practice and good judgement a light meter and your reference are not reqiured. Personally I think better accuracy and consistency can be obtained with a light meter than by judgement alone, at least for me anyway. However when it comes to a reference I simply don't find any need at all. I view a grey card like a comfort blanket for those who haven't quite mastered their metering, calibration and judgement.

 

[frank]

I have to admit to thinking disparaging thoughts about a photographer who would pull out a grey card to assist with metering.

 

[Bill Burk]

My cat didn't care (see Grayscale and Cat in my gallery) while I was putting it together, but my best friend made a disparaging gesture when I asked him to hold up my home-made grayscale.

 

[markbarendt]

When metering, have you ever; used the palm of your hand, your wife's face, your camera bag, a lawn, a black camera, a sheet of paper, a clear northern sky, or some other "known to you" reference point to check yourself or to find exposure?

If so, you have essentially "pulled out a gray card to assist with metering", it was just disguised as something else.

 

[RobC]

why would I meter something else when I can meter the actual subject and place more or less exactly on the film curve where I want it. Just doesn't make sense to meter the wrong thing. Why would anyone do that and assume its the same reflectance as a grey card which isn't the middle of anything anyway. You might as well stick your finger in the air, see if the winds in the east and then pick a random number.

If you ain't going to use the Zone system as accurately as possible then you would be far better off using an incident meter which will give you more consistent results than badly applied zone system technique.

 

[frank]

I usually just aim the meter towards the ground (unless its snow) that is in the same light as the subject, take a meter reading, and adjust if I feel it needs adjusting. No need to carry around a piece of gray cardboard.

The adjustment is based on my preference to over expose b+w negative film (by up to 2 stops), rather than to under expose (by 1/2 stop).

 

[markbarendt]

I usually just aim the meter towards the ground (unless its snow) that is in the same light as the subject, take a meter reading, and adjust if I feel it needs adjusting. No need to carry around a piece of gray cardboard.

The adjustment is based on my preference to over expose b+w negative film (by up to 2 stops), rather than to under expose (by 1/2 stop).

And from experience you know where the ground normally falls on the film curve. The ground is your gray card. That is a perfectly valid way of doing things.

 

[frank]

And from experience you know where the ground normally falls on the film curve. The ground is your gray card. That is a perfectly valid way of doing things.

It works for me. But I'm not so much into film curves.

 

[removed account4]

And from experience you know where the ground normally falls on the film curve. The ground is your gray card. That is a perfectly valid way of doing things.

( frank i am not referring to you or anything you or anyone else in this thread has posted )
but from all conversations i have read regarding on the zone system, an metering for the zone system &c
personal experience has nothing to do with metering and SHOULD have nothing to do with it
one uses the meter to be the judge, jury and executioner to determind what zone middle grey might be
and decide what to do with the scene from there ...

sorry if i have minunderstood all i have read ( the negative about 4 times ) as well as
countless conversations/threads on line..