A Primer on Incident Metering.

AndreasT said:

Mark could you clarify a bit more on this what you mean.
I changed over to the BTZS a few years ago and got terrible results in the beggining. Forcing me to push up my development.
Yes I know we are talking about exposure, but that was fine on my films.
The clarifications about flare, (which I ignored a bit) I fell are badly discussed in his book.
With the help of a few other here I could clear this problem. Those crazy guys who dream about this stuff.
Using the Incident light meter gives more secure results.

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Well we are talking about incident metering, not just exposure. Measuring contrast is a distinct part of that.

What I'm talking about is that systems: BTZS, ZS, mine, yours; are all based on assumptions.

BTZS assumes lots of shadow detail is important, similarly it wants to keep good highlight detail; that describes a longer than normal SBR. To fit the extra detail that the system expects into a straight print on given grade of paper the negative needs to be developed to a lower contrast than normal.

Phil's bias brings with it a generic reduction in film contrast to make room for more shadow detail without losing highlight detail, but that bias has a side effect, the local contrast across the mid tones gets flatter, loses some snap if you will.

This loss of snap is I'm guessing is part of the problem you fixed with a bit more development. Yes?

You applied a patch to the system by making a decision "outside the rules". You eliminated Phil's bias and applied your own

In Davis's BTZS he writes about a normal contrast of 0,5 which I used in the beginning and got flat negatives. He talks about an average flare of 0,02 which I based my exposure on.
I quickly moved back to a "normal contrast of .57 and got better results like the time before I tried out BTZS. This confused me for a long time.
Then I read a few things from Stephen Beskins (a lot I still do not understand), explaining a bit more about flare.
As I understand it the flare mostly showed by BTZS is just too small. At least the way I understood it.
Although flare is discussed, the dominant usage of such a small amount of flare made me use too small a amount.

markbarendt said:

In the third coulomb he goes on to assert that a full sunlight measurement "almost inevitably results in of underexposure in the shadow areas of the subject".................IMO in both of these assertions Phil is personally trying to "fill in the banks" and by doing so he was unintentionally creating myths/misinformation.

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Been following the thread-----I would have to disagree on this, I think Davis is merely stating a fact about incident metering in general, IMO. An incident reading in full sunlight can't account for the shadow area in that same full sunlight. Since an incident reading is based only on the intensity of the light falling on the subject, it stands to reason that all the various reflective surfaces would be properly exposed, only if they are all receiving the same illumination. This can't possibly be the case in a high contrast lighting such as full sun with distinct shadows. As it's been stated, I think, averaging the sun and shadow reading is a step in the right direction, but by itself, the full sun reading can leave the shadows hugely unprotected.

In the attachment with Ross---in A, the meter is placed in a full sun area, the shadows are under exposed; in B, it's placed in a full shade area, the high values are over exposed; in C, the meter is placed in an area that is not full sun nor is it full shade, the exposure is about right; and in D, the exposure is based off an in-camera reflective reading, the exposure pretty much matches that in C.

Nice illustrations Mr. Porter...thanks. I agree that averaging the highlight values might well be the best way to proceed. As noted below, it is wonderful that you are obtaining great results using your metering methods. Nicely done.

I think one must recall that incident metering via Phil Davis/BTZS does NOT involve only one meter reading. I respectfully state that those who are commenting about the BTZS system as being based on a single highlight incident meter reading are not accurately reporting on the BTZS system/ method. Please consider avaiing yourselves to the material available on the BTZS system and incident metering.

If those who are using only a single incident shadow or highlight reading are obtaining the results that they desire...well done! I have nothing further to add or suggest. Add a stop, subtract a stop...heck, who cares if your results are as you desire.

Mark: In his text Phil David clearly states that if absolutely necessary, basing an exposure on a single highlight reading will be incorrect unless one INCREASES the exposure. Without such action, the meter will render the single highlight reading as middle grey. If one MUST base one's exposure on a single shadow reading then one must decrease the exposure so that the tones are moved down from middle gray. Increasing the exposure will raise the highlights to a more "correct value"....and will of course increase the shadow exposure. As to the 18% illumination and 5 stop range that Mr. Davis alludes to: Everything is very well explained starting on page 131 of the 4th edition of his book. The 5 stop range is based upon studies that are well detailed. Of course, feel free to differ from Mr. Davis's conclusions. You are entitled to your opinions and surely such are based upon sound experimental or empiric data as reflected in your negatives and prints. I am very pleased that your "system" works for you, and that most of your film exposures and prints meet all of your expectations. Well done. As to your comments about teachers: All teachers synthesize material and interpret such. Students can decide to reject or modify the material presented. However, if they do so then such changes should be accompanied with illustrations and information as to the methods that they have used to deconstruct the information presented, and the data illustrating the validity of the reinterpretation that they have applied to the chemical and physical realities of the photographic process. Of course, if you are at the stage that teachers cannot offer you any useful information and advice then you have progressed far beyond what must of us here have attained. Again, well done!

For emphasis: The BTZS system depends upon two readings, highlights and shadows. The zone system depends upon two readings-highlights and shadows- with appropriate placement so as to determine the time of development. In both systems one can manipulate exposure so as to render the negative with the tones that one desires. In both "systems" a single reading will not suffice.

If one wants to use a single shadow reading then another approach might be developing by inspection in which one will expose for the shadows and then determine development by the appearance of the highlights under a green safe light in the darkroom. DBI is fun, and many will eventually acquire the necessary visual skills. Moreover, one can learn to develop 4-6 sheets of film at one time.

Exposing for the highlights was a method that at one time the late Fred Picker advocated. Perhaps those who have studied with Fred can comment.

Andreas-I am sorry that you are having some problems with the BTZS system. As I offered, we can discuss any issues via PM. I and others are ready to help.

Some are successful using reflected metering and the ZS, others using BTZS and incident metering, and still others use whatever "system" they have devised based upon their material and metering methods that work for them.

Now it's time to once more go into the field and take some photos. Use whatever "exposure system" works for you; if you are getting the results that you desire, who cares.


Elliot

CPorter said:

An incident reading in full sunlight can't account for the shadow area in that same full sunlight.

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Thanks for chiming in.

The first question I have for you regarding the scene in your example is "what is the subject(s) and where do you want them to fall?" Tough to make a metering plan without knowing what the expectation for the print is.

I will quote myself though:

markbarendt said:

Meters of all types simply provide exposure suggestions. Just because incident meters are really good at giving you the camera setting as a direct reading doesn't mean that's the only way to use it.

If your subject is in sun and you are in open shade, or vice versa, and you can't get into the same light, you can simply apply an offset as you would with a spot meter reading. Sure this is a bit subjective but the world isn't perfect and its much better than just guessing.

Use your imagination and you can find simple ways to make any meter do what you need.

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The examples you provided are a good example of where I'd use that principle and some imagination.

My second question for you is "what are you willing to compromise in this situation; highlight detail, shadow detail, print contrast?" And "what other tools are you willing to bring to bear, reflectors, strobes, scrims, pre-flashing, dodge, burn?"

I ask because the scene in your examples probably won't straight print acceptably, seriously if you want detail in the door and the shaded bark which are how many stops apart?

I used the example with Ross only to react to the sentament that Davis is creating myth or misinformation about the incident meter. The statement he makes is true----- using a full sunlight incident reading to determine exposure can result in shadows that are horribly underexposed.

I chimed in, but not as one who uses an incident meter with any regularity at all as I am completely in the "reflective" camp------metering the intensity of the reflected light that will actually be striking the film is far more intuitive to me and more important to my way of doing things. But in the BTZS way of doing things, it is obvious that the incident meter can be used with that system quite successfully. Any answer I gave toward those questions would be totally biased toward reflective readings with a spot meter.

So, the concept of "place" and "fall" is entirely different, to my mind, between the two methods, I place a luminance value, you would place a light intensity. I know exactly what to expect from that value in my negative and how it should render on the paper, the same idea with the "fall" of the highlight. I can see in my mind a print value for the "placed" luminance and the luminance of the "fall", and knowing the luminance of all the values in between helps to complete the visualization. For me, I can't predict or visualize the same for those luminances when only "placing" the light intensity of the shadow or just knowing where the intensity of the sunlight "falls".

As to question #2, again, relative to the spot meter, I would compromise next to nothing-----within the limits------of my tested system. Any luminances that "fall" below the "placed" value, are entirely precictable and thus ok with me, and any luminances that "fall" higher on the scale after the planned development scheme that favors the textured high value are also predictable. As far as other tools, dodge and burn of course (always, no negative is that perfect! when it comes to meeting the visualization, IMO), perhaps intensification of the negative, perhaps pre-exposure of the negative, etc......

Bottom line, I feel I can achieve the most optimum negative to meet the desired print by reflective metering than with incident, but that is just my feelings on it.

Considering Camera and lens flare, the normal 7 stop range is reduced to 6 stops. So taking reading on any where is "assumed" to be 5 stops by the meter.

With that, reducing a stop for shadows when reading is based on shadow(meter placement is critical) and opening one stop for highlight with bring it to an effective six stops.

Roll film(Negative): I personally have good shadow detail.

Again, testing and placement of meter are essential and critical.

CPorter said:

I used the example with Ross only to react to the sentament that Davis is creating myth or misinformation about the incident meter. The statement he makes is true----- using a full sunlight incident reading to determine exposure can result in shadows that are horribly underexposed.

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IMO Davis is using an assumption and a special case to make his point.

First, Davis is assuming that deep shadow detail is important in most shots, that seems true for you and I'm sure a fair number of people at APUG, but it is far from a given in photography as a whole.

Second Davis is describing a special case measurement, pointing directly at the light source, that measurement is typically only used with a second measurement (as in duplexing) or with modification (as you might with your spot meter placing zone III). Ruling out the normal way to use the incident meter may help win a debate but it doesn't help us understand what's really happening or help us make better pictures.

If instead we allow the normal cases into the argument we easily get good results, no muss, no fuss.

An incident meter used in the follow-the-manufacturer's-directions manner, dome pointed at the camera, meter held at Ross's nose, one reading taken, will in the grand majority of situations reasonably place Ross's face nicely on a print. That is assuming we pose Ross a bit in your scene to avoid mottling the light on his face.

A duplexed incident meter reading using the readings from say frames a&b in your example would provide plenty of info to decide on how to adjust contrast and the average of the two readings would give you a camera setting that should protect both shadows and highlights quite nicely.

Mahler_one said:

Mark: In his text Phil David clearly states that if absolutely necessary, basing an exposure on a single highlight reading will be incorrect unless one INCREASES the exposure. Without such action, the meter will render the single highlight reading as middle grey.

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I have a problem with the affirmation I underlined.

The nature of incident reading is that it leaves aside the reflectance of the subject. In the same light, with the same incident reading, a middle grey subject will be rendered middle grey, a bright subject will be rendered bright, and a dark subject will be rendered dark.

This happens "automatically" by the sheer fact that the bright object reflects more light than the middle grey objects and thus renders brighter on the slide than the middle grey object, given the same exposure. It works very well until the "whiteness" or "blackness" of the subject is within the dynamic range of the film.

With reflective reading the photographer must be aware of how different the subject is from middle grey and adjust the exposure based on that, thus "placing" the subject in the film curve. With incident metering the photographer doesn't need to do the thinking. The subject will be "placed" in the film curve by its own reflective power "automagically".

Diapositivo said:

I have a problem with the affirmation I underlined.

The nature of incident reading is that it leaves aside the reflectance of the subject. In the same light, with the same incident reading, a middle grey subject will be rendered middle grey, a bright subject will be rendered bright, and a dark subject will be rendered dark.

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Provided it is uniformly lit by a source. Since, blackest black is approximately 4.5 stops to whitest white. So incident metering will render blacks and whites satisfactorily.

It needs some adjustment when there are shadows and highlights.

markbarendt said:

IMO Davis is using an assumption and a special case to make his point.

First, Davis is assuming that deep shadow detail is important in most shots, that seems true for you and I'm sure a fair number of people at APUG, but it is far from a given in photography as a whole.

Second Davis is describing a special case measurement, pointing directly at the light source, that measurement is typically only used with a second measurement (as in duplexing) or with modification (as you might with your spot meter placing zone III). Ruling out the normal way to use the incident meter may help win a debate but it doesn't help us understand what's really happening or help us make better pictures.

If instead we allow the normal cases into the argument we easily get good results, no muss, no fuss.

An incident meter used in the follow-the-manufacturer's-directions manner, dome pointed at the camera, meter held at Ross's nose, one reading taken, will in the grand majority of situations reasonably place Ross's face nicely on a print. That is assuming we pose Ross a bit in your scene to avoid mottling the light on his face.

A duplexed incident meter reading using the readings from say frames a&b in your example would provide plenty of info to decide on how to adjust contrast and the average of the two readings would give you a camera setting that should protect both shadows and highlights quite nicely.

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Mark, I'm only responding to the Davis quote you referenced earlier. His statement is accurate with regard to a single incident measurement in full sun and the corresponding poor exposure to be had in the shadows----there's no myth involved with that statement, it's a fact. I'm making no other challenge.

Fair enough.

I am confused about the OP's remarks about dome in and dome out. Lets take an example where a landscape is being taken where one cannot get to the subject. Example shooting across the Grand Canyon or at Sentinel Bridge in Yosemite photographing Half Dome with snow.

I believe dome in gives the local reading of light falling on the subject when local source of illuminate is considered.

In real-world(landspace) the shadow may have global source. So, reading dome out may be appropriate.

Invercone in: flat, single dimension subjects such as copying, overhead direct shots.
Invercone out: directional 3 dimensional light.

A landscape reading with the invercone recessed will be erroneous.

Sirius Glass said:

For incident meters: Dome in the meter is where would be standing and aiming away from the subject. Dome out, aimed the same way would be an average reflective reading pointing directly away from the subject. The dome in reading measures the incoming light to the subject; the dome out reading is the light reflected by the anti-subject. If I am understanding the OP, I fail to see the use of the dome out reading. Please correct anything I have said where my understanding differs from the OP and then explain how the dome out measurement would be useful in this example or any additional example.
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So even though the incident dome out is the same configuration the reflected configuration in the anti-subject direction, there is great value in the dome out measurement for landscapes.

Sirius the dome out allows the sensor to "see" more of the light sources effecting the subject matter.

The sky, the reflection off the other walls of the of the canyon and the valley floor each contribute to the exposure in addition to the sun.

Sirius Glass said:

So even though the incident dome out is the same configuration the reflected configuration in the anti-subject direction, there is great value in the dome out measurement for landscapes.

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Yes

1. The limiting factor is the paper.
2. Most papers will hold about a 7 zone (or stop if you want to call it that) range of tones. The rest is black and white.
3. An incident meter will provide an exposure to render middle gray as middle gray only if you have previously tested your meter to provide a proper exposure called the speed point that is a certain amount about film base plus fog. (You need a rock to stand on).

kevinjk said:

1. The limiting factor is the paper.
2. Most papers will hold about a 7 zone (or stop if you want to call it that) range of tones. The rest is black and white.
3. An incident meter will provide an exposure to render middle gray as middle gray only if you have previously tested your meter to provide a proper exposure called the speed point that is a certain amount about film base plus fog. (You need a rock to stand on).

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Welcome kevinjk,

On 1&2 could you provide a bit of context on what you are responding to.

On #3 the incident meter provides a scientifically objective reference point that we regularly refer to as middle gray from the scene. The testing for the film has already been done, its simply the ISO speed. If our tools are working correctly and we are reasonably capable in processing our films, then our results will be very close to ISO standard and mid tones will fall nicely on the paper with reasonable shadow and highlight detail.

There are several wild cards here. Our skills and understanding are the biggest factors. The other big one is creative choice.

Testing is valuable in refining our skills, understanding, and creative vision but the baseline is already there.

I read this when Mark first posted it, and it swished over my head leaving not a trace.

I revisited it a couple of weeks ago, when I understood most of it.

Accordingly, I've started clicking the dome across on my little Digisix where appropriate.

Result: many more beautifully exposed and easy-to-print negatives in the past fortnight.

This is a slightly long-winded way of saying thank you to Mark for the article.

(the zonology in the discussion still swishes past though )

One thing which bothered me for very long time was, which grey is dome's grey? Though it is secret from the manufacturer but it has brought an insurmountable confusion in understanding a rather very simple concept.

Now, I am quite happy to meter(dome facing the camera) the shadow illuminance at box speed and developing it with a speed loosing developer.

Thank you pdeeh.

baachitraka said:

One thing which bothered me for very long time was, which grey is dome's grey? Though it is secret from the manufacturer but it has brought an insurmountable confusion in understanding a rather very simple concept.

Now, I am quite happy to meter(dome facing the camera) the shadow illuminance at box speed and developing it with a speed loosing developer.

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i believe that one of the reasons many people feel more comfortable with reflective metering is that they get the connection between subject and exposure, or in your case between tone and exposure.

To a great extent this explains for me why so many people enjoy and get respectable results from center weighted camera meters.

The same can't be said of spot metering. The results can be significantly better than CW'd metering but spot metering is a true step into abstraction (visualization), subjective judgement, accuracy in aiming, and the world of film sensitometry and physics. If you don't "get" the concepts of zoning and how that carries through from scene, through film, onto paper (and I honestly believe that only a minority ever do) spot metering will be a frustration.

Incident metering for the first timer requires a small leap of faith, that a given amount of light will create a given reflection off our subject. The "problem" for incident metering is that it is "almost too easy". It is so good at getting a high quality camera setting with the dome pointed at the camera that most people never step past that and use it creatively.

BTZS concepts on metering may be interesting but I took it little more serious and for me that was one the biggest source of confusion.

Now, I ignore the most and concentrate on spicing the illumination of the concerned area with incident meter.