Understanding Incident Metering [Archive] - Large Format Photography Forum

IanBarber

3-Jul-2018, 03:23

I would like to get a better understanding on how to properly use incident metering for subjects other than portraits.

Scenario: Street scene where subject maybe in both sunlight and shade.
I would normally walk into the sunlit area, point the meter back at the camera and take a reading and dial those numbers into the camera.

What I am trying to learn, is what this is doing and how can I visualise what the negative is going to look like. On the gray scale, where will the bright and dark areas be for example.

Scenario: Indoor still life with even North light, no shadows visible.
How can I visualise what the negative is going to look like. On the gray scale, where will the bright and dark areas be for example.

Thanks, Ian

Pere Casals

3-Jul-2018, 04:38

Scenario: Street scene where subject maybe in both sunlight and shade.
I would normally walk into the sunlit area, point the meter back at the camera and take a reading and dial those numbers into the camera.

The incident metering is based on the light that arrives to the subject, so if the meter is illuminated by direct sunlight you know the exposure for all subjects than are sunbathed.

If the meter is in the shadow then you know the exposure for all subjects that are in the shadow.

We can add that the angle of the subject surface and of the canmera to the light (if it is directional) will have an impact.

...so if you have subjects both in the shadow and also sunbathed in the same scene then you have to take 2 readings, and then decide.

In general shadows are 4 stops under the sunlit area, so if you place the sunny area at +2 you should have shadows at -2, if shadows are open, this is not in a hole, but illuminated well by the blue sky.

Anyway sun always delivers the similar light power, it can vary a bit depending on time, latitude, etc but as Sunny 16 rule states a sunbathed subject needs at ISO 100 some 1/100s at f16

Scenario: Indoor still life with even North light, no shadows visible.
How can I visualise what the negative is going to look like. On the gray scale, where will the bright and dark areas be for example.

It depends on the situation, what window size, distance to the window, white or dark wall ?

Again you have to take 2 readings, for the bright area and for the dark one, then decide exposure.

IanBarber

3-Jul-2018, 06:32

In general shadows are 4 stops under the sunlit area, so if you place the sunny area at +2 you should have shadows at -2, if shadows are open, this is not in a hole, but illuminated well by the blue sky.

So thinking in terms of Zones, anything in the sunny area is effectively on Zone VII and anything in the shadows will fall on Zone V ?

Luis-F-S

3-Jul-2018, 06:56

Use a spot meter. An incident meter gives you no information about the scene, only the light striking it! It will reference this light to middle gray, or zone V.

Pere Casals

3-Jul-2018, 07:06

So thinking in terms of Zones, anything in the sunny area is effectively on Zone VII and anything in the shadows will fall on Zone V ?

No... it will fall around 4 zones, if something is 2 stops overexposed in the sunny area (Z VII) it will be at around Z III if placed in the shadow and if conserving the same exposure.

Also remember that the Zone in what a subject will be (if exposing what incident metering says) depends o the subject's reflectiveness, as subject can be white, grey or black...

So imagine your subject is a 18% grey card, you read the incident light to expose... the card will be at Z V. If instead the card you have a subject that reflects 36% of light towards the camera then (with same exposure) the subject will be in Z VI.

IanBarber

3-Jul-2018, 07:15

No... it will fall around 4 zones, if something is 2 stops overexposed in the sunny area (Z VII) it will be at around Z III if placed in the shadow and if conserving the same exposure.

Also remember that the Zone in what a subject will be (if exposing what incident metering says) depends o the subject's reflectiveness, as subject can be white, grey or black...

So imagine your subject is a 18% grey card, you read the incident light to expose... the card will be at Z V. If instead the card you have a subject that reflects 36% of light towards the camera then (with same exposure) the subject will be in Z VI.

Thanks Peter. So just to confirm I have this correct...

incident meter reading from in-front of a textured tree would roughly put it on ZV, I would simply close down 2 stops to effectively put it n the region of Zone III and anything in the scene which is reflecting the same light would also be in the same zone

Pere Casals

3-Jul-2018, 07:15

Use a spot meter. An incident meter gives you no information about the scene, only the light striking it! It will reference this light to middle gray, or zone V.

I also prefer spot metering, but incident metering is also nice.

IMHO incident metering is not for all situations, but when we have homogeneous illumination it delivers very consistent results: what is recorded is subject's refectiveness, because metered exposure does not depend on if we take the reading in a darker or lighter surface.

If one is used to incident metering then it's easier to use flash meters in the studio, because regular flash meters are a kind of incident meters...

Bill Burk

3-Jul-2018, 07:47

So thinking in terms of Zones, anything in the sunny area is effectively on Zone VII and anything in the shadows will fall on Zone V ?

A flat 2-dimensional picture has only 5 stops of Subject Luminance Range (SLR). (Zones IV to VIII).

Even when it's in the shade, a part of a 3-dimensional picture has only 5 stops of SLR (but it's down in the shade in the range of Zone II to Zone VI).

Taken together (both sunny and shady parts of the picture), the camera might see a scene with 7 stops of SLR.

The incident meter will try to position the exposure over the whole 7 stops of range (Zone II to Zone VIII).

IanBarber

3-Jul-2018, 08:37

A flat 2-dimensional picture has only 5 stops of Subject Luminance Range (SLR). (Zones IV to VIII).

Even when it's in the shade, a part of a 3-dimensional picture has only 5 stops of SLR (but it's down in the shade in the range of Zone II to Zone VI).

Taken together (both sunny and shady parts of the picture), the camera might see a scene with 7 stops of SLR.

The incident meter will try to position the exposure over the whole 7 stops of range (Zone II to Zone VIII).

Is this why with the BTZS system, you take a shadow and highlight reading and add the different to 5 ?

Pere Casals

3-Jul-2018, 08:58

Thanks Peter. So just to confirm I have this correct...

incident meter reading from in-front of a textured tree would roughly put it on ZV, I would simply close down 2 stops to effectively put it n the region of Zone III and anything in the scene which is reflecting the same light would also be in the same zone

Ian, it depends on the texturized tree. If it reflects 18% of the light, then it will end in Z-V. If the bark of the tree is very dark then it would end perhaps be in Z-III or IV.

So if you use the incident reading to set exposure then all that's illuminated by that light will be in Z-V if it is has same bright than an standard grey card, if it is darker or lighter, of course, it will be recorded in another zone.

To get practice with the incident light meter I'd suggest you an exercise with different scenes. Take the incident metering of the light that's illuminating your sujects, place also an standard grey card in the scene. Then take an SLR or DSLR (an F65 for example) and with its spot meter measure different spots of the scene. You will see that the grey card will be at Zero of the SLR meter, and some dark spots will read -1 or -2, this is Z-IV or III, while white spots will be perhaps at +2.5.

After the incident meter recommends an exposure you just set same ISO and exposure in the SLR and then you can check in what zones will end any spot of the scene. In this way you will get necessary practice to predict the result.

Note that we can meter incident light without having an incident meter, we just need to place the standard grey card in the scene and taking an spot metering of the card. Note that incident metering depends on the direction we meter, in the same way it also depedends on the grey card orientation. Kodak grey card instructions sheet explains how to use the grey card.

3-Jul-2018, 09:11

In situation 2, just incident meter and use the exposure.

In situation 1, if you're into "expose for the shadows and develop for the highlights" you'd set exposure based on the incident reading taken while in the shadows of the scene. Some people would then read or estimate the sun brightness and make a note to develop accordingly. The other option is to use a compensating developer and tmax film and not worry about the highlights.

Jac@stafford.net

3-Jul-2018, 10:02

IanBarber

3-Jul-2018, 10:42

There have been mentions of placing the incident meter in shadow, then also in sunlight and figuring from there. No harm, but that's more of ratio metering; incident meters usually have a dome which is supposed to properly compensate for shadow and highlight when pointed at the camera from the subject. Some meters come with an optional flat translucent disk (or a dome which retracts) and is intended for measuring lighting ratios.

I had a situation only the other day where I was photographing a building, 3/4 of the building was in shade and the other 1/4 was in direct sunlight. I am guessing that in this situation, I should have put the dome of the light meter on the sunny side and maybe open up 2 stops ?

alexmuir

3-Jul-2018, 12:54

I don’t think incident meters are best suited to zone system methods. I use one quite often when out and about. It’s good for scenes where the overall lighting is quite even, and you want a quick reading. I read the instructions for one of the models I’ve owned, possibly Gossen? If I recall correctly, it suggested that, where the main subject was brighter than average, you open up one or two stops, and where darker, you close down one or two. I also use an incident flash meter in studio situations. There are more sophisticated methods of using them in this type of situations. I have some books that describe these methods;
The Photographers Studio Manual by Michael Freeman
Perfect Exposure from theory to practice by Roger Hicks and Frances Schultz
Learning to Light by R. Hicks and F. Schultz
These might be of some interest, especially for indoor work, although Perfect Exposure covers all situations.
Alex

Sent from my iPhone using Tapatalk Pro

Luis-F-S

3-Jul-2018, 16:26

I had a situation only the other day where I was photographing a building, 3/4 of the building was in shade and the other 1/4 was in direct sunlight. I am guessing that in this situation, I should have put the dome of the light meter on the sunny side and maybe open up 2 stops ?

Actually, you should have used a spot meter, then you would not be guessing as to what to put in what zone. The Zone dial pretty much tells you what is doable and what isn't. The film may not have enough latitude for the scene and you may need to expand or contract the tonal range. But without a spot meter you’re guessing in the dark or asking a bunch of “experts” who may or may not really know and could be making up scenarios in their heads. Some people like to make this way more difficult than it is! L

Colin Robertson

4-Jul-2018, 01:36

Good Morning Ian.
First, some thoughts for you. If you're shooting black and white negative the film has some tolerance of 'incorrect' exposure. In printing we can vary the paper grade, and use selective dodging and burning to recover highlights or relieve shadows. When I use 120 folders, or any of my manual 35mm cameras I don't carry a meter. I sunny 16 everything, and can't think when I last had an unprintable negative.
I don't do any colour now, but when I shot 5x4 Velvia (which is much less tolerant of exposure error) I used a Sekonic incident meter without problem. And, as stated, flash metering is incident, and nobody would suggest you can't take successful images by flash due to lack of spot metering. How others work may not be right for you, what matters is achieving a result which satisfies you.

To take your scenario 2. Just put the incident meter in front of main subject, dome towards camera, and go with the suggested reading. With North facing window you'll have low contrast lighting. The meter will do it's best to make sure any object within the scene which is 18% grey falls on ZV and the highlights and shadows will fall comfortably around it.

Scenario 1- Imagine taking two incident readings. One in the area of full sun, the other in open shade. They'll be different by about 2 stops, and both are correct. Each reading is doing the job of placing your 18% grey on ZV for that degree of illumination. You have to decide what you want from that image. If the main interest is the shaded area, go with that. The highlights just might overexpose, but the latitude of film will probably save you. If the highlight area is more important go with that and accept lower shadow values. Or, split the difference; for years the majority of photographs were taken on cameras using reflective metering which averaged the scene.

Here you'll find plenty of advocates for spot-metering, but that really comes into its own when you have complete control of your process. Well used, a spot meter will give you the brightness range of the scene. If you've previously calculated developing times for Normal, High and Low contrast scenes you can then use film speed, exposure, and development time to make a negative that reflects what you want the scene to look like.
However, you also need to really grasp what the spot meter does; the meter 'thinks' whatever you point it at is ZV. Back to scenario 1- even metering within just the shaded area of your scene could produce a range of exposure readings depending on the tone of the area you point the meter at. Same within the bright area.
You could think of your incident meter as giving you the equivalent of pointing your spot meter at an area of ZV in each area of that scene. In fact, if your usual method was to try and find a ZV tone to meter from, the incident meter will be just as effective. Perhaps even more reliable, if you have difficulty deciding which area to meter from to find ZV.
Good luck.

IanBarber

4-Jul-2018, 01:57

Good Morning Ian.

Scenario 1- Imagine taking two incident readings. One in the area of full sun, the other in open shade. They'll be different by about 2 stops, and both are correct. Each reading is doing the job of placing your 18% grey on ZV for that degree of illumination. You have to decide what you want from that image. If the main interest is the shaded area, go with that. The highlights just might overexpose, but the latitude of film will probably save you. If the highlight area is more important go with that and accept lower shadow values. Or, split the difference; for years the majority of photographs were taken on cameras using reflective metering which averaged the scene.

Morning Colin.
Thanks for the detailed explanation, much appreciated.

If the important information of the scene is illuminated by the bright sun, would you meter in the sun and then possibly open up 2 stops to move that away from zoneV or just go with the reading

Pere Casals

4-Jul-2018, 02:08

One in the area of full sun, the other in open shade. They'll be different by about 2 stops

This may vary, but normally we have more than 2 stops, 3 is common and sometimes we have 4.

The Sunny 16 rule recipe says 4 stops: f/16, ISO 100, 1/100s for sunlit area, but f/4 for open shade https://en.wikipedia.org/wiki/Sunny_16_rule

chassis

4-Jul-2018, 05:35

The two scenarios are quite different. The street scene is relatively high contrast. The still life with north window light is relatively low contrast.

No matter the contrast, the film capability and scene lighting need to be understood, primarily through experience (accumulated trial and error).

I use the Zone System as a very loose idea. I recommend avoiding an overly rigid adherence to it. Read the websites of Sekonic, Profoto and PocketWizard. AdoramaTV has some good videos on lighting and metering.

I would meter the scenes in these ways:

Street: from the camera position: spot meter the shadows, spot meter the highlights, choose the exposure based on experience and desired aesthetic.

Still life: from the subject position: incident meter with dome facing the north window on the highlight side of the subject, incident meter with dome facing away from window on the shadow side of the subject, incident meter with the dome facing the camera, directly between the camera and subject. Choose the exposure based on experience and desired aesthetic.

In both cases, it's common to have chosen the aperture, independent of lighting, based on desired depth of field, lens choice and subject to camera distance. This leaves only shutter speed to select, based on the metering measurements, experience and desired aesthetic. Film development and subsequent positive image manipulation are the remaining controls to produce the desired aesthetic. Get out there and shoot!

chassis

4-Jul-2018, 06:48

This addresses the points well, in my view:

https://www.sekonic.com/united-states/classroom/articles/metering-for-large-format-film.aspx

The article addresses examples of metering higher contrast and lower contrast scenes, with 4x5 film.

Jac@stafford.net

4-Jul-2018, 15:33

I don’t think incident meters are best suited to zone system methods.

I think the Zone System desperately needs a thorough revision considering modern films.
.

Pere Casals

4-Jul-2018, 18:57

I think the Zone System desperately needs a thorough revision considering modern films.
.

Jac, why do you think this ? is it because extended dynamic range ?

Randy

5-Jul-2018, 04:08

Pere Casals

5-Jul-2018, 07:49

Our own Ken Lee has a couple articles about metering on his Tech page (http://www.kennethleegallery.com/html/tech/index.php). They may be helpful. Scroll down to the heading - A Simpler Approach to Metering and just below that is The Myth of the 18% Gray Card.

Let me say my view, a grey card is a 18% fact, used as an standard. We know that the spot meter on the card will give the same exposure than an incident meter. If we set the exposure for the grey card then a white subject reflecting 90% will be some 2 1/3 stops over exposed, and black subjects reflecting 2% will be just outside the toe, as defined by ISO. From here we can decide what we do for each particular situation.

Also a grey card is spectrally flat, so it is a good reference for color corrections to restore neutral greys.

So IMHO a 18% grey card is simply a calibrated tool, an amazing one, but it's up to us how we use it.

Jac@stafford.net

5-Jul-2018, 14:39

So IMHO a 18% grey card is simply a calibrated tool, an amazing one, but it's up to us how we use it.

My friend, you have challenged my presumptions. Are you certain that a grey card (12% or 18%) does not reflect, for example, blue skylight? I am on the tilting point of the question, and I thought I understood color reflection. Burst my bubble if I am wrong!

Pere Casals

5-Jul-2018, 15:02

My friend, you have challenged my presumptions. Are you certain that a grey card (12% or 18%) does not reflect, for example, blue skylight?

A Kodak R-27 gray card have a flat spectrum, reflecting all visible colors in a consistent way, blue included, of course.

Jac@stafford.net

5-Jul-2018, 17:04

A Kodak R-27 gray card have a flat spectrum, reflecting all visible colors in a consistent way, blue included, of course.

Thanks to your input I have learned that I am not yet old enough to be right. :)

Bill Burk

5-Jul-2018, 19:19

Is this why with the BTZS system, you take a shadow and highlight reading and add the different to 5 ?

Yes, that is the relationship "Beyond the Zone System" (BTZS) takes advantage of with their steps that use incident meters to find a "Subject Luminance Range" (SLR).

Bill Burk

5-Jul-2018, 19:50

We know that the spot meter on the card will give the same exposure than an incident meter.

We could talk about specific circumstances where that's true, but many people find a 1 stop difference between spot meter and incident meter.

I just check it both ways and make an exposure determination based on which type of meter I think was more likely to give me the correct exposure in that situation. (For example for a backlighted scene I would think the exposure recommended by my spot meter is more likely to be correct, in ordinary light, I would lean towards using the recommendation of the incident meter - unless I was making careful use of Zone System).

Pere Casals

6-Jul-2018, 02:16

We could talk about specific circumstances where that's true

R-27 Kodak grey card includes precise instructions about how to use the grey card, it has to be angled depending light direction...

I find that the instructions in the R-27 are very well explained, this was really helpful to me.

but many people find a 1 stop difference between spot meter and incident meter.

For caucasian skin there is just one stop difference, but this is very easy to explain.

1) I take indicent metering of lingt illuminating a wall

2) If I take an spot metering... if the wall is white I'll real +2, it is black I'll read -2.

As we normally place white skin in Z-VI we have to overexposr +1 what we read with spot meter in the cheek. If it is black skin we well place the face in Z IV.

The spot metering depends on subjects reflectance, so we'll need to correct exposure depending on the zone we what that spot...

I just check it both ways and make an exposure determination based on which type of meter I think was more likely to give me the correct exposure in that situation.

I agree...

IMHO it's just about understanding what we are doing. If we use the incident metering (with uniform light) we know that subjects reflecting 18% like a grey card will be in Z-V, and that white things will be in Z VII.

Instead, if we explore the scene with the spot meter we will know in what zone we will have each spot, for each exposure we consider.

Pere Casals

6-Jul-2018, 02:30

We could talk about specific circumstances where that's true

R-27 Kodak grey card includes precise instructions about how to use the grey card, it has to be angled depending light direction...

I find that the instructions in the R-27 are very well explained, this was really helpful to me.

but many people find a 1 stop difference between spot meter and incident meter.

For caucasian skin there is just one stop difference, but this is very easy to explain.

1) I take incident metering of light illuminating a wall

2) If I take an spot metering... if the wall is white I'll real +2, it is black I'll read -2.

As we normally place white skin in Z-VI (or VII) we have to overexpose (+1 or +2) what we read with spot meter in the cheek. If it is black skin we will place the face in Z IV (or V).

The spot metering depends on subject's reflectance, so we'll need to correct exposure depending on the zone we what that spot... what is true is that a well done spot metering on the gray card should match the incident meter

I just check it both ways and make an exposure determination based on which type of meter I think was more likely to give me the correct exposure in that situation.

I agree...

IMHO it's just about understanding what we are doing. If we use the incident metering (with uniform light) we know that subjects reflecting 18% like a grey card will be in Z-V, and that white things will be in Z VII or VIII, from that we can take decisions to modify exposure.

Instead, if we explore the scene with the spot meter we will know in what zone we will have each spot, for each exposure we consider, so we can change exposure and/or plan a custom development...

Bill Burk

7-Jul-2018, 09:27

For caucasian skin there is just one stop difference, but this is very easy to explain.

1) I take incident metering of light illuminating a wall

2) If I take an spot metering... if the wall is white I'll real +2, it is black I'll read -2.

As we normally place white skin in Z-VI (or VII) we have to overexpose (+1 or +2) what we read with spot meter in the cheek. If it is black skin we will place the face in Z IV (or V).

The spot metering depends on subject's reflectance, so we'll need to correct exposure depending on the zone we what that spot...

I agree... and it is easy to agree when we talk about reading a subject and placing it on an appropriate Zone. So long as we aren't talking about that dratted gray card.

What is true is that a well done spot metering on the gray card should match the incident meter

This is the point on which I'd like to bring you around to my way of thinking.

I teach that it doesn't necessarily match the incident meter.

We are talking about a minor difference, around 5%.

So it's not something to get up in arms over.

I mean, 5% is a small difference. In terms of Zone System, the gray card remains in Zone V.

But you might want me to answer how can it be possible that a gray card is not Zone V when you meter and place it at Zone V for camera tests?

Isn't a gray card Zone V by definition?

Yeah I guess so. I'm going to have some trouble explaining that.

I think the 2/3 stop difference that people often find in film speed tests has something to do with using the gray card for Zone V.

Try this experiment, for a 400 speed film, such as Tri-X set the incident meter at 400 and the spot meter at 250 and wander around taking meter readings...

Let me know if you don't find readings of the gray card and the incident readings are in agreement more often.

Alan Klein

7-Jul-2018, 09:55

I found that gray card reflective vs. incident measurements are up to a stop difference. I read somewhere that the meter manufacturers design has 18% vs. 12% for gray and that Ansel Adams may have had something to do with that. I don't have time right now but maybe someone could do the research and report the results here.

Bill Burk

7-Jul-2018, 17:40

Isn't a gray card Zone V by definition?

I think the 2/3 stop difference that people often find in film speed tests has something to do with using the gray card for Zone V.

Try this experiment, for a 400 speed film, such as Tri-X set the incident meter at 400 and the spot meter at 250 and wander around taking meter readings...

Let me know if you don't find readings of the gray card and the incident readings are in agreement more often.

Scratch that. Though the experiment would work, it's not the same 2/3 stop.

All three kinds of light meters are calibrated to give the same resulting reading, but they all "intend" to place an amount of light that is 10 times the speed point on film.

Zone System "intends" to place 4 stops more than the speed point (Zone I) on film.

That's where 2/3 stop comes from directly.

The difference of an 18% gray card and the standard scene that all the meters are calibrated to...

That's what we are searching for. It's not the 2/3 stop I mention above.

Pere Casals

7-Jul-2018, 17:41

I teach that it doesn't necessarily match the incident meter.

We are talking about a minor difference, around 5%.

So it's not something to get up in arms over.

I mean, 5% is a small difference. In terms of Zone System, the gray card remains in Zone V.

But you might want me to answer how can it be possible that a gray card is not Zone V when you meter and place it at Zone V for camera tests?

Isn't a gray card Zone V by definition?

Yeah I guess so. I'm going to have some trouble explaining that.

I think the 2/3 stop difference that people often find in film speed tests has something to do with using the gray card for Zone V.

Try this experiment, for a 400 speed film, such as Tri-X set the incident meter at 400 and the spot meter at 250 and wander around taking meter readings...

Let me know if you don't find readings of the gray card and the incident readings are in agreement more often.

Bill,

First, the calibration of photographic light meters is covered by ISO 2720:1974. Calibration of in camera internal meters is specified by ISO 2721:1982.

Recommended ISO calibration factors spread 1/6 EV, with recommended K in the range 10.6 to 13.4 (cd/m²). A manufacturer is free to use what K he wants, but there is that recommended range.

IMHO with on axis illumination the incident metering and the spot metering on a card should match... The ISO 2720:1974 recommendations are aiming that.

...but we can have practical situations that would make the readings differ because illumination is not like the one in the calibration conditions:

> Light is more or less directional/diffuse, and the imperfect lambertian card reacts different than the imperfect half dome in the incident meter.

> Light is not on axis, for example sunlight illuminating a face in a portrait or a building can come from sun elevated 30º, or 45º or even 90º. Because that Kodak recommends increasing the exposure by 1/2 stop when sun is high in the sky. Again a half dome and the card react different to out of axis illumination.

the meter manufacturers design has 18% vs. 12% for gray

What I understood from the reading of the ISO norms is that incident light meters are calibrated to a 12% reflectance in the formulas while reflective meters are calibrated to around 18% (16-18%), in that way both kinds of meter deliver the same reading with on axis illumination on a 18% lambertian reflective subject.

So IMHO that 18% vs 12% difference in the calibrations is introduced just to make the incident reading match the spot reading, but this is with on-axis illumination, when illumination is well not on-axis we'll have a difference coming from different reaction to the off-axis light depending on if we read throught a dome or from a naked photocell.

please correct me if I'm wrong...

____________________________

Then we have spectral sensitivity of the sensors, some are spectrally flatter than other... and depending on the light, subject color and filtering...

Alan Klein

7-Jul-2018, 19:04

Bill,

First, the calibration of photographic light meters is covered by ISO 2720:1974. Calibration of in camera internal meters is specified by ISO 2721:1982.

Recommended ISO calibration factors spread 1/6 EV, with recommended K in the range 10.6 to 13.4 (cd/m²). A manufacturer is free to use what K he wants, but there is that recommended range.

IMHO with on axis illumination the incident metering and the spot metering on a card should match... The ISO 2720:1974 recommendations are aiming that.

...but we can have practical situations that would make the readings differ because illumination is not like the one in the calibration conditions:

> Light is more or less directional/diffuse, and the imperfect lambertian card reacts different than the imperfect half dome in the incident meter.

> Light is not on axis, for example sunlight illuminating a face in a portrait or a building can come from sun elevated 30º, or 45º or even 90º. Because that Kodak recommends increasing the exposure by 1/2 stop when sun is high in the sky. Again a half dome and the card react different to out of axis illumination.

What I understood from the reading of the ISO norms is that incident light meters are calibrated to a 12% reflectance in the formulas while reflective meters are calibrated to around 18% (16-18%), in that way both kinds of meter deliver the same reading with on axis illumination on a 18% lambertian reflective subject.

So IMHO that 18% vs 12% difference in the calibrations is introduced just to make the incident reading match the spot reading, but this is with on-axis illumination, when illumination is well not on-axis we'll have a difference coming from different reaction to the off-axis light depending on if we read throught a dome or from a naked photocell.

please correct me if I'm wrong...

____________________________

Then we have spectral sensitivity of the sensors, some are spectrally flatter than other... and depending on the light, subject color and filtering...

I'm not familiar with the technical. So I assume you're correct. What I am more interested in is practical application. So when I'm trying to determine exposure, what would be your recommendations when using my meter that has a dome for incident and a 10% angle for reflectance? How to aim and how to interpret? Any suggestions would be appreciated.

Bill Burk

7-Jul-2018, 21:41

Recommended ISO calibration factors spread 1/6 EV, with recommended K in the range 10.6 to 13.4 (cd/m²). A manufacturer is free to use what K he wants, but there is that recommended range.

IMHO with on axis illumination the incident metering and the spot metering on a card should match...

So IMHO that 18% vs 12% difference ...please correct me if I'm wrong...

____________________________

Then we have spectral sensitivity of the sensors, some are spectrally flatter than other... and depending on the light, subject color and filtering...
I think you will find manufacturers and standards never talk of 18%. That is just a card that we have for our own use.

As I understand the manufacturers use the spread for K and C to try to make the exposure recommendations identical. They use the spread to account for sensor differences, etc.

10 times the speed point is the target amount of light that a meter will try to put on film. And they all want to do that. Unless you do Zone System tests. Then the aim is four stops above the speed point instead of ten times.

Pere Casals

8-Jul-2018, 03:59

I think you will find manufacturers and standards never talk of 18%. That is just a card that we have for our own use.
As I understand the manufacturers use the spread for K and C to try to make the exposure recommendations identical. They use the spread to account for sensor differences, etc.

My guess is that manufacturers follow the ISO recommendation that specifies a range, so the we have that 1/6 EV differences between meters, so they are in fact aiming a 16% to 18% reflectance.

10 times the speed point is the target amount of light that a meter will try to put on film. And they all want to do that. Unless you do Zone System tests. Then the aim is four stops above the speed point instead of ten times.

You are right, this comes from the ASA PH2.5-1960 speed change.

In 1960 Box Speed was doubled without any film manufacturing change, so in theory the Zone System experimented a shift.

Divine ZS inspiration came to AA while shooting the Half Dome with an Adon, this was 1927, 33 years later ASA changed the 20x to 10x.

Anyway IMHO it can be debated if we should or not rate film at its half speed to use the ZS, now we have accurate meters and we don't need that additional safety factor of one stop that was removed in 1960.

AA says Z-II is "Textured black; the darkest part of the image in which slight detail is recorded". Z-II is -3 stops in the meter, so outside the toe if using the post 1960 box speed.

My view is that ZS is not a recipe but a methodology & visualization and we need to ajust it to our process, depending on how we meter and develop we have an impact, so IMHO at the end we just need to consider a personal ISO for a film that makes our Z-II "the darkest part of the image in which slight detail is recorded".

This is what I personally concluded...

Pere Casals

8-Jul-2018, 06:42

meter that has a dome for incident and a 10% angle for reflectance? How to aim and how to interpret?

It should deliver a good exposure for normal conditions. The Problem with incident metering is when not all the scene is bathed with the same incident light, then may take more readings to decide what you over/under expose.

Also while with spot metering you know what over/under exposure you have in each spot of the scene, with incident metering (when illumination is challenging) you have to guess how the deep shadows and highlights will be, and particular film toe/shoulder has an impact.

One thing it can be done is shooting a 35mm test roll of the same film you use for LF. You just select some meaningful scenes to make bracketings, and taking notes of the incident metering, the SLR metering in matrix, ponderated and spot modes, with the spot readings of highlights and shadows. This would give you a solid criterion about when you should modify the indicent metering to suit your visualization, from the results you bracketed.

IanBarber

8-Jul-2018, 08:06

It should deliver a good exposure for normal conditions. The Problem with incident metering is when not all the scene is bathed with the same incident light, then may take more readings to decide what you over/under expose.

Very interesting thread, this to me sounds like even with roll film when "not all the scene is bathed with the same incident light" it may be just as easy to spot meter.

Bill Burk

8-Jul-2018, 08:27

My guess is that manufacturers follow the ISO recommendation that specifies a range, so the we have that 1/6 EV differences between meters, so they are in fact aiming a 16% to 18% reflectance.

You are right, this comes from the ASA PH2.5-1960 speed change.

In 1960 Box Speed was doubled without any film manufacturing change, so in theory the Zone System experimented a shift.

Divine ZS inspiration came to AA while shooting the Half Dome with an Adon, this was 1927, 33 years later ASA changed the 20x to 10x.

Anyway IMHO it can be debated if we should or not rate film at its half speed to use the ZS, now we have accurate meters and we don't need that additional safety factor of one stop that was removed in 1960.

AA says Z-II is "Textured black; the darkest part of the image in which slight detail is recorded". Z-II is -3 stops in the meter, so outside the toe if using the post 1960 box speed.

My view is that ZS is not a recipe but a methodology & visualization and we need to ajust it to our process, depending on how we meter and develop we have an impact, so IMHO at the end we just need to consider a personal ISO for a film that makes our Z-II "the darkest part of the image in which slight detail is recorded".

This is what I personally concluded...
This is a well-considered conclusion.

I’ll state as fact manufacturers do not consider any gray card percentage in their calibration. As casual experimenters, we are interested in gray card percentage, with 18% the most easily obtained for our experiments.

They don’t deliberately relate, but we might discover a relationship, or theorize about a relationship we can test.

I have a Sekonic gray card with 1/6 stop differences between a set of seven chips that span one f/stop. That gray card is good for tests like this.

For example I could shoot this card in a scene that I photograph according to careful metering, with incident dome or with flat disc, with reflective light mode. I could hold the card correctly and I could position it wrong.

The results could be compared easily by finding the chip that placed 10x the speed point... And that would be the deduced gray card percentage which exactly matches the exposure meter.

In a practical outcome from this test, we could write “scenarios” - in it we could talk about how we positioned the card, what meter was used and then the usable information...

How much to adjust a reading that was taken with an 18% gray card.

(Sounds like we intend to write some instructions which will be like the Kodak instructions, doesn’t it?)

Pere Casals

8-Jul-2018, 09:42

(Sounds like we intend to write some instructions which will be like the Kodak instructions, doesn’t it?)

Yes, a bit ! :)

The test you propose would be interesting to be performed...

chassis

8-Jul-2018, 18:33

This image was made on July 5, and was metered with both incident and spot with a Sekonic L-758DR. Setting sun (golden hour) was to camera rear. Camera to subject distance was approximately 50 feet/15 meters. f/45 was chosen with ISO 100 for Kodak Ektar 100. Spot metering on the shadows in the cornstalks gave 1/2 second exposure, and from memory the sky was 3 stops brighter, again using spot metering. Incident metering from the camera position with the dome facing the sun gave 1/4 second exposure. The sun was fully exposed, no clouds were diffusing its light.

I set the shutter to 1/4 second, and made the exposure.

I'm satisfied with the result. Overall brightness of the image could be higher, but that is a postprocessing choice, not the exposure. There is more than enough density on the negative. For my aesthetic, "black" is an acceptable color, and often occurs in the shadows. ;)

http://www.wilmarcoimaging.com/img/s/v-3/p2955794824-4.jpg (http://www.wilmarcoimaging.com/p426252683/eb02dd988)

Pere Casals

8-Jul-2018, 19:08

the sky was 3 stops brighter

Excellent example !

One thing I found with saturated subjects with color film is that while a spot reading may look moderately overexposed it can happen that the amount of light of the saturated color may overexpose a lot one of the channels.

This effect is seen well in the 3-color histograms in the DSLRs.

One interesting feature of the Nikon F5 is that its photometer is RGB, beyond the 1k points, the smart neural network and the DX code informing the system about film latitude, the RGB information allows the camera to not overexpose a particular color channel.

I think this is an issue when shooting Velvia if not wanting a washed sky... Negative film is more bullet proof.

The F5 (and 6) is an excellent LF photometer, if not considering weight :)

Alan Klein

8-Jul-2018, 19:27

It should deliver a good exposure for normal conditions. The Problem with incident metering is when not all the scene is bathed with the same incident light, then may take more readings to decide what you over/under expose.

Also while with spot metering you know what over/under exposure you have in each spot of the scene, with incident metering (when illumination is challenging) you have to guess how the deep shadows and highlights will be, and particular film toe/shoulder has an impact.

One thing it can be done is shooting a 35mm test roll of the same film you use for LF. You just select some meaningful scenes to make bracketings, and taking notes of the incident metering, the SLR metering in matrix, ponderated and spot modes, with the spot readings of highlights and shadows. This would give you a solid criterion about when you should modify the indicent metering to suit your visualization, from the results you bracketed.

Well, I have to admit that when shooting (MF 6x7 120 roll film), I cheat by bracketing +1 and -1 stop with Velvia 50 and with Tmax 100. SInce I'm shooting landscapes, this works for me. And roll film makes it simple to do and relatively cheap assurance I get at least one good exposure. I think on the BW film I probably should bracket -1 and +2.

cowanw

9-Jul-2018, 06:18