Different measurements with spot vs incident metering on same target?

Beanzu · Feb 28, 2016

Hi all,

Apologies if this doesn't quite fit in this forum; I figured, though, it's a question that might be helpful to some future medium format shooter...

My question: why am I getting two different meter readings when I take an incident reading of my subject, and then a spot reading off a grey card at my subject? I'm consistently getting differing readings of one full stop difference - with the spot metering off the grey card metering one stop less than incident.

My setup: Hasselblad 501c, 80mm glass, Sekonic L-508 (has both spot and incident capabilities). My subject was a flower pot about 10' away from my tripod mounted camera, outside, under overcast skies (the light was omnidirectional, no harsh shadows). I take the incident reading with the diffuser dome up: 1/30th at 5.8, ISO 400. I take the 1° spot reading back at the camera (at 1°, the grey card not only covers the metering circle, but the entire viewfinder): 1/60th at 5.8, ISO 400.

So, am I missing something fundamental about using a spot meter? Or does my used-from-ebay-meter need to be calibrated? This is the first time I've used a meter (if that wasn't obvious!)

Sirius Glass · Feb 28, 2016

The incident meter should be at the location of the subject and pointed to the camera. Do not use a gray card for incident readings.
The reflective reading [average, matrix or spot] should be a the location of the camera and aimed at the subject.

Incident readings are of the received light.
Reflected readings are of the reflected light.
They may not be the same.

Example:

a reading of a white dish or snow with a reflective reading will be under exposed and come out as grey. The incident reading will be properly exposed.
a reading of a black cat against a black background with a reflective reading will be over exposed and come out as grey. The incident reading will be properly exposed.

Beanzu · Feb 28, 2016

Sirius Glass said:
The incident meter should be at the location of the subject and pointed to the camera. Do not use a gray card for incident readings.
The reflective reading [average, matrix or spot] should be a the location of the camera and aimed at the subject

Hi Sirius,

Yep, that's exactly what I did: incident reading at my subject (diffuser dome up; not retracted) pointed at the camera, and then a spot reading from my position at the camera off of a grey card located at my subject. My spot reading was one stop under my incident reading. They should be the same, no? The light was even, and I was very careful not to cast a shadow on the dome during my incident reading.

MattKing · Feb 28, 2016

An 18% grey card isn't the same reflectance as an average scene, so a straight reading off one should usually vary from an incident reading. In addition, there is a particular technique that you should use with a grey card (a particular angle you should use).

An 18% grey card is useful, because it provides a standardized reflectance.

Sirius Glass · Feb 28, 2016

The first part on my post:

Sirius Glass said:
The incident meter should be at the location of the subject and pointed to the camera. Do not use a gray card for incident readings.
The reflective reading [average, matrix or spot] should be a the location of the camera and aimed at the subject.

Beanzu said:
Hi Sirius,

Yep, that's exactly what I did: incident reading at my subject (diffuser dome up; not retracted) pointed at the camera, and then a spot reading from my position at the camera off of a grey card located at my subject. My spot reading was one stop under my incident reading. They should be the same, no? The light was even, and I was very careful not to cast a shadow on the dome during my incident reading.

The second part on my post:

Sirius Glass said:
Incident readings are of the received light.
Reflected readings are of the reflected light.
They may not be the same.
Example:

a reading of a white dish or snow with a reflective reading will be under exposed and come out as grey. The incident reading will be properly exposed.

a reading of a black cat against a black background with a reflective reading will be over exposed and come out as grey. The incident reading will be properly exposed.

No, they may not be the same depending on the reflectance of the subject. Also the gray card is a standard for reflectance and may not reflect [pun] the light as a subject reflects light.

benjiboy · Feb 28, 2016

If they were producing the same reading, that's when you worry.

RobC · Feb 28, 2016

Also you are making assumptions that they should be the same. What makes you think that?
A spot meter needs to be understood and the learning curve with one is much longer and steeper than with an incident meter if you want to use it with accuracy.

But just for you, the middle of a 7 1/3 stop range, which is close enough to what the B+W film manufacturers (and Colour Film manufacturers for all I know) deem to be the average range of a subject from its darkest to lightest parts, is 8% reflectance and not 18% reflectance. 18% reflectance is about 1 stop lighter than the middle of the standard range.
That means both your meters are correct which may seem counter intuitive but is not becasue the meters are reading different things and assuming different things, read on....
Because an 18% grey card is not the middle and is 1 stop lighter than the middle it makes sound sense that your spot meter which is trying to put the exposure in the middle, will give a reading of 1 stop less required exposure than an incident meter reading which is not pointing at the grey card and is trying to render it exactly as it is which is 1 stop lighter than the middle(8%) at 18% reflectance(twice the middle reflectance near enough). So both meters are giving correct readings. You just need to get your head around why, how to interpret that and how you intend to use that information when setting your camera exposure.

In short all reflectance meters try and place whatever you point them at in the middle of a curve which is designed to hold a range of 7 1/3 stops. The reflectance meters are calibrated to do this NOT based on anything to do with a kodak grey card or 18%, but on reflectance of 8% which equates to a K Factor of 12.5 if you understand the formula that reflectance meters use. If what you point it at is white it will turn out grey. If what you point it at is black it will turn out grey.
Incident meters try and render everything at their actual reflectance which is great if the subject range does happen to fit the average 7 1/3 stop range. Its not so clever if it is doesn't which is where a spot meter is more useful but only if you really know your film and how to use the spot properly otherwise you're better of with and incident meter.

AND

Just to complicate things for you, if you are going down the zone system route of a 10 stop range using zones 0 to 10, then the reflectance of the middle of that range is only 3.125% reflectance. Just bear that in mind because it WILL take you a while to get your head around it becasue most of the books are wrong since they all copied the information from each other without understanding it.

pentaxuser · Feb 28, 2016

I must admit that I too had always thought that a reflected reading from a grey card with a spot meter would produce the same or close to the same reading as an incident reading. Bear in mind that the OP says it was overcast so the angle of the grey card is largely irrelevant, isn't.

The incidence meter accurately gathers all the light on the subject in this case a grey card and translates that to an accurate reading for that subject, in this case the grey card, so reproduces the grey card accurately. The reflectance reading does likewise but bases its reading on the same grey card.

Shouldn't the two readings in this case be very close or at least closer than a stop apart?

If it was an overcast day and for the sake of argument whole scene was a mid grey wall then shouldn't the incident reading produce an exposure and a print of said grey wall that was very close if not identical to the same neg and print produced from a spot reflectance reading of the same grey wall?

As long as the grey card is the same shade as the grey wall then the readings will be identical, will they not?

pentaxuser

RobC · Feb 28, 2016

of course not. What happens if the wall is pure white or pure black. Incident reads light falling on subject which remains the same regardless of whether part of wall is black and part is white. Reflectance meter tries to put what you point it at in middle of film curve.
Take three readings, one with incident meter, next with spot meter pointing at black part of wall and last with spot meter pointing white part of wall. Are you seriously suggesting they should all give the same reading?

benjiboy · Feb 29, 2016

I just tried this with a Kodak 18% Grey Card and there was just ovet a stop difference between the incidental and reflected spot readings which is about what it should be.

markbarendt · Feb 29, 2016

Beanzu said:
Hi Sirius,

Yep, that's exactly what I did: incident reading at my subject (diffuser dome up; not retracted) pointed at the camera, and then a spot reading from my position at the camera off of a grey card located at my subject. My spot reading was one stop under my incident reading. They should be the same, no? The light was even, and I was very careful not to cast a shadow on the dome during my incident reading.

No.

With a reflective meter of any type, spot, center weighted, or whatever, the reflectivity of the target must be considered and an offset applied to determine the camera exposure.

With a normal gray card that offset is about 1 stop.

The silly question I have is "why didn't Kodak (or anybody) mass produce a card that would provide the right reading without extra maths?"

RalphLambrecht · Feb 29, 2016

RobC said:
Also you are making assumptions that they should be the same. What makes you think that?
A spot meter needs to be understood and the learning curve with one is much longer and steeper than with an incident meter if you want to use it with accuracy.

But just for you, the middle of a 7 1/3 stop range, which is close enough to what the B+W film manufacturers (and Colour Film manufacturers for all I know) deem to be the average range of a subject from its darkest to lightest parts, is 8% reflectance and not 18% reflectance. 18% reflectance is about 1 stop lighter than the middle of the standard range.
That means both your meters are correct which may seem counter intuitive but is not becasue the meters are reading different things and assuming different things, read on....
Because an 18% grey card is not the middle and is 1 stop lighter than the middle it makes sound sense that your spot meter which is trying to put the exposure in the middle, will give a reading of 1 stop less required exposure than an incident meter reading which is not pointing at the grey card and is trying to render it exactly as it is which is 1 stop lighter than the middle(8%) at 18% reflectance(twice the middle reflectance near enough). So both meters are giving correct readings. You just need to get your head around why, how to interpret that and how you intend to use that information when setting your camera exposure.

In short all reflectance meters try and place whatever you point them at in the middle of a curve which is designed to hold a range of 7 1/3 stops. The reflectance meters are calibrated to do this NOT based on anything to do with a kodak grey card or 18%, but on reflectance of 8% which equates to a K Factor of 12.5 if you understand the formula that reflectance meters use. If what you point it at is white it will turn out grey. If what you point it at is black it will turn out grey.
Incident meters try and render everything at their actual reflectance which is great if the subject range does happen to fit the average 7 1/3 stop range. Its not so clever if it is doesn't which is where a spot meter is more useful but only if you really know your film and how to use the spot properly otherwise you're better of with and incident meter.

AND

Just to complicate things for you, if you are going down the zone system route of a 10 stop range using zones 0 to 10, then the reflectance of the middle of that range is only 3.125% reflectance. Just bear that in mind because it WILL take you a while to get your head around it becasue most of the books are wrong since they all copied the information from each other without understanding it.

You got me confused now too. As the OP, I would have expected the two readings to be the same and when I do this with my metwrs and my card,they are.However, my card has a reflectance of 18% or a density reading of 0.75.This makes sense to me and in my mind is in alignment with AA's books and strategy.We may have a different understanding of 'middle'.

pentaxuser · Feb 29, 2016

RalphLambrecht said:
You got me confused now too. As the OP, I would have expected the two readings to be the same and when I do this with my metwrs and my card,they are.However, my card has a reflectance of 18% or a density reading of 0.75.This makes sense to me and in my mind is in alignment with AA's books and strategy.We may have a different understanding of 'middle'.

Yes, Ralph I too felt that I had seen something to the effect that if you cut out the problem of the grey card's correct angle which you can here as it was overcast and shadowless then grey card measuring by spot and incident meters was the one scenario in which the two meters agreed.

As Mark has said if this isn't the case then why produce a grey card that isn't a standard for the middle

It appears that in the scenario as described by the OP the incident meter gives the correct exposure but the spot meter reading on the grey card has to be then adjusted to apply the correction that gives the "true exposure"

Given that this makes the spot meter somewhat cumbersome to use compared to the incident meter then maybe someone will gives examples of where the spot give a more accurate exposure?

pentaxuser

markbarendt · Feb 29, 2016

pentaxuser said:
Given that this makes the spot meter somewhat cumbersome to use compared to the incident meter then maybe someone will gives examples of where the spot give a more accurate exposure?

pentaxuser

Neither is more or less accurate, used appropriately they are equals. They simply measure different light sources.

Ps.

Where I think incident and reflective meters differ is in a "sense of touch". That saying is being used as an allegory here.

The upside with a spot meter is that you can reach out and "touch" a spot in the scene and tie that to a physical point on a film curve. For many people that provides a nice tangible way to think about metering. The downside of spot metering is that a human judgement generally needs to be made about the reflectivity of the target. Even with a gray card we have to know if we are using the white side or the gray side and apply the correct offset.

Part of this challenge can be illustrated by the typical "shoot to the shadows" advice. The subject in the shadow still has to be judged and corrected for, for example if the subject snow or dark black lava? The offset one would choose would be very different for each.

The upside with an incident meter is that no judgement about reflectivity is needed, the downside for some is that it requires a leap of faith, you don't "touch" a point in the scene to get the camera setting.

RobC · Feb 29, 2016

RalphLambrecht said:
You got me confused now too. As the OP, I would have expected the two readings to be the same and when I do this with my metwrs and my card,they are.However, my card has a reflectance of 18% or a density reading of 0.75.This makes sense to me and in my mind is in alignment with AA's books and strategy.We may have a different understanding of 'middle'.

Precisely, reflectance meters target middle of film curve or more precisely 8% (= 12.5K Factor) of whatever is metered which is approx 3 2/3 stops less than metered and is the middle of a 7 1/3 stop range.

The mistake everyone makes is they think reflection meters target middle grey. They don't, they target middle of film curve which has transaprency and no reflection %age or colour and is not the same thing as an opaque surface which has a reflection percentage and colour. Film has neither, it has transaparency.

The OPs meter is sekonic which uses a K factor of 12.5. The K factor is used as a divisor. if you divide 100 by 12.5 what do you get? You get 8. If you divide something by 12.5 you get 8% of its value which is the same as multiplying it by 8%. i.e. current sekonic meters are calibrated to 8% of whatever they read and 8% is 3 2/3 stops less tahn what is metered which is in the middle of the film curve range of 7 1/3 stops.
And 8% is roughly 1 stop less than 18%, i.e. half the brighness.

The formula for a reflection meter is:

2^Ev= (B*S)/K

as you can see the K factor is a divisor and a divisor of 12.5 is the same as taking 8% of the value. Hopefully this will quell any ideas of reflection meters being calibrated to 18%. Today they are not. Years ago what they were calibrated to I don't care becasue I'm not using old meters and neither is OP. But if you are using one then you better say so and say what its calibrated to and which standard it was using becasue they aren't the same as today.

AND

its important to understand that the 8% is 8% of whatever ever is metered and not an 8% reflectance. It is 3 2/3 stops less than what is metered. That equates to a certain reflectance but only if you factor in the standards used to determine film speed which you can read Stephen Benskins writings to find out more. I lost the will to live while reading them but got the general idea.

Jim Noel · Feb 29, 2016

"Just to complicate things for you, if you are going down the zone system route of a 10 stop range using zones 0 to 10, then the reflectance of the middle of that range is only 3.125% reflectance. Just bear that in mind because it WILL take you a while to get your head around it becasue most of the books are wrong since they all copied the information from each other without understanding it.[/QUOTE]

A 10 stop range is 0-IX. Not 0-X which is 11 stops. this is another error commonly repeated. Ansel's and Fred Archer's original writings talked about 10 zones, 0-IX.

RobC · Feb 29, 2016

Jim Noel said:
"Just to complicate things for you, if you are going down the zone system route of a 10 stop range using zones 0 to 10, then the reflectance of the middle of that range is only 3.125% reflectance. Just bear that in mind because it WILL take you a while to get your head around it becasue most of the books are wrong since they all copied the information from each other without understanding it.

A 10 stop range is 0-IX. Not 0-X which is 11 stops. this is another error commonly repeated. Ansel's and Fred Archer's original writings talked about 10 zones, 0-IX.

Bollocks, you should try actually reading the Negative and NOT some other persons interpretation.

I grant you that AA used a differeent range to start with but he evolved it to zones 0 thru 10 which is what he writes about in the negative and clearly shows it. So if you get picky and ignore "The Negative" then you have an argument but where he finished was with 0-X zones and you should state at what point in that evolution you are refering to.

RobC · Feb 29, 2016

OMG we're into another AA argument. I'm opting out as I've been here to often and it never ends pretty. Just accept everyone has their own interpretation.

macfred · Feb 29, 2016

Do you meter still or do you take photographs already ??

Diapositivo · Feb 29, 2016

Hallo,
certain very stressing circumstances of life brought me away from these shores for three years or so. I plan to begin developing film again in a few months. I greet everybody and I am glad to find some old acquaintances. Hallo everybody.

Yes the theory published a bit everywhere in books and magazines says that the incident reading and the reading of the light reflected by the middle gray card should give the same value, and that's more or less right, with some caveats which have already been discussed in the past years on this distinguished and very technically minded forum.

The net result that I distilled from the forum wisdom, about why the actual readings differ can be boiled down to some circumstances that are normally not taken into account:

1) The light meters in the camera deliver an exposure calculation for the "average greyness of the world" which, according to some research dating back many years, was found to be around 12% - 14% and not 18%.

18% is the middle grey in the printing industry, which goes from dark black on paper ( around 2% of reflectivity) to very white paper (corresponding to around 90% of reflectivity). "Middle" of this range is considered 18%.

Real world around us shows very little pure black, very little pure white, and a different middle grey. So 18% is not, in principle, the "average reflectance" for which light meters are set in factory.

Kodak instructions with grey cards say to measure on the grey card, and then open half a stop.

2) There is a certain risk of specular reflection when measuring the light falling on a flat smooth surface. This "mirror effect" might increase the exposure value read on the grey card.

The two combined effects should produce a difference in exposure of more than 0,5 EV (and maybe more than 1 EV), the reflected reading leading to less exposure than the incident reading.

In theory, though, there must be coincidence if the two mistakes are eliminated.

If we had a corrugated 12% - 14% grey card, and supposing our reflected lightmeter is factory set for that same 12% - 14% reflectance world, the two measurements should and must coincide.

In practice, an incident light meter is much easier to use than a grey card, which is prone to many mistakes in use (angle of use, shadow of the photographer, "not really middle grey as the average world", your subject not having hands etc.) and their real use, in modern world, is mainly to check whether the measurement obtained by it is the same as the one obtained with the incident light meter, finding it is not and then asking on APUG why it is not (I did that myself, if memory serves).

pentaxuser · Feb 29, 2016

A good learning thread is this one for me. To continue with this practical learning theme let's assume that I only have a spot meter and a Kodak grey card but want to get the same accuracy in metering as given by an incident meter. For simplicity lets assume that it is overcast conditions and I can ignore getting the angle of the card right and the scene is a 71/3 stop range.

In practical terms what do I need to do after spot measuring the grey card to get the correct(a la the incident meter) exposure ?

Thanks

pentaxuser

RobC · Feb 29, 2016

18% reflectance is the middle of a 5 stop range and slide film accepts approx a 5 stop range.
18% reflectance is not the middle of a 7 1/3 stop range which is what black and white film standard works to and pretty much colour film too.
Until people understand the difference between film which is not a medium we take reflectance readings from and that the reflectance meters aim to put the exposure in the middle of the film curve more or less and which is nothing to do with 18% then people are not going to understand that 18% reflectance is meaningless to exposure.
A kodak grey card is just over 1 stop lighter than the middle of a 7 1/3 stop range which is what current black and white film standard works to.

RobC · Feb 29, 2016

pentaxuser said:
A good learning thread is this one for me. To continue with this practical learning theme let's assume that I only have a spot meter and a Kodak grey card but want to get the same accuracy in metering as given by an incident meter. For simplicity lets assume that it is overcast conditions and I can ignore getting the angle of the card right and the scene is a 71/3 stop range.

In practical terms what do I need to do after spot measuring the grey card to get the correct(a la the incident meter) exposure ?

Thanks

pentaxuser

It's a moot point becasue it all depends on your film development but if you use manufacturers film ISO speed and dev times and temps, then I would meter the card and open up 1 stop. Change to your own EI and and/or mess with the dev strength times or temps then you're on your own.
If you go down the zone system route and use approx half ISO speed and reduce dev by 30% or so, then you should throw your grey out and just meter a zone 7 subject area and open up 2 stops. Or meter a zone 3 area and close down 2 stops. metering the zone 7 area will give you much more consistent print times and will work in all cases except where the subject range is greater than 10 stops in which case meter a zone 3 area and close down 2 stops.

If you are doing colour work then stick with manufacturers ISO speed, normal dev, meter the card and open up 1 stop.

I suggest you try it out and see what you get. The proof of the pudding is in the eating...

pentaxuser · Feb 29, 2016

RobC said:
It's a moot point becasue it all depends on your film development but if you use manufacturers film ISO speed and dev times and temps, then I would meter the card and open up 1 stop. Change to your own EI and and/or mess with the dev strength times or temps then you're on your own.

Thanks for that RobC. Can I also take it that the opening up one stop correction applies under any and all light conditions( accepting that in sunlight the angle of the card will be important)

Can I move on now and ask: under what circumstances might the spot meter produce a "better exposure than an incident meter which I think you mentioned in one of your earlier posts?

pentaxuser

DREW WILEY · Feb 29, 2016

Whatever system you use, you simply have to become accustomed to it. I've been using strictly Pentax or Minolta spotmeters for the last thirty years, for everything. They read identically but differ in controls. What does Hollywood primarily use, when serious money is at stake and you have to do it right the first time? The same thing. These need to be recalibrated from time to time; for me about once a decade. And don't assume that commercial gray cards are correctly made. They rarely are. A few expensive gray scales like those from MacBeath are quite good. But I can't speak to "sorta"
spot options on either general purpose meters or TTL in-camera. I never fully trust those - too many other things potentially going on other than a
simple reading.

Different measurements with spot vs incident metering on same target?

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