Is it possible to train oneself to accurately interpret light intensity?

David Lyga · Dec 29, 2013

Of course, Michael, one MUST understand that meter's parameters and its 'lack of sentience'. It has no feel for subject matter; that missing link has to be gotten from your brain.

A really good exercise to adopt that wastes no film is to practice guessing at proper exposure and then meter that scene to see if you are correct. Obviously, you have to make the adjustment for the meter's 'neutral' reading (if that scene is not so 'neutral') and come up with a FINAL evaluation as to proper EV. And an aid in this regard is to start thinking in terms of EVs, not standardized shutter speeds or f-stops. This way, only one (combined) number is needed to define the exposure needed for the scene, and, importantly, that single number can offer a panoply of choices as to shutter speeds and apertures. For example, if one resolves to use '16' for a cloudy-bright day with Tri-X, that simple number offers a range of aperture and shutter EVs that allow easy interchangeability because all that is necessary is that they 'add up' to '16'. -David Lyga

Alan Klein · Dec 29, 2013

I've noticed that there is a stop difference between an incident reading and reflected light reading off a gray card all in the same light. Have you guys found the same thing?

Bill Burk · Dec 29, 2013

That's because the reflected light meter is not calibrated to 18% gray.

Try metering the palm of your hand and opening up a stop (place on Zone VI). And compare _that_ to an incident reading.

Alan Klein · Dec 29, 2013

If its not calibrated to 18% gray, why do people use it? What's the point? Exposures are going to be different between reflected and incidence. The answer can't be read the palm of your hand. What if you're black? You should be able to use the meter to get the right exposure with a reflected meter. How do you use a reflected light meter? What is the purpose of the gray card?

MattKing · Dec 30, 2013

Are you following the directions for the grey card?

They are quite specific with respect to the angles of incidence.

Bill Burk · Dec 30, 2013

Alan,

It's frustrating I know. I think it used to make Ansel Adams mad, and is the basis of a lot of internet discussions.

The gray card is a "known" reflectance, 18%, from which you take a reading and then calculate the appropriate exposure for an 18% reflectance. Circular reasoning I know.

The standard calibration of exposure meters, on the other hand, is not an arbitrary or mathematically centered gray. It's not 18%. The exposure meter is actually calibrated to pictures. Scientists studied sets of photographs. They put these photographs in front of people and asked them to pick the best ones. From the research, they established a standard that would lead to a statistically high percentage of successful exposures.

So you "are supposed to" just point and shoot.

kintatsu · Dec 30, 2013

Bill Burk said:
That's because the reflected light meter is not calibrated to 18% gray.

Try metering the palm of your hand and opening up a stop (place on Zone VI). And compare _that_ to an incident reading.

My understanding was that both types were calibrated the same, modified for some K factor that varies between manufacturers. That K factor is part of the reason we use our working meters in film speed and developing tests.

It's also possible that some manufacturers use a 12% reference, as that represent middle grey on a scale of 60:1 white to black ratio, and is used by ANSI, if I remember correctly.

It would seem that measuring incident light would give a higher reading, as you're measuring the light source. It follows that a reflected reading would indicate less light, as the full intensity of the source is not involved. At least, that's my understanding.

If I'm wrong, I apologize, and am glad to learn something new.

David Lyga · Dec 30, 2013

There is a case to be made for scrapping that '18%' card and using a '90%' pure white one, instead. That way you get to meter more scenes' ultra low lighting environments. You simply subtract a few stops to then get your actual reading. - David Lyga

E. von Hoegh · Dec 30, 2013

David Lyga said:
There is a case to be made for scrapping that '18%' card and using a '90%' pure white one, instead. That way you get to meter more scenes' ultra low lighting environments. You simply subtract a few stops to then get your actual reading. - David Lyga

Or you can just use a LunaPro.

Alan Klein · Dec 30, 2013

Kintatsu: I believe you're correct. I recall reading about the 12% vs the 18%. But of course, that begs the question. If the meters are set for 12% standard, of what value is using an 18% gray card? Shouldn't we be using a 12% gray card so our reflective and incident readings will match? Does anyone make such a thing?

E. von Hough: Have you actually tried 18% gray card measurement and incident readings with the Luna Pro in the same light? Do you get the same readings? If so, it appears then that your meter is calibrated to the 18% reading not the industry standard 12%.

E. von Hoegh · Dec 30, 2013

Alan Klein said:
Kintatsu: I believe you're correct. I recall reading about the 12% vs the 18%. But of course, that begs the question. If the meters are set for 12% standard, of what value is using an 18% gray card? Shouldn't we be using a 12% gray card so our reflective and incident readings will match? Does anyone make such a thing?

E. von Hough: Have you actually tried 18% gray card measurement and incident readings with the Luna Pro in the same light? Do you get the same readings? If so, it appears then that your meter is calibrated to the 18% reading not the industry standard 12%.

I was commenting on the Lunapro's low light capabilities, rather than it's calibration.

kintatsu · Dec 30, 2013

Alan Klein said:
Kintatsu: I believe you're correct. I recall reading about the 12% vs the 18%. But of course, that begs the question. If the meters are set for 12% standard, of what value is using an 18% gray card? Shouldn't we be using a 12% gray card so our reflective and incident readings will match? Does anyone make such a thing?

I'm not sure if anyone makes one, but it would make sense.

I remember reading. though, and forgot to mention, that not only is there a K factor, but also a C factor and what value the manufacturer uses seems to be up to them. I believe the 18% gray came from old print/publishing. If I recall correctly, the reason Ansel Adams and others pushed for 18% had to do with being able to print their negatives and have them published at the correct levels.

You may want to check a printing house for a 12% card. I just test using my meter and go with that. I've found with my Starlite 2, I get box speed for FP4+. The difference between 12% and 18% is about 2/3 stop, I think. I'm sure your meter can be checked by shooting 2 shots of the same gray card 2/3 stop apart and printing it out. I never tried it, so it's just a theory.

benjiboy · Dec 30, 2013

To interpret light yes, to judge light intensity no, because the human eye reacts to quickly to changing light intensity for the brain to register it accurately , that's why light meters were invented, you take the meter reading first then interpret it with your knowledge and experience before setting the exposure .

Vaughn · Dec 30, 2013

benjiboy said:
To interpret light yes, to judge light intensity no, because the human eye reacts too quickly to changing light intensity for the brain to register it accurately....

This is why I recommend to our students not to look at their prints just as they walk out of the darkroom, and instead wait several seconds for their eyes to adjust to the light. If a student looks at his/her print as they exit the darkroom, their pupils are still dialated and that first impression is made. They then tend to add a little more time as the prints will seem a little light -- even though their eyes adjust and they continue to look at the print. Sort of a psychological dry-down effect.

BrianShaw · Dec 30, 2013

kintatsu said:
... not only is there a K factor, but also a C factor and what value the manufacturer uses seems to be up to them. ...

K factor is for reflected light.

C factor is for incident light.

kintatsu · Dec 30, 2013

BrianShaw said:
K factor is for reflected light.

C factor is for incident light.

But they are calibrated to produce the same 12-18% depending on manufacturer, which leads to different readings on the meter, and between manufacturers.

Gossen uses K=11.37, so if C=250 for a flat diffusor, we end up with a reflected gray value of about 14.3%. Other values for C exist for hemisphere diffusor, 320 is common. So using C=320, k=11.37, gray= 11.36%. The reflected percentage formaula is % reflectance for grey= pi(3.1416) x k/c. It would seem to me, that both numbers are factored to get the final reflectance value, and are dependant on each other and other mathematical gymnastics.

BrianShaw · Dec 30, 2013

kintatsu said:
But they are calibrated to produce the same 12-18% depending on manufacturer, which leads to different readings on the meter, and between manufacturers.

Gossen uses K=11.37, so if C=250 for a flat diffusor, we end up with a reflected gray value of about 14.3%. Other values for C exist for hemisphere diffusor, 320 is common. So using C=320, k=11.37, gray= 11.36%. The reflected percentage formaula is % reflectance for grey= pi(3.1416) x k/c.

That is my understanding too. I wish I understood more about the designer INTENT of the engineering values they use in their design models.

No matter, I really believe that Bill Burk's bottom line (post #57) is correct. The design intent was for a photographer to be able to meter, use the provided exposure recommendation, and have a reasonably successful exposure MOST OF THE TIME. Knowing when that will and won't work is the difference between a good photographer and a photographer who is risking failure.

kintatsu · Dec 30, 2013

BrianShaw said:
That is my understanding too. I wish I understood more about the designer INTENT of the engineering values they use in their design models.

No matter, I really believe that Bill Burk's bottom line (post #57) is correct. The design intent was for a photographer to be able to meter, use the provided exposure recommendation, and have a reasonably successful exposure MOST OF THE TIME. Knowing when that will and won't work is the difference between a good photographer and a photographer who is risking failure.

That about sums it all up. I would add that that's precisely why we do testing with our gear and our film. It gives us the knowledge to get repeatable results without the randomness of these factors.

Sirius Glass · Dec 30, 2013

E. von Hoegh said:
Or you can just use a LunaPro.

Luna Pro SBC, Nikon built in, Hasselblad 45° PME

Bill Burk · Dec 30, 2013

Alan Klein said:
The answer can't be read the palm of your hand. What if you're black?

What if the most reasonable answer IS to read the palm of your hand? I'm not a person of color, but I believe human palms are universally Zone VI.

Same can't be said of the 18% gray card. It's not necessarily Zone V, and if you treat it like that, then you have discrepancies to frustrate you. People who calibrate their Zone System to camera tests with gray card as Zone V rate their EI differently than box speed. Certainly it will not agree with the incident meter at that point.

These frustrations can interfere with your desire to "accurately interpret light intensity". You'll ask yourself "Why don't I get the same results when incident reading compared to when I meter the gray card?" Haa, I think you did ask that.

I have a graycard from Sekonic that has several patches of gray. Trying to remember which one of those patches reads same as incident reading in same light has distracted me before. It's somewhere near the 12.7% patch. But remembering to pick the right patch - or using White paper and remembering how many stops that is from gray, is beyond my ability when taking pictures.

Placing my palm on Zone VI is not that hard. And on top of that... I find it pretty well matches my incident readings.

Alan Klein · Dec 30, 2013

Well, I honestly have to say I don't use a gray card even though I have one. If I can read the light I need with a gray card, then I can use the incidence meter which is easier.

Of mcourse, people shooting cameras with built in meters don't have that luxury. How are they calibrated by the factories?

BrianShaw · Dec 30, 2013

Alan Klein said:
... people shooting cameras with built in meters don't have that luxury. How are they calibrated by the factories?

Posts 57, 67, and 68.

Plus a few additional tricks to improve the likelihood of good exposure such as center-weighting and matrix metering.

Bill Burk · Dec 30, 2013

The weird thing is that you "know" that if you point at the sky your reading won't be the same as if you just aim and shoot straight ahead. Even though you know that the two pictures require the same exposure to look good together (for example in a slide show or diptych).

But... If you deliberately aim slightly down to reduce the influence of the sky... you are messing with the intended calibration. You can deliberately aim down all the time. But then you would have to recalibrate to account for your new unusual metering method.

This is one of the reasons I appreciate manual cameras more and more these days. I don't even use built-in meters any more, and remove batteries if the camera will operate without them, because I want to choose the exposure myself.

Bill Burk · Dec 30, 2013

Cameras with built-in meters can rule out a couple variables that a hand-held meter has to estimate. For example, with Through-the-lens metering, a camera can directly measure the flare from the lens, because the meter is looking through the taking lens.

kintatsu · Dec 31, 2013

Alan Klein said:
Well, I honestly have to say I don't use a gray card even though I have one. If I can read the light I need with a gray card, then I can use the incidence meter which is easier.

Of mcourse, people shooting cameras with built in meters don't have that luxury. How are they calibrated by the factories?

I remember reading that Canon and Nikon use 12.5 for their K, which means with c=250, they come out at around 15.7%.

Is it possible to train oneself to accurately interpret light intensity?

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