Comparing Spectral Sensitivities

When comparing sensitivities of films, meters and the eye, one must compare not the peaks but rather the area under the entire curve.

Also, the eye is linear and has the capability to extend its sensitivity in darkness and bright light by changing sensitivity and effective "speed". In addition, the meter and films are not linear and have a pronounced toe and shoulder which the eye does not have by virtue of its variable sensitivity.

This has no simple answer and requires a lot of understanding of the entire topic. Some of it is loaded with concepts that are best explained by math.

PE

In the conversation that brought rise to this thread, some reference was made to the Zone VI Meter Modifications to the Pentax Spotmeter. I thought this take on those modifications was quite interesting: http://www.butzi.net/articles/zone%2...worth%20it.htm

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Originally Posted by Leigh B

The only comparisons that are meaningful are between the film sensitivity curves and those of various exposure meters.

While the meter sensor technology has a major impact on its spectral sensitivity, it does not define same.
Meter manufacturers incorporate filters to modify the sensor response, in an attempt to duplicate that of film.

For a meaningful analysis you need to obtain the sensitivity curves for the actual meters (make and model) of interest,
and compare those with the curves for the actual films.

- Leigh

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You're right Leigh, comparing the spectral response of the fully assembled meter versus the specific film in question is important ... especially for example the Zone VI Modified meters which tried to emulate the spectral response of Tri-X.

The human eye response is relevant when using the SEI photometer, which uses the eye as a sensor. But I think Matt may be more interested to know why results don't match what he "felt" at the time of the shoot.

I dislike the conclusions in that butzi.net article that rely on a Macbeth Color Checker test.

I believe the relevant way to test colors that don't meter "linearly", such as Periwinkles, Greens and Shadows, is by using live subjects - for example green pigments don't reflect infrared to the same extent that living foliage does...

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Originally Posted by Bill Burk

I dislike the conclusions in that butzi.net article that rely on a Macbeth Color Checker test.

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The Macbeth chart is widely used for tests because it renders colors accurately and consistently over a wide range of light sources.

That is not true of other color swatch products.

- Leigh

I'll have to agree with Bill.

The Color Checker does not "handle" IR - or for that matter, UV - the same as the real outdoors.

That said, it is a very useful tool and can be used for determining spectral sensitivities and speeds in comparisons. One must be careful to use the right conditions though.

PE

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Originally Posted by Photo Engineer

The Color Checker does not "handle" IR - or for that matter, UV -

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Why should it? It's a visual standard. By definition 'visual' does not include IR nor UV.

We may need to revise the definition after the Denisovans take over the planet, since they have extended visible range.

- Leigh

Leigh;

The problem is that you must then meter and photograph the real world with an IR and UV filter to make things even out. See my point?

PE

Quote:

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Originally Posted by Leigh B

The Macbeth chart is widely used for tests because it renders colors accurately and consistently over a wide range of light sources.

That is not true of other color swatch products.

- Leigh

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Hi Leigh,

The color checker is fine for standards - for color reproduction for copywork or studio illumination, calibration etc. But it is made of ink pigments.

When photographing nature, living things have anomalous ultraviolet and infrared reflections that the color checker doesn't represent. If you tell me the Macbeth company deliberately selected metameric inks representing "foliage green" and "periwinkle blue", then I'd concede the point, because it is possible to select substances that reflect light "the same way". But I think they chose certain Pantone Matching System inks and left it at that.

These are the things that come out looking illogical in photographs because the "system" (un-modified meter, color checker, film) didn't properly evaluate them.

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Originally Posted by Bill Burk

But it is made of ink pigments.

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Bill,

All colored objects that are not self-luminescent derive their color(s) from pigments.
The number thereof available in the printing industry is staggering.

Many of these are inorganic, and can exhibit very stable colors over centuries or millenia.
One recent example is a particular blue that was used by the Mayan culture in Central America 2000 years ago.
It is still as bright and vibrant as when it was originally mixed. The same is true of their red pigment.

The choice of specific pigment(s) to use for a particular application will take into account many factors.
When one is designing a standard, stability and longevity are top priorities. Macbeth did its homework.

- Leigh