Can someone explain Equivalent Neutral Density?

Hi,
Ive been reading the technical data sheets for the Kodak films, and for the slide films, on the spectral sensitivity curve chart, it says "densitometry: E.N.D.". Now I know the E.N.D. stands for "Equivalent Neutral Density", however I have done a very thorough google search and read books on photographic processes and no where have I been able to find and explanation of what this means. Could someone on here please explain to me what this means? I would like to know how to measure and calculate equivalent neutral density. Please get as technical as you need to and it would even be helpful to throw in some math if necessary (Im very mathematically knowledgeable so I will know what you are talking about if you throw in calculus terms).

Also, on the characteristic curve chart, the vertical axis says "density", but doesnt say how this density is measured (is it visual density? is it measuring the total amount of light that passes through the film without using any special filters? It doesnt specifiy, however I believe the density is refering to "visual equivalent neutral density". I would also like an explanation on how to intepret the characteristic curves.

I believe there is a poster by the name of Photo Engineer who will probably be able to answer all of my questions, but if anyone else can help me I would appreciate it.

Thanks, Retro Film

There are basically three types of colour densities in colour photography:

-) the analytical one, by which each dye-forming layer is made seperately and its density over the spectrum is measured,
-) the integral one, by which the stack of these layers is measured the same way

-) the neutral density by which a positive film is spectrally exposed in a way, that when viewed, with the light the positive is designed for, a neutral (grey) impression is made. It is typically shown as well as three analytical grey densities and as one integral grey density.

All exposures can be made to form different peak levels of density.

The density of the characteristic curve

is the density obtained after exposure to light of a specified spectrum over a continuously varied magnitude, measured at the processed film by lighting with light again of a specified spectrum.

This relation/graphic is intended in many cases vor evaluation/comparison of films only. In case of materials serving as as final, viewed material that latter light should be of a kind typical for viewing this material. However, in data sheets you won't find any details of this light.

Okay thanks for the reply.
So lets see if I understand this. Equivalent Neutral density means that we have the correct ratios of each dye layer so that when the human eye is looking at it with the correct illuminant, it will appear grey? So how do we quantitatively quantify the amount of "equivalent Neutral density"? Are we just measuring log(total light projected/absolute total light transmitted) or do we use a visual filter. Depending on how I measure the transmitted light through the film, I can get different density readings, this is one of the things Im confused about and why I ask about the visual filter.

Also, I have seen sources discuss an equivalent neutral density in reference to an individual layer. For example "the cyan layer has an E.N.D. of 1", how can one single layer be said to have an E.N.D. reading? It seems to me that E.N.D. refers to all 3 layers collectively so that you can get a grey color. you cant get grey with just one layer. Im struggling to understand this.

The characteristic curves gives you basically information about the way a light sensitive material reacts to exposure. You can read

the sensitivity (speed)
the average contrast (gamma) of the formed image
the exposure range (min. to max. exposure)
the dynamic range (min. to max. density)
the varying way it renders the image over the exposure (bendings of the curve)
the possible variation in exposure with unchanged contrast of the images (latitude)

etc.


The form of the END curves tells you something of the quality of colour reproduction.

Visual grey can either be examined by means of a human or the human vision can be substituted by sensors.

As I said the END curve can be shown added with analytic curves. Of course no single analytical curve out of an exposure to render neutral grey can tell the whole story.


For a Holga shooter you are quite demanding...