Simple Question Concerning Negative Density

[al_divenuti]

If negatives produced from two different films possess identical

densities in a specific region - will these regions print with the

same tonality assuming that each negative is printed identically?

 

Let me put this another way...

 

If I have two negatives - both of which have a uniform (makes this

easy) density of 0.7 - and one negative is Tri-X film and one

negative is HP5+ - can I expect that both negatives will render the

same tone on the print if all aspects of the printing of both

negatives are identical?

 

Probably a silly question, but related implications can drive you to

distraction...

[conrad_hoffman]

The absolutely "considering every arcane detail" scientific answer is no. It has to do with light scattering, how the transmission densitometer was designed, the enlarger light source vs. the densitometer light source, and various other things, not to mention grain patterns and visual perception. The practical answer is probably yes, within some small tolerance. An even more interesting question is, will prints from different films and developers look significantly different if the negatives are processed to exactly the same contrast index? That's something I'm (slowly) working on right now. I have a suspicion that developer differences are far smaller than is usually believed.

[andrew_hull]

Conrad:

 

wasn't there an example of two shots of the same subject, one developed in Xtol and one in Diafine that was posted a while back here? I recall the differences were minute.

[george_pappas1]

Al,

 

The answer is no; differences will be present even if they are subtle. The real issue here is whether the difference is enough for you to choose one or the other. The only way to find out is to experience the difference for yourself.

 

A ton of testing and comparison of films, developers, enlarger light sources, etc. has been done over the years by many people. Usually the results are described as "not significantly different", or "sensitometrically identical", or some other comparative phrase.

 

However, Photography is a subtle medium and photographers exercise their preferences through their choices. That is why many people gravitate to a certain camera/format/film/developer/paper/enlarger, etc.

 

I wouldn't let the "implications drive you to distraction". So many of us spent too much time testing versus photographing (myself included). Pick a combination that you like; try new variations from time to time if you are unsatisfied or to see if there is any improvement to be had.

[jay_de_fehr]

My suspicion is that the two films would produce the same tone in a print, but your question ignores the many significant factors that contribute to the character and performance differences in films, not least of which is their differing characteristic curves. To isolate a single density/tone in the films' curve is not indicative of the differences in those films curves overall, or spectral sensitivity, or grain pattern etc., etc. My suspicion is that X density will produce Y tone regardless of the films speed, spectral sensitivity, developer, etc., unless one of the films was developed in a staining developer and the other was developed in a non-staining developer, and the printing paper was variable contrast, in which case the density of the film developed in the staining developer is composed of two separate components that combine to make up the total density, and the color of that density will affect the tone in the print. Make sense?

[pascal_miele]

with same density you will have same tone....

 

but only tone, grain can be different.

 

When you measure the density you must use a densitometer with a sensibility equal to the paper sensibility (blue and not visual)

 

If you compare a chromogenic and a silver film the silver has Callier effect with condenser enlarger.

 

conclusion : the experience need many controls and prove nothing !!!!

 

Yes the same level of light give the same photographic effect, and it's good for photography !

[doremus_scudder1]

Al,

 

Some of the respondents above seem to have missed a thing or two.

 

First, your assumption is correct for all practical purposes. If you have two negatives with equal and uniform densities, they are capable of reproducing exactly the same tone in a print (sure, the grain pattern may be different, but the tone in the print will be the same). What people seem to forget is that these same two negatives can be made to render ANY tone that a given photographic paper is capable of. Also uniformly exposed negatives of widely differing densities can be made to render the exact same tone in a print. Simply expose for the proper amount to achieve the tone you want, regardless of the negative density. You can even do this without a negative at all, just by giving the photo paper the requisite amount of light.

 

All this is well and good, but not much help in choosing film or getting to know the characteristics of film.

 

The important difference in films, besides grain size and structure, is the distribution of tones (densities) in relation to exposure. The general range of density in a negative can be exaggerated or minimized by extending or shortening development time respectively (read increase or decrease contrast). However, it is important to realize that each film has a unique characteristic curve that describes exactly how the densities over a large exposure range will be rendered in relation to each other. This characteristic distribution pattern is not easily changed. Finding a book or a comprehensive film data sheet with a couple of characteristic curves that you can compare will help in understanding this. I'll try to explain it as well as I can without visual aids.

 

An ideal film would give precisely the same change in density for a given change in exposure, from shadows to highlights. Its characteristic curve would be a straight line on a graph with the vertical axis being density change and the horizontal axis exposure change. One can change the contrast of this ideal film by developing longer for more contrast or less for lower contrast. This affects the steepness of the straight line in the graph. Steeper equals more contrast, flatter equals less contrast. This means proportionally more or less density change for given differences in exposure. With this ideal film, regardless of development, a given exposure change results in a given density change throughout the entire range of exposure.

 

This ideal straight line film does not exist. It is not only impossible to achieve, but not always desirable. All films have flatter toe areas (the area that corresponds to the shadows, is the least dense in the negative and is just above the point where the film begins to respond to exposure to light). Some films have toes that are very long and flat. These can record a long scale in the shadows, but with less separation. Other films have toes that become steeper more quickly. These will give more shadow separation, but record less exposure range.

 

The corresponding section for the high-density parts of the film is the shoulder of the curve, which corresponds to the highlight (densest) areas of the negative. Many films have a marked flattening of the curve in the shoulder, which reduces separation between the highlight values. Some films have little or no shoulder at all. These films have relatively more separation in the highlights, sometimes separating them too much for aesthetically pleasing printing on modern photo papers.

 

The middle portion of the characteristic curve can also have different shapes for different films: Some are relatively straight, some concave, some convex, some wavy, etc. Whatever the shape, the steeper the line, the more density change there is for a given difference in exposure. For example, concave mid-section will render mid tones with increasingly more separation (i.e. more density difference for a given exposure) as the exposure increases. A convex curve results in decreasing separation as the exposure increases. Some curve shapes are better for some things than others...

 

In other words, the same exposure of a scene with a wide range subject brightnesses on films with different-shaped curves will give a different distribution of tones.

 

Common differences are: more and less separation in the shadows and highlights (corresponding to different toe and shoulder shapes of the characteristic curves of different films), mid-tone relationships and the accompanying different densities in these areas.

 

As an example, I often prefer a film with a marked shoulder for photographing on overcast days when the overcast sky is relatively bright, but the other elements relatively dark. A convex curve with little toe and a relatively flat shoulder gives lots of separation in the shadows and lower mid tones, a higher relative value (brighter in the final print) for the mid-tones, but compression (less bright in the print) of the highlights and, especially, that too-bright overcast sky. A film with little or no shoulder would be better for situations where highlights are flat and require more "snap".

 

Add to all this the facts that printing papers all have curves too, and they vary from brand to brand and grade to grade, that different developers can affect curve shape (more or less shoulder for example) and that altering development time, can change contrast and that different film exposures change where important areas of the scene fall on the characteristic curve, and you see what a can of worms this whole issue can be.

 

However, if one concentrates on the general differences and characteristics of film curves and doesn't get swamped in the details, this knowledge can become a useful tool. For example, I routinely use two different films, one with a marked shoulder and one without. Many subjects I shoot on both, but many times I choose one film over the other based on the subject (as in the examples above). Also, I will intentionally overexpose when shadow detail is of paramount importance. This moves those shadows from the toe onto a steeper portion of the curve resulting in more separation. This all in addition to the normal development controls inherent in the Zone System.

 

I hope this over-simplified explanation helps a little.

 

Regards,

 

[peter_svensson]

Is the base density included in the 0.7? If not, differing base densities will ensure different tone.

[al_divenuti]

Yup, assume that the density of 0.7 represents NET density (i.e. subtract density due to film base + fog)