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Photography Question 
Jackie Robins

member since: 1/15/2008
 

cause of losing detail in shadow of negative


The cause of losing detail in the shadow of the negative

1/15/2008 12:50:02 PM

 
Gregory LaGrange
BetterPhoto Member
gregorylagrange.org

member since: 11/11/2003
  Loose your text book?

1/15/2008 1:11:03 PM

 
Alan N. Marcus

member since: 3/4/2006
  Hi Jackie,

Helping you with your homework is my pleasure.

We humans have been truly granted a remarkable vision system consisting of an eye/brain combination. As we look about on a bright sunny day we see a reflection of the sun gleaming off an automobile chrome trim. In the same vista we see the black automobile tires and upon close inspection, we can see that the tire is treaded. What I am trying to say is; the human eye/brain can observe and make out detail over quite a wide brightness range. We call this dynamic range. If you were to measure the black tire in shadow you will likely find it measures 1 foot candle brilliance. Measure the gleaming chrome and likely it measures 2000 foot candles. Thatís a dynamic range of 2000:1.

Now with film and even digital chips science has fallen far short as compared to human vision. Likely (there are exceptions) but the best we can consistently do is record a range of about 8 f/stops. Each f/stop represents a 2x change in light so we express this as 2x2x2x2x2x2x2x2=256 meaning the dynamic range of film is 256:1. Thatís 2 to the eight power. After that range the film is totally unexposed at one end meaning the film is void of detail and thus completely clear. At the other end the film is totally blackened and thus also void of detail.

You are asking about the shadow end of this range. You want to know why the shadows are void of detail. They are void because film requires an exposure to light to record an image. The lens projects an image of the outside world onto the film. This image is not uniform in brilliance, it has both bright and dim areas. The dim areas are the shadow areas and if they are void of detail it means the amount of light that played on these areas is below the threshold of energy needed by the film or chip. Thus the film or chip was unable to make any record.

When an area of the negative is grossly underexposed it remains clear and void of detail. It would have been possible to record detail in the shadows. One could bring in supplemental fill light. One could have used a slower shutter speed allowing more time for this low level of light to play on the film. Longer exposure improved the likelihood that shadow detail will record because film and chips have the ability to accumulate light energy over time. One could use a lens with a larger diameter opening. A larger diameter (aperture) allows more light to play on the film. One could choose a more light sensitive film (higher ISO). One could over-develop the film a technique that reduces the necessary threshold light energy required..

No one said itís easy!
Alan Marcus (marginal technical gobbledygook)

1/15/2008 2:17:50 PM

 
Christopher A. Walrath
BetterPhoto Member

member since: 4/25/2006
  One addendum to the infinite wisdom of the gobbledygook god (I love that word) is the old addage to expose for the shadows and develop/print for the highlights. At least with negative film (assuming film as of where you posted this thread). A film negative is comprised of silver halide crystals encapsulated in a gelatin container. When light strikes the crystals it turns to black metallic silver and, when developed, the negative retains the black metallic siver in these areas and the remaining unexposed silver halides are washed away. So, where there's light, there is negative detail. Where there is no light exposure, there is no detail. No chance to get anything back. So it is of the utmost importance to make sure that you shadow areas of importance get the proper exposure to record the detail you envision in the final print. The highlights will have negative densities no matter what. Not to neglect them, because overexposure in these areas will result in washed out prints. So expose for the shadows.

Develop/print for the highlights. Well, there is information on the negative and through decreased exposure (N-1, etc.) or dodging during the printing process, the highlight areas can be brought down to a more managable area. As long as there is tonal seperation there to work with in the first place.

1/16/2008 7:24:43 AM

 
Alan N. Marcus

member since: 3/4/2006
  Hi Chris, if I had to choose one area of photo science to hang my hat on it would be what happens when light sticks film. So this old lecturer wants to add -- Films are made using crystals of silver salts. Theses salts are held on the film using a binder consisting of highly purified unflavored gelatin. Only three salts of silver are practical. They consists of the metal silver combined with one members of the halogen family (salt maker) silver bromine (highest sensitivity) Ė silver chlorine Ė silver iodine (lowest sensitivity).

When a silver salt crystal is struck by a sufficient number of photons (light energy) the bond holding the crystal together (silver with halogen) is weakened. The developer is a reducing agent able to liberate metals from its salt. Further, the developer is able to differentiate between exposed and unexposed silver salts crystals. After chemically identifying that a crystal is unexposed, the developer reduces the crystal to its two component parts. The metallic silver portion is not soluble thus it remains behind embedded in the gelatin, it appearance is that of an unpolished metal and its opaque thus it emerges as the black stuff that makes up the image. The halogen portion of the crystal is water soluble and quickly dissolves away into the waters of the developer. After the developer stage, to make the image permanent, we must remove all traces unexposed and thus undeveloped silver crystals as they will blacken over time causing the image to fad away. The fixer step follows the developer (a stop-bath step is stuck between) takes care of this task. The fixer renders undeveloped silver crystals solvent thus they dissolve in the waters of the fixer. The image which now consisting exclusively of metallic silver which is not soluble, remains infused in the gelatin.

Alan Marcus, Anaheim, CA (more gobbledygook)
ammarcus@earthlink.net

1/16/2008 8:35:36 AM

 
Alan N. Marcus

member since: 3/4/2006
  Correction:
After chemically identifying that a crystal is unexposed, the developer reduces the crystal to its two component parts.

After chemically identifying that a crystal is exposed, the developer reduces the crystal to its two component parts.

1/16/2008 9:20:49 AM

 
Christopher A. Walrath
BetterPhoto Member

member since: 4/25/2006
  I learn something everytime this man sets down in front of a monitor. I love to have photografia further defined past what I know. Helps us all grow. Thank you, Alan. Good work.

Thanks
Chris

1/16/2008 2:49:22 PM

 

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