Tuesday, August 13, 2013

A Note On Long Exposures, Reciprocity Failure For Kodak Ektar And City Night Shooting

Kodak Ektar

City Streets At Night

General for a city light scene you can ignore reciprocity failure and expose at ISO 100 f8 or f11 to capture car light trails. These f stops work with almost any ISO 100 film when trying to capture slowly moving light trial. It does depend a bit on the lens and the angle and speed of motion. I usually use f8 for wider angle lenses and f11 for standard lenses.

Exposure time depends on whether or not you want to blow out street lights and how bright you want mid greys to be.


Reciprocity


Without going into the The Gurney-Mott Theory or subsequent rewrites the standard characteristic curves can be used to accurately predict how much compensation is needed for what part of the image. Of course you have to do conversions to put it in terms that you can use it on the different zones of your image.. When you do long exposures you are effectively working in the lower part of the curve how much of the image and by how much depends on where each zone lies on the curve and where you want it to end up in terms of density.

Note that each colour curve (blue having lower failure) has a different shape hence explaining the inconsistent colour balance in the lower extremes of the chart.

To form a stable latent images multiple photons (light as a particle) must impinge on the same area of the film within a very short time frame or the latent image doesn't form and the crystal site decays (relaxes) back to it's ground state. When the number of photons per second goes down to a low enough number the chances of the next photon hitting the crystal before it relaxes starts to decrease. No amount of pre-flashing or witchcraft will bypass reciprocity failure except maybe for processes like hypersensitization.

Reciprocity Failure For Ektar (1)

metered time -- adjusted time
1 ----------------- 1.4
2 ----------------- 2.9
4 ----------------- 6.3
8 ----------------- 14.1
15 ----------------- 29.7
30 ----------------- 68.4
60 ----------------- 159.0
90 ----------------- 261.2
120 ----------------- 371.7
240 ----------------- 871.5

(1)Table came from Lee (see comments) measured by his standards and should be considered as a starting point where adapting exposure to account for reciprocity failure is important.

Toronto Nights Old Style
10 second exposure at  f11 in a 1953 Kodak Retina IIa
Efke KB21 Expired April 1977

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