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Reciprocity Failure

Published : 13th July 2004


Does reciprocity fail in the same way with digital sensors as it does with film? Are CCD and CMOS different in this regard?

Just curious...

Dan Drasin
Producer/DP
Marin County, CA



Dan,

Digital doesn't have real RF but instead builds up noise in underexposed frames.

I did a 4 hour exposure on stars recently on Velvia 35mm with my Nikon an got severe shift in color- but don’t even know how to do this digitally. would like to try it.

Caleb Crosby, s.o.c.
DP/Cameraman
www.calebcrosby.com



Generally, CCD sensors don't have "reciprocity failure", where the sensitivity changes with exposure time. But correction is needed for changes in the image due to bias and dark current:

http://www.astrosurf.com/re/ccd.html

"The most basic steps in pre-processing a raw CCD image involve subtracting bias and dark frames and dividing by a flat field frame. The result is a pre-processed or calibrated image. Individual dark and flat fielded frames or calibrated images can be added or averaged for better results. I use the "master dark technique". This technique assumes that the rate at which dark current accumulates in each pixel is a linear function of the temperature in such a way that dark frames for different temperatures can be made by linearly scaling a "master" dark frame."

Kodak sensors are widely used for ultra high resolution applications and astrophotography. Many satellite imaging systems use Kodak sensors and optics. Kodak sensors and technical information:

http://www.kodak.com/global/en/digital/ccd/sensorsMain.jhtml

John Pytlak
Eastman Kodak Company
http://www.kodak.com/go/motion



Caleb,

I don't think Velvia is a good choice for any long exposure work where you're looking for color fidelity as it's not a very unforgiving stock. If I'm doing time - lapse night filming I sometimes shoot long exposure stills at the same time, to save me from boredom and for possible later After Effects type use or for my own stock.

I tend to use 4x5 camera for stars, because you need the resolution for the small stars, with a 90 or 65mm wide angle for a big chunk of sky and use one of several of the Kodak or Fuji 100 speed stocks and get good color.

Transparency still film is sharper and has less grain than negative, although at 5x4 size it matters less.

Paul Hicks