BetterPhoto Q&A
Category: New Questions

Photography Question 
Donald R. Curry
BetterPhoto Member Since: 3/2/2006


Are pixels created by scanning a slide effective pixels or real pixels?

To love this question, log in above
11/8/2006 6:07:19 PM

Bob Fately   I'm not certain of this, but I'm responding as much to read what others say...

Anyway, the "effective pixel" thing we hear about with digital cameras reltes to the factor that (except for the Foveon CCD used in the Sigma DSLRs) CCD and CMOS chips are monochromatic - each individual light sensor only sees photons, irrespective of color.

Thus, manufacturers overlay the individual sensors with colored filters in the oft-mentioned Bayer pattern - 50% green, 25% red and 25% blue. So, for a sensor sitting under a blue filter, the photons that hit will be only those that pass through the blue filter (red and green, etc. are absorbed by the filter). The camera's onboard computer then builds an image by taking all these individual readings and interpolating to give the colors recorded in the RAW or JPEG file. THus the term, effective pixels - the image you record is not precisely what the CCD read, but the result of this post-read calculation stuff.

The Foveon CCD differs in that each light-sensing cell actually reads the R, G and B values thanks to some clever design - the Foveon has layers much like color film so each of the primary colors is read in a layer.

Now as for scanning, if the scanner inquestion does a 3-pass job, it means that the same CCD (or PMT) reader will "read" a given spot of the film, each time with a different colored filter overlaid on it. As a result, I think it's more appropriate to state that for (most) film scanners the pixel count is not "effective" but "actual".

At least that's my understanding of things - again, perhaps more knowledgeable folk will correct me.

To love this comment, log in above
11/9/2006 5:49:29 AM

Log in to respond or ask your own question.