Plane of Best Focus of Flatbed Scanners

[Kirk Gittings]

Maybe I am confusing terms. I say that because a 1-2mm difference in height of the negative can effect the sharpness of the scan. Other things being equal, what would it be but shallow depth of field? I can think of nothing else which would explain this. Though I have not played with this on the new V750, I have certainly found this to be true on earlier models. Epson acknowledges this on the new models by giving you spacers which can reset the distance above the glass from 2.5 to 3.5 mm. Also noted by: http://www.photo-i.co.uk/Reviews/int...00/page_13.htm

[sanking]

Maybe I am confusing terms. I say that because a 1-2mm difference in height of the negative can effect the sharpness of the scan. Other things being equal, what would it be but shallow depth of field? I can think of nothing else which would explain this. Though I have not played with this on the new V750, I have certainly found this to be true on earlier models. Epson acknowledges this on the new models by giving you spacers which can reset the distance above the glass from 2.5 to 3.5 mm. Also noted by: http://www.photo-i.co.uk/Reviews/int...00/page_13.htm

I don't know why Epson provides the spacers. It may me that the spacers help at certain *sweet* spots over the scanning surface. But spacing does not appear to improve resolution over the entire surface in my testing with the Epson 4870 and Microtek 9800XL. Spacers appear to improve resolution in some areas, but degrade it in others. I suspect that the improvement or degradation is due to the fact that the the lens is covering a wide area and by necessity light from some areas is reaching the lens aperture at very acute angles. I would speculate that this is potentially important because the net effect is to effectively increase or decrease the size or effective aperture of the lens opening, thus significantly varying the effects of diffraction.

In any event much of the literature on the subject suggests that the lenses in CCD sensor type scanners are short focus and are used at very small apertures which provide great depth of field, up to several cms. In fact, some people have even used them for scanning of 3-D objects. See some of the following sites.

http://www.normankoren.com/Tutorials..._flatbeds.html

(See NLK's comments in response to email from Dennis Wilkins)

http://csdl2.computer.org/persagen/D...1109/38.824535

http://www.microscopy-uk.org.uk/mag/...dwscanner.html



Sandy King

[Kirk Gittings]

Sandy, Interesting information. I stand corrected on the depth of field reason for sure.

I would speculate that this is potentially important because the net effect is to effectively increase or decrease the size or effective aperture of the lens opening, thus significantly varying the effects of diffraction.

Fascinating.

One cras possibility is also that the spacers allow Epson to igore a level of QC by putting critical focusing back in the hands of the consumer.

[sanking]

Sandy, Interesting information. I stand corrected on the depth of field reason for sure.



Fascinating.

One cras possibility is also that the spacers allow Epson to igore a level of QC by putting critical focusing back in the hands of the consumer.

Kirk,

I am just speculating, not stating anything as a fact. However, there must be a reason why some people see an improvement in changing the position of the film plane, and others do not, and given the large DoF of these scanners, and the wide area covered by the lens, the diffraction concept seems a viable explanation. It would be nice if someone with a good engineering knowledge of how these scanners work would comment on the DoF question. I just don't know enough about the optics involved to make the calcuations to determine how much difference in resolution would result from changing the plane of focus 1-4mm.

Sandy King