Re: scanner density-value relation

----- Original Message -----
From: "Michael Koch-Schulte" <mkochsch@shaw.ca>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Tuesday, March 08, 2005 9:09 PM
Subject: Re: scanner density-value relation

> Ryuji Suzuki wrote:
>> If you use step tablet of a large enough patch size, you
>> can average a
>> large region within the patch to reduce the measurement
>> error. This is
>> how I figured out my scanner's linearity limit. I know
>> the noise is
>> one important issue but my question is related to
>> linearity not the
>> noise.
>
> There's also light fall off at the edges to consider the
> effect is present
> even with a diffusion source. My Epson 3200 is advertised
> as 3.4 but it
> barely makes it to 3 if you are using a step wedge to
> judge. I did
> rudimentary tests using step wedges to find this out. I
> also noticed that if
> I rotated the step wedge 90 degrees or only scanned a
> small portion (the
> light lid only turned on for a brief amount of time) my
> results would
> drastically change for the better or worse. For the most
> part the graph was
> fairly linear except for the extremes (beginning and end
> portions). I found
> that by using Vuescan as my scanner driver and increasing
> the exposure from
> the nominal value of 1 to 1.2 help to straighten out the
> line a bit too.
>
 I wonder if flare is a factor. The test is to mask off the
area of the step wedge so that the rest of the field is not
illuminated.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@ix.netcom.com