Re: Step wedges

Peter Marshall (petermarshall@cix.compulink.co.uk)
Fri, 22 May 1998 19:22 +0000

In-Reply-To: <01bd8551$9d2f2640$LocalHost@newmicronpc>
> >What is the practical difference between the two mentioned above?
> Could I
> >produce something myself cheaper?
> >
> >Dwight
>
>
> Okay, Dwight. I've been thinking about this all day and I think I've
> got it.
> You need a sheet of film with even density of 0.15 cut into 20 strips of
> whatever width you like - let's say 1". Then let's make each step 1/4"
> wide
> so then we cut the 1" wide strips into pieces of 1/4, 1/2, 3/4, 1, 1
> 1/4, 1
> 1/2, 1 3/4, 2, ..., and 5". There are only 20 because for step 1 (which

I've done this before and it works more or less for a few steps. However
the real problem if you haven't go a densitometer is in deciding what is
density 0.15.

When you get a lot piled up I think reflections on each surface start to
cause a problem, as does dust, scratches etc.

You would also need to develop so that your straight line has the correct
gradient.

Its actually easier to do a test strip or strips at various times,
develop, measure the densities, draw a curve of density v time and read
off the times you need for the 0.15 steps under the same conditions.

As I said before if you want a reasonably accurate wedge pay the 8
dollars!

Peter Marshall

On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/